mm: migrate: add more comments for selecting target node randomly

[linux-2.6-block.git] / mm / migrate.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Memory Migration functionality - linux/mm/migrate.c
 *
 * Copyright (C) 2006 Silicon Graphics, Inc., Christoph Lameter
 *
 * Page migration was first developed in the context of the memory hotplug
 * project. The main authors of the migration code are:
 *
 * IWAMOTO Toshihiro <iwamoto@valinux.co.jp>
 * Hirokazu Takahashi <taka@valinux.co.jp>
 * Dave Hansen <haveblue@us.ibm.com>
 * Christoph Lameter
 */

#include <linux/migrate.h>
#include <linux/export.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/pagemap.h>
#include <linux/buffer_head.h>
#include <linux/mm_inline.h>
#include <linux/nsproxy.h>
#include <linux/pagevec.h>
#include <linux/ksm.h>
#include <linux/rmap.h>
#include <linux/topology.h>
#include <linux/cpu.h>
#include <linux/cpuset.h>
#include <linux/writeback.h>
#include <linux/mempolicy.h>
#include <linux/vmalloc.h>
#include <linux/security.h>
#include <linux/backing-dev.h>
#include <linux/compaction.h>
#include <linux/syscalls.h>
#include <linux/compat.h>
#include <linux/hugetlb.h>
#include <linux/hugetlb_cgroup.h>
#include <linux/gfp.h>
#include <linux/pagewalk.h>
#include <linux/pfn_t.h>
#include <linux/memremap.h>
#include <linux/userfaultfd_k.h>
#include <linux/balloon_compaction.h>
#include <linux/mmu_notifier.h>
#include <linux/page_idle.h>
#include <linux/page_owner.h>
#include <linux/sched/mm.h>
#include <linux/ptrace.h>
#include <linux/oom.h>
#include <linux/memory.h>
#include <linux/random.h>

#include <asm/tlbflush.h>

#define CREATE_TRACE_POINTS
#include <trace/events/migrate.h>

#include "internal.h"

int isolate_movable_page(struct page *page, isolate_mode_t mode)
{
        struct address_space *mapping;

        /*
         * Avoid burning cycles with pages that are yet under __free_pages(),
         * or just got freed under us.
         *
         * In case we 'win' a race for a movable page being freed under us and
         * raise its refcount preventing __free_pages() from doing its job
         * the put_page() at the end of this block will take care of
         * release this page, thus avoiding a nasty leakage.
         */
        if (unlikely(!get_page_unless_zero(page)))
                goto out;

        /*
         * Check PageMovable before holding a PG_lock because page's owner
         * assumes anybody doesn't touch PG_lock of newly allocated page
         * so unconditionally grabbing the lock ruins page's owner side.
         */
        if (unlikely(!__PageMovable(page)))
                goto out_putpage;
        /*
         * As movable pages are not isolated from LRU lists, concurrent
         * compaction threads can race against page migration functions
         * as well as race against the releasing a page.
         *
         * In order to avoid having an already isolated movable page
         * being (wrongly) re-isolated while it is under migration,
         * or to avoid attempting to isolate pages being released,
         * lets be sure we have the page lock
         * before proceeding with the movable page isolation steps.
         */
        if (unlikely(!trylock_page(page)))
                goto out_putpage;

        if (!PageMovable(page) || PageIsolated(page))
                goto out_no_isolated;

        mapping = page_mapping(page);
        VM_BUG_ON_PAGE(!mapping, page);

        if (!mapping->a_ops->isolate_page(page, mode))
                goto out_no_isolated;

        /* Driver shouldn't use PG_isolated bit of page->flags */
        WARN_ON_ONCE(PageIsolated(page));
        __SetPageIsolated(page);
        unlock_page(page);

        return 0;

out_no_isolated:
        unlock_page(page);
out_putpage:
        put_page(page);
out:
        return -EBUSY;
}

static void putback_movable_page(struct page *page)
{
        struct address_space *mapping;

        mapping = page_mapping(page);
        mapping->a_ops->putback_page(page);
        __ClearPageIsolated(page);
}

/*
 * Put previously isolated pages back onto the appropriate lists
 * from where they were once taken off for compaction/migration.
 *
 * This function shall be used whenever the isolated pageset has been
 * built from lru, balloon, hugetlbfs page. See isolate_migratepages_range()
 * and isolate_huge_page().
 */
void putback_movable_pages(struct list_head *l)
{
        struct page *page;
        struct page *page2;

        list_for_each_entry_safe(page, page2, l, lru) {
                if (unlikely(PageHuge(page))) {
                        putback_active_hugepage(page);
                        continue;
                }
                list_del(&page->lru);
                /*
                 * We isolated non-lru movable page so here we can use
                 * __PageMovable because LRU page's mapping cannot have
                 * PAGE_MAPPING_MOVABLE.
                 */
                if (unlikely(__PageMovable(page))) {
                        VM_BUG_ON_PAGE(!PageIsolated(page), page);
                        lock_page(page);
                        if (PageMovable(page))
                                putback_movable_page(page);
                        else
                                __ClearPageIsolated(page);
                        unlock_page(page);
                        put_page(page);
                } else {
                        mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON +
                                        page_is_file_lru(page), -thp_nr_pages(page));
                        putback_lru_page(page);
                }
        }
}

/*
 * Restore a potential migration pte to a working pte entry
 */
static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma,
                                 unsigned long addr, void *old)
{
        struct page_vma_mapped_walk pvmw = {
                .page = old,
                .vma = vma,
                .address = addr,
                .flags = PVMW_SYNC | PVMW_MIGRATION,
        };
        struct page *new;
        pte_t pte;
        swp_entry_t entry;

        VM_BUG_ON_PAGE(PageTail(page), page);
        while (page_vma_mapped_walk(&pvmw)) {
                if (PageKsm(page))
                        new = page;
                else
                        new = page - pvmw.page->index +
                                linear_page_index(vma, pvmw.address);

#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
                /* PMD-mapped THP migration entry */
                if (!pvmw.pte) {
                        VM_BUG_ON_PAGE(PageHuge(page) || !PageTransCompound(page), page);
                        remove_migration_pmd(&pvmw, new);
                        continue;
                }
#endif

                get_page(new);
                pte = pte_mkold(mk_pte(new, READ_ONCE(vma->vm_page_prot)));
                if (pte_swp_soft_dirty(*pvmw.pte))
                        pte = pte_mksoft_dirty(pte);

                /*
                 * Recheck VMA as permissions can change since migration started
                 */
                entry = pte_to_swp_entry(*pvmw.pte);
                if (is_writable_migration_entry(entry))
                        pte = maybe_mkwrite(pte, vma);
                else if (pte_swp_uffd_wp(*pvmw.pte))
                        pte = pte_mkuffd_wp(pte);

                if (unlikely(is_device_private_page(new))) {
                        if (pte_write(pte))
                                entry = make_writable_device_private_entry(
                                                        page_to_pfn(new));
                        else
                                entry = make_readable_device_private_entry(
                                                        page_to_pfn(new));
                        pte = swp_entry_to_pte(entry);
                        if (pte_swp_soft_dirty(*pvmw.pte))
                                pte = pte_swp_mksoft_dirty(pte);
                        if (pte_swp_uffd_wp(*pvmw.pte))
                                pte = pte_swp_mkuffd_wp(pte);
                }

#ifdef CONFIG_HUGETLB_PAGE
                if (PageHuge(new)) {
                        unsigned int shift = huge_page_shift(hstate_vma(vma));

                        pte = pte_mkhuge(pte);
                        pte = arch_make_huge_pte(pte, shift, vma->vm_flags);
                        if (PageAnon(new))
                                hugepage_add_anon_rmap(new, vma, pvmw.address);
                        else
                                page_dup_rmap(new, true);
                        set_huge_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
                } else
#endif
                {
                        if (PageAnon(new))
                                page_add_anon_rmap(new, vma, pvmw.address, false);
                        else
                                page_add_file_rmap(new, false);
                        set_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
                }
                if (vma->vm_flags & VM_LOCKED && !PageTransCompound(new))
                        mlock_vma_page(new);

                if (PageTransHuge(page) && PageMlocked(page))
                        clear_page_mlock(page);

                /* No need to invalidate - it was non-present before */
                update_mmu_cache(vma, pvmw.address, pvmw.pte);
        }

        return true;
}

/*
 * Get rid of all migration entries and replace them by
 * references to the indicated page.
 */
void remove_migration_ptes(struct page *old, struct page *new, bool locked)
{
        struct rmap_walk_control rwc = {
                .rmap_one = remove_migration_pte,
                .arg = old,
        };

        if (locked)
                rmap_walk_locked(new, &rwc);
        else
                rmap_walk(new, &rwc);
}

/*
 * Something used the pte of a page under migration. We need to
 * get to the page and wait until migration is finished.
 * When we return from this function the fault will be retried.
 */
void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
                                spinlock_t *ptl)
{
        pte_t pte;
        swp_entry_t entry;
        struct page *page;

        spin_lock(ptl);
        pte = *ptep;
        if (!is_swap_pte(pte))
                goto out;

        entry = pte_to_swp_entry(pte);
        if (!is_migration_entry(entry))
                goto out;

        page = pfn_swap_entry_to_page(entry);
        page = compound_head(page);

        /*
         * Once page cache replacement of page migration started, page_count
         * is zero; but we must not call put_and_wait_on_page_locked() without
         * a ref. Use get_page_unless_zero(), and just fault again if it fails.
         */
        if (!get_page_unless_zero(page))
                goto out;
        pte_unmap_unlock(ptep, ptl);
        put_and_wait_on_page_locked(page, TASK_UNINTERRUPTIBLE);
        return;
out:
        pte_unmap_unlock(ptep, ptl);
}

void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
                                unsigned long address)
{
        spinlock_t *ptl = pte_lockptr(mm, pmd);
        pte_t *ptep = pte_offset_map(pmd, address);
        __migration_entry_wait(mm, ptep, ptl);
}

void migration_entry_wait_huge(struct vm_area_struct *vma,
                struct mm_struct *mm, pte_t *pte)
{
        spinlock_t *ptl = huge_pte_lockptr(hstate_vma(vma), mm, pte);
        __migration_entry_wait(mm, pte, ptl);
}

#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
void pmd_migration_entry_wait(struct mm_struct *mm, pmd_t *pmd)
{
        spinlock_t *ptl;
        struct page *page;

        ptl = pmd_lock(mm, pmd);
        if (!is_pmd_migration_entry(*pmd))
                goto unlock;
        page = pfn_swap_entry_to_page(pmd_to_swp_entry(*pmd));
        if (!get_page_unless_zero(page))
                goto unlock;
        spin_unlock(ptl);
        put_and_wait_on_page_locked(page, TASK_UNINTERRUPTIBLE);
        return;
unlock:
        spin_unlock(ptl);
}
#endif

static int expected_page_refs(struct address_space *mapping, struct page *page)
{
        int expected_count = 1;

        /*
         * Device private pages have an extra refcount as they are
         * ZONE_DEVICE pages.
         */
        expected_count += is_device_private_page(page);
        if (mapping)
                expected_count += compound_nr(page) + page_has_private(page);

        return expected_count;
}

/*
 * Replace the page in the mapping.
 *
 * The number of remaining references must be:
 * 1 for anonymous pages without a mapping
 * 2 for pages with a mapping
 * 3 for pages with a mapping and PagePrivate/PagePrivate2 set.
 */
int folio_migrate_mapping(struct address_space *mapping,
                struct folio *newfolio, struct folio *folio, int extra_count)
{
        XA_STATE(xas, &mapping->i_pages, folio_index(folio));
        struct zone *oldzone, *newzone;
        int dirty;
        int expected_count = expected_page_refs(mapping, &folio->page) + extra_count;
        long nr = folio_nr_pages(folio);

        if (!mapping) {
                /* Anonymous page without mapping */
                if (folio_ref_count(folio) != expected_count)
                        return -EAGAIN;

                /* No turning back from here */
                newfolio->index = folio->index;
                newfolio->mapping = folio->mapping;
                if (folio_test_swapbacked(folio))
                        __folio_set_swapbacked(newfolio);

                return MIGRATEPAGE_SUCCESS;
        }

        oldzone = folio_zone(folio);
        newzone = folio_zone(newfolio);

        xas_lock_irq(&xas);
        if (!folio_ref_freeze(folio, expected_count)) {
                xas_unlock_irq(&xas);
                return -EAGAIN;
        }

        /*
         * Now we know that no one else is looking at the folio:
         * no turning back from here.
         */
        newfolio->index = folio->index;
        newfolio->mapping = folio->mapping;
        folio_ref_add(newfolio, nr); /* add cache reference */
        if (folio_test_swapbacked(folio)) {
                __folio_set_swapbacked(newfolio);
                if (folio_test_swapcache(folio)) {
                        folio_set_swapcache(newfolio);
                        newfolio->private = folio_get_private(folio);
                }
        } else {
                VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
        }

        /* Move dirty while page refs frozen and newpage not yet exposed */
        dirty = folio_test_dirty(folio);
        if (dirty) {
                folio_clear_dirty(folio);
                folio_set_dirty(newfolio);
        }

        xas_store(&xas, newfolio);
        if (nr > 1) {
                int i;

                for (i = 1; i < nr; i++) {
                        xas_next(&xas);
                        xas_store(&xas, newfolio);
                }
        }

        /*
         * Drop cache reference from old page by unfreezing
         * to one less reference.
         * We know this isn't the last reference.
         */
        folio_ref_unfreeze(folio, expected_count - nr);

        xas_unlock(&xas);
        /* Leave irq disabled to prevent preemption while updating stats */

        /*
         * If moved to a different zone then also account
         * the page for that zone. Other VM counters will be
         * taken care of when we establish references to the
         * new page and drop references to the old page.
         *
         * Note that anonymous pages are accounted for
         * via NR_FILE_PAGES and NR_ANON_MAPPED if they
         * are mapped to swap space.
         */
        if (newzone != oldzone) {
                struct lruvec *old_lruvec, *new_lruvec;
                struct mem_cgroup *memcg;

                memcg = folio_memcg(folio);
                old_lruvec = mem_cgroup_lruvec(memcg, oldzone->zone_pgdat);
                new_lruvec = mem_cgroup_lruvec(memcg, newzone->zone_pgdat);

                __mod_lruvec_state(old_lruvec, NR_FILE_PAGES, -nr);
                __mod_lruvec_state(new_lruvec, NR_FILE_PAGES, nr);
                if (folio_test_swapbacked(folio) && !folio_test_swapcache(folio)) {
                        __mod_lruvec_state(old_lruvec, NR_SHMEM, -nr);
                        __mod_lruvec_state(new_lruvec, NR_SHMEM, nr);
                }
#ifdef CONFIG_SWAP
                if (folio_test_swapcache(folio)) {
                        __mod_lruvec_state(old_lruvec, NR_SWAPCACHE, -nr);
                        __mod_lruvec_state(new_lruvec, NR_SWAPCACHE, nr);
                }
#endif
                if (dirty && mapping_can_writeback(mapping)) {
                        __mod_lruvec_state(old_lruvec, NR_FILE_DIRTY, -nr);
                        __mod_zone_page_state(oldzone, NR_ZONE_WRITE_PENDING, -nr);
                        __mod_lruvec_state(new_lruvec, NR_FILE_DIRTY, nr);
                        __mod_zone_page_state(newzone, NR_ZONE_WRITE_PENDING, nr);
                }
        }
        local_irq_enable();

        return MIGRATEPAGE_SUCCESS;
}
EXPORT_SYMBOL(folio_migrate_mapping);

/*
 * The expected number of remaining references is the same as that
 * of folio_migrate_mapping().
 */
int migrate_huge_page_move_mapping(struct address_space *mapping,
                                   struct page *newpage, struct page *page)
{
        XA_STATE(xas, &mapping->i_pages, page_index(page));
        int expected_count;

        xas_lock_irq(&xas);
        expected_count = 2 + page_has_private(page);
        if (page_count(page) != expected_count || xas_load(&xas) != page) {
                xas_unlock_irq(&xas);
                return -EAGAIN;
        }

        if (!page_ref_freeze(page, expected_count)) {
                xas_unlock_irq(&xas);
                return -EAGAIN;
        }

        newpage->index = page->index;
        newpage->mapping = page->mapping;

        get_page(newpage);

        xas_store(&xas, newpage);

        page_ref_unfreeze(page, expected_count - 1);

        xas_unlock_irq(&xas);

        return MIGRATEPAGE_SUCCESS;
}

/*
 * Copy the flags and some other ancillary information
 */
void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
{
        int cpupid;

        if (folio_test_error(folio))
                folio_set_error(newfolio);
        if (folio_test_referenced(folio))
                folio_set_referenced(newfolio);
        if (folio_test_uptodate(folio))
                folio_mark_uptodate(newfolio);
        if (folio_test_clear_active(folio)) {
                VM_BUG_ON_FOLIO(folio_test_unevictable(folio), folio);
                folio_set_active(newfolio);
        } else if (folio_test_clear_unevictable(folio))
                folio_set_unevictable(newfolio);
        if (folio_test_workingset(folio))
                folio_set_workingset(newfolio);
        if (folio_test_checked(folio))
                folio_set_checked(newfolio);
        if (folio_test_mappedtodisk(folio))
                folio_set_mappedtodisk(newfolio);

        /* Move dirty on pages not done by folio_migrate_mapping() */
        if (folio_test_dirty(folio))
                folio_set_dirty(newfolio);

        if (folio_test_young(folio))
                folio_set_young(newfolio);
        if (folio_test_idle(folio))
                folio_set_idle(newfolio);

        /*
         * Copy NUMA information to the new page, to prevent over-eager
         * future migrations of this same page.
         */
        cpupid = page_cpupid_xchg_last(&folio->page, -1);
        page_cpupid_xchg_last(&newfolio->page, cpupid);

        folio_migrate_ksm(newfolio, folio);
        /*
         * Please do not reorder this without considering how mm/ksm.c's
         * get_ksm_page() depends upon ksm_migrate_page() and PageSwapCache().
         */
        if (folio_test_swapcache(folio))
                folio_clear_swapcache(folio);
        folio_clear_private(folio);

        /* page->private contains hugetlb specific flags */
        if (!folio_test_hugetlb(folio))
                folio->private = NULL;

        /*
         * If any waiters have accumulated on the new page then
         * wake them up.
         */
        if (folio_test_writeback(newfolio))
                folio_end_writeback(newfolio);

        /*
         * PG_readahead shares the same bit with PG_reclaim.  The above
         * end_page_writeback() may clear PG_readahead mistakenly, so set the
         * bit after that.
         */
        if (folio_test_readahead(folio))
                folio_set_readahead(newfolio);

        folio_copy_owner(newfolio, folio);

        if (!folio_test_hugetlb(folio))
                mem_cgroup_migrate(folio, newfolio);
}
EXPORT_SYMBOL(folio_migrate_flags);

void folio_migrate_copy(struct folio *newfolio, struct folio *folio)
{
        folio_copy(newfolio, folio);
        folio_migrate_flags(newfolio, folio);
}
EXPORT_SYMBOL(folio_migrate_copy);

/************************************************************
 *                    Migration functions
 ***********************************************************/

/*
 * Common logic to directly migrate a single LRU page suitable for
 * pages that do not use PagePrivate/PagePrivate2.
 *
 * Pages are locked upon entry and exit.
 */
int migrate_page(struct address_space *mapping,
                struct page *newpage, struct page *page,
                enum migrate_mode mode)
{
        struct folio *newfolio = page_folio(newpage);
        struct folio *folio = page_folio(page);
        int rc;

        BUG_ON(folio_test_writeback(folio));    /* Writeback must be complete */

        rc = folio_migrate_mapping(mapping, newfolio, folio, 0);

        if (rc != MIGRATEPAGE_SUCCESS)
                return rc;

        if (mode != MIGRATE_SYNC_NO_COPY)
                folio_migrate_copy(newfolio, folio);
        else
                folio_migrate_flags(newfolio, folio);
        return MIGRATEPAGE_SUCCESS;
}
EXPORT_SYMBOL(migrate_page);

#ifdef CONFIG_BLOCK
/* Returns true if all buffers are successfully locked */
static bool buffer_migrate_lock_buffers(struct buffer_head *head,
                                                        enum migrate_mode mode)
{
        struct buffer_head *bh = head;

        /* Simple case, sync compaction */
        if (mode != MIGRATE_ASYNC) {
                do {
                        lock_buffer(bh);
                        bh = bh->b_this_page;

                } while (bh != head);

                return true;
        }

        /* async case, we cannot block on lock_buffer so use trylock_buffer */
        do {
                if (!trylock_buffer(bh)) {
                        /*
                         * We failed to lock the buffer and cannot stall in
                         * async migration. Release the taken locks
                         */
                        struct buffer_head *failed_bh = bh;
                        bh = head;
                        while (bh != failed_bh) {
                                unlock_buffer(bh);
                                bh = bh->b_this_page;
                        }
                        return false;
                }

                bh = bh->b_this_page;
        } while (bh != head);
        return true;
}

static int __buffer_migrate_page(struct address_space *mapping,
                struct page *newpage, struct page *page, enum migrate_mode mode,
                bool check_refs)
{
        struct buffer_head *bh, *head;
        int rc;
        int expected_count;

        if (!page_has_buffers(page))
                return migrate_page(mapping, newpage, page, mode);

        /* Check whether page does not have extra refs before we do more work */
        expected_count = expected_page_refs(mapping, page);
        if (page_count(page) != expected_count)
                return -EAGAIN;

        head = page_buffers(page);
        if (!buffer_migrate_lock_buffers(head, mode))
                return -EAGAIN;

        if (check_refs) {
                bool busy;
                bool invalidated = false;

recheck_buffers:
                busy = false;
                spin_lock(&mapping->private_lock);
                bh = head;
                do {
                        if (atomic_read(&bh->b_count)) {
                                busy = true;
                                break;
                        }
                        bh = bh->b_this_page;
                } while (bh != head);
                if (busy) {
                        if (invalidated) {
                                rc = -EAGAIN;
                                goto unlock_buffers;
                        }
                        spin_unlock(&mapping->private_lock);
                        invalidate_bh_lrus();
                        invalidated = true;
                        goto recheck_buffers;
                }
        }

        rc = migrate_page_move_mapping(mapping, newpage, page, 0);
        if (rc != MIGRATEPAGE_SUCCESS)
                goto unlock_buffers;

        attach_page_private(newpage, detach_page_private(page));

        bh = head;
        do {
                set_bh_page(bh, newpage, bh_offset(bh));
                bh = bh->b_this_page;

        } while (bh != head);

        if (mode != MIGRATE_SYNC_NO_COPY)
                migrate_page_copy(newpage, page);
        else
                migrate_page_states(newpage, page);

        rc = MIGRATEPAGE_SUCCESS;
unlock_buffers:
        if (check_refs)
                spin_unlock(&mapping->private_lock);
        bh = head;
        do {
                unlock_buffer(bh);
                bh = bh->b_this_page;

        } while (bh != head);

        return rc;
}

/*
 * Migration function for pages with buffers. This function can only be used
 * if the underlying filesystem guarantees that no other references to "page"
 * exist. For example attached buffer heads are accessed only under page lock.
 */
int buffer_migrate_page(struct address_space *mapping,
                struct page *newpage, struct page *page, enum migrate_mode mode)
{
        return __buffer_migrate_page(mapping, newpage, page, mode, false);
}
EXPORT_SYMBOL(buffer_migrate_page);

/*
 * Same as above except that this variant is more careful and checks that there
 * are also no buffer head references. This function is the right one for
 * mappings where buffer heads are directly looked up and referenced (such as
 * block device mappings).
 */
int buffer_migrate_page_norefs(struct address_space *mapping,
                struct page *newpage, struct page *page, enum migrate_mode mode)
{
        return __buffer_migrate_page(mapping, newpage, page, mode, true);
}
#endif

/*
 * Writeback a page to clean the dirty state
 */
static int writeout(struct address_space *mapping, struct page *page)
{
        struct writeback_control wbc = {
                .sync_mode = WB_SYNC_NONE,
                .nr_to_write = 1,
                .range_start = 0,
                .range_end = LLONG_MAX,
                .for_reclaim = 1
        };
        int rc;

        if (!mapping->a_ops->writepage)
                /* No write method for the address space */
                return -EINVAL;

        if (!clear_page_dirty_for_io(page))
                /* Someone else already triggered a write */
                return -EAGAIN;

        /*
         * A dirty page may imply that the underlying filesystem has
         * the page on some queue. So the page must be clean for
         * migration. Writeout may mean we loose the lock and the
         * page state is no longer what we checked for earlier.
         * At this point we know that the migration attempt cannot
         * be successful.
         */
        remove_migration_ptes(page, page, false);

        rc = mapping->a_ops->writepage(page, &wbc);

        if (rc != AOP_WRITEPAGE_ACTIVATE)
                /* unlocked. Relock */
                lock_page(page);

        return (rc < 0) ? -EIO : -EAGAIN;
}

/*
 * Default handling if a filesystem does not provide a migration function.
 */
static int fallback_migrate_page(struct address_space *mapping,
        struct page *newpage, struct page *page, enum migrate_mode mode)
{
        if (PageDirty(page)) {
                /* Only writeback pages in full synchronous migration */
                switch (mode) {
                case MIGRATE_SYNC:
                case MIGRATE_SYNC_NO_COPY:
                        break;
                default:
                        return -EBUSY;
                }
                return writeout(mapping, page);
        }

        /*
         * Buffers may be managed in a filesystem specific way.
         * We must have no buffers or drop them.
         */
        if (page_has_private(page) &&
            !try_to_release_page(page, GFP_KERNEL))
                return mode == MIGRATE_SYNC ? -EAGAIN : -EBUSY;

        return migrate_page(mapping, newpage, page, mode);
}

/*
 * Move a page to a newly allocated page
 * The page is locked and all ptes have been successfully removed.
 *
 * The new page will have replaced the old page if this function
 * is successful.
 *
 * Return value:
 *   < 0 - error code
 *  MIGRATEPAGE_SUCCESS - success
 */
static int move_to_new_page(struct page *newpage, struct page *page,
                                enum migrate_mode mode)
{
        struct address_space *mapping;
        int rc = -EAGAIN;
        bool is_lru = !__PageMovable(page);

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);

        mapping = page_mapping(page);

        if (likely(is_lru)) {
                if (!mapping)
                        rc = migrate_page(mapping, newpage, page, mode);
                else if (mapping->a_ops->migratepage)
                        /*
                         * Most pages have a mapping and most filesystems
                         * provide a migratepage callback. Anonymous pages
                         * are part of swap space which also has its own
                         * migratepage callback. This is the most common path
                         * for page migration.
                         */
                        rc = mapping->a_ops->migratepage(mapping, newpage,
                                                        page, mode);
                else
                        rc = fallback_migrate_page(mapping, newpage,
                                                        page, mode);
        } else {
                /*
                 * In case of non-lru page, it could be released after
                 * isolation step. In that case, we shouldn't try migration.
                 */
                VM_BUG_ON_PAGE(!PageIsolated(page), page);
                if (!PageMovable(page)) {
                        rc = MIGRATEPAGE_SUCCESS;
                        __ClearPageIsolated(page);
                        goto out;
                }

                rc = mapping->a_ops->migratepage(mapping, newpage,
                                                page, mode);
                WARN_ON_ONCE(rc == MIGRATEPAGE_SUCCESS &&
                        !PageIsolated(page));
        }

        /*
         * When successful, old pagecache page->mapping must be cleared before
         * page is freed; but stats require that PageAnon be left as PageAnon.
         */
        if (rc == MIGRATEPAGE_SUCCESS) {
                if (__PageMovable(page)) {
                        VM_BUG_ON_PAGE(!PageIsolated(page), page);

                        /*
                         * We clear PG_movable under page_lock so any compactor
                         * cannot try to migrate this page.
                         */
                        __ClearPageIsolated(page);
                }

                /*
                 * Anonymous and movable page->mapping will be cleared by
                 * free_pages_prepare so don't reset it here for keeping
                 * the type to work PageAnon, for example.
                 */
                if (!PageMappingFlags(page))
                        page->mapping = NULL;

                if (likely(!is_zone_device_page(newpage)))
                        flush_dcache_page(newpage);

        }
out:
        return rc;
}

static int __unmap_and_move(struct page *page, struct page *newpage,
                                int force, enum migrate_mode mode)
{
        int rc = -EAGAIN;
        bool page_was_mapped = false;
        struct anon_vma *anon_vma = NULL;
        bool is_lru = !__PageMovable(page);

        if (!trylock_page(page)) {
                if (!force || mode == MIGRATE_ASYNC)
                        goto out;

                /*
                 * It's not safe for direct compaction to call lock_page.
                 * For example, during page readahead pages are added locked
                 * to the LRU. Later, when the IO completes the pages are
                 * marked uptodate and unlocked. However, the queueing
                 * could be merging multiple pages for one bio (e.g.
                 * mpage_readahead). If an allocation happens for the
                 * second or third page, the process can end up locking
                 * the same page twice and deadlocking. Rather than
                 * trying to be clever about what pages can be locked,
                 * avoid the use of lock_page for direct compaction
                 * altogether.
                 */
                if (current->flags & PF_MEMALLOC)
                        goto out;

                lock_page(page);
        }

        if (PageWriteback(page)) {
                /*
                 * Only in the case of a full synchronous migration is it
                 * necessary to wait for PageWriteback. In the async case,
                 * the retry loop is too short and in the sync-light case,
                 * the overhead of stalling is too much
                 */
                switch (mode) {
                case MIGRATE_SYNC:
                case MIGRATE_SYNC_NO_COPY:
                        break;
                default:
                        rc = -EBUSY;
                        goto out_unlock;
                }
                if (!force)
                        goto out_unlock;
                wait_on_page_writeback(page);
        }

        /*
         * By try_to_migrate(), page->mapcount goes down to 0 here. In this case,
         * we cannot notice that anon_vma is freed while we migrates a page.
         * This get_anon_vma() delays freeing anon_vma pointer until the end
         * of migration. File cache pages are no problem because of page_lock()
         * File Caches may use write_page() or lock_page() in migration, then,
         * just care Anon page here.
         *
         * Only page_get_anon_vma() understands the subtleties of
         * getting a hold on an anon_vma from outside one of its mms.
         * But if we cannot get anon_vma, then we won't need it anyway,
         * because that implies that the anon page is no longer mapped
         * (and cannot be remapped so long as we hold the page lock).
         */
        if (PageAnon(page) && !PageKsm(page))
                anon_vma = page_get_anon_vma(page);

        /*
         * Block others from accessing the new page when we get around to
         * establishing additional references. We are usually the only one
         * holding a reference to newpage at this point. We used to have a BUG
         * here if trylock_page(newpage) fails, but would like to allow for
         * cases where there might be a race with the previous use of newpage.
         * This is much like races on refcount of oldpage: just don't BUG().
         */
        if (unlikely(!trylock_page(newpage)))
                goto out_unlock;

        if (unlikely(!is_lru)) {
                rc = move_to_new_page(newpage, page, mode);
                goto out_unlock_both;
        }

        /*
         * Corner case handling:
         * 1. When a new swap-cache page is read into, it is added to the LRU
         * and treated as swapcache but it has no rmap yet.
         * Calling try_to_unmap() against a page->mapping==NULL page will
         * trigger a BUG.  So handle it here.
         * 2. An orphaned page (see truncate_cleanup_page) might have
         * fs-private metadata. The page can be picked up due to memory
         * offlining.  Everywhere else except page reclaim, the page is
         * invisible to the vm, so the page can not be migrated.  So try to
         * free the metadata, so the page can be freed.
         */
        if (!page->mapping) {
                VM_BUG_ON_PAGE(PageAnon(page), page);
                if (page_has_private(page)) {
                        try_to_free_buffers(page);
                        goto out_unlock_both;
                }
        } else if (page_mapped(page)) {
                /* Establish migration ptes */
                VM_BUG_ON_PAGE(PageAnon(page) && !PageKsm(page) && !anon_vma,
                                page);
                try_to_migrate(page, 0);
                page_was_mapped = true;
        }

        if (!page_mapped(page))
                rc = move_to_new_page(newpage, page, mode);

        if (page_was_mapped)
                remove_migration_ptes(page,
                        rc == MIGRATEPAGE_SUCCESS ? newpage : page, false);

out_unlock_both:
        unlock_page(newpage);
out_unlock:
        /* Drop an anon_vma reference if we took one */
        if (anon_vma)
                put_anon_vma(anon_vma);
        unlock_page(page);
out:
        /*
         * If migration is successful, decrease refcount of the newpage
         * which will not free the page because new page owner increased
         * refcounter. As well, if it is LRU page, add the page to LRU
         * list in here. Use the old state of the isolated source page to
         * determine if we migrated a LRU page. newpage was already unlocked
         * and possibly modified by its owner - don't rely on the page
         * state.
         */
        if (rc == MIGRATEPAGE_SUCCESS) {
                if (unlikely(!is_lru))
                        put_page(newpage);
                else
                        putback_lru_page(newpage);
        }

        return rc;
}


/*
 * node_demotion[] example:
 *
 * Consider a system with two sockets.  Each socket has
 * three classes of memory attached: fast, medium and slow.
 * Each memory class is placed in its own NUMA node.  The
 * CPUs are placed in the node with the "fast" memory.  The
 * 6 NUMA nodes (0-5) might be split among the sockets like
 * this:
 *
 *      Socket A: 0, 1, 2
 *      Socket B: 3, 4, 5
 *
 * When Node 0 fills up, its memory should be migrated to
 * Node 1.  When Node 1 fills up, it should be migrated to
 * Node 2.  The migration path start on the nodes with the
 * processors (since allocations default to this node) and
 * fast memory, progress through medium and end with the
 * slow memory:
 *
 *      0 -> 1 -> 2 -> stop
 *      3 -> 4 -> 5 -> stop
 *
 * This is represented in the node_demotion[] like this:
 *
 *      {  nr=1, nodes[0]=1 }, // Node 0 migrates to 1
 *      {  nr=1, nodes[0]=2 }, // Node 1 migrates to 2
 *      {  nr=0, nodes[0]=-1 }, // Node 2 does not migrate
 *      {  nr=1, nodes[0]=4 }, // Node 3 migrates to 4
 *      {  nr=1, nodes[0]=5 }, // Node 4 migrates to 5
 *      {  nr=0, nodes[0]=-1 }, // Node 5 does not migrate
 *
 * Moreover some systems may have multiple slow memory nodes.
 * Suppose a system has one socket with 3 memory nodes, node 0
 * is fast memory type, and node 1/2 both are slow memory
 * type, and the distance between fast memory node and slow
 * memory node is same. So the migration path should be:
 *
 *      0 -> 1/2 -> stop
 *
 * This is represented in the node_demotion[] like this:
 *      { nr=2, {nodes[0]=1, nodes[1]=2} }, // Node 0 migrates to node 1 and node 2
 *      { nr=0, nodes[0]=-1, }, // Node 1 dose not migrate
 *      { nr=0, nodes[0]=-1, }, // Node 2 does not migrate
 */

/*
 * Writes to this array occur without locking.  Cycles are
 * not allowed: Node X demotes to Y which demotes to X...
 *
 * If multiple reads are performed, a single rcu_read_lock()
 * must be held over all reads to ensure that no cycles are
 * observed.
 */
#define DEFAULT_DEMOTION_TARGET_NODES 15

#if MAX_NUMNODES < DEFAULT_DEMOTION_TARGET_NODES
#define DEMOTION_TARGET_NODES   (MAX_NUMNODES - 1)
#else
#define DEMOTION_TARGET_NODES   DEFAULT_DEMOTION_TARGET_NODES
#endif

struct demotion_nodes {
        unsigned short nr;
        short nodes[DEMOTION_TARGET_NODES];
};

static struct demotion_nodes *node_demotion __read_mostly;

/**
 * next_demotion_node() - Get the next node in the demotion path
 * @node: The starting node to lookup the next node
 *
 * Return: node id for next memory node in the demotion path hierarchy
 * from @node; NUMA_NO_NODE if @node is terminal.  This does not keep
 * @node online or guarantee that it *continues* to be the next demotion
 * target.
 */
int next_demotion_node(int node)
{
        struct demotion_nodes *nd;
        unsigned short target_nr, index;
        int target;

        if (!node_demotion)
                return NUMA_NO_NODE;

        nd = &node_demotion[node];

        /*
         * node_demotion[] is updated without excluding this
         * function from running.  RCU doesn't provide any
         * compiler barriers, so the READ_ONCE() is required
         * to avoid compiler reordering or read merging.
         *
         * Make sure to use RCU over entire code blocks if
         * node_demotion[] reads need to be consistent.
         */
        rcu_read_lock();
        target_nr = READ_ONCE(nd->nr);

        switch (target_nr) {
        case 0:
                target = NUMA_NO_NODE;
                goto out;
        case 1:
                index = 0;
                break;
        default:
                /*
                 * If there are multiple target nodes, just select one
                 * target node randomly.
                 *
                 * In addition, we can also use round-robin to select
                 * target node, but we should introduce another variable
                 * for node_demotion[] to record last selected target node,
                 * that may cause cache ping-pong due to the changing of
                 * last target node. Or introducing per-cpu data to avoid
                 * caching issue, which seems more complicated. So selecting
                 * target node randomly seems better until now.
                 */
                index = get_random_int() % target_nr;
                break;
        }

        target = READ_ONCE(nd->nodes[index]);

out:
        rcu_read_unlock();
        return target;
}

/*
 * Obtain the lock on page, remove all ptes and migrate the page
 * to the newly allocated page in newpage.
 */
static int unmap_and_move(new_page_t get_new_page,
                                   free_page_t put_new_page,
                                   unsigned long private, struct page *page,
                                   int force, enum migrate_mode mode,
                                   enum migrate_reason reason,
                                   struct list_head *ret)
{
        int rc = MIGRATEPAGE_SUCCESS;
        struct page *newpage = NULL;

        if (!thp_migration_supported() && PageTransHuge(page))
                return -ENOSYS;

        if (page_count(page) == 1) {
                /* page was freed from under us. So we are done. */
                ClearPageActive(page);
                ClearPageUnevictable(page);
                if (unlikely(__PageMovable(page))) {
                        lock_page(page);
                        if (!PageMovable(page))
                                __ClearPageIsolated(page);
                        unlock_page(page);
                }
                goto out;
        }

        newpage = get_new_page(page, private);
        if (!newpage)
                return -ENOMEM;

        rc = __unmap_and_move(page, newpage, force, mode);
        if (rc == MIGRATEPAGE_SUCCESS)
                set_page_owner_migrate_reason(newpage, reason);

out:
        if (rc != -EAGAIN) {
                /*
                 * A page that has been migrated has all references
                 * removed and will be freed. A page that has not been
                 * migrated will have kept its references and be restored.
                 */
                list_del(&page->lru);
        }

        /*
         * If migration is successful, releases reference grabbed during
         * isolation. Otherwise, restore the page to right list unless
         * we want to retry.
         */
        if (rc == MIGRATEPAGE_SUCCESS) {
                /*
                 * Compaction can migrate also non-LRU pages which are
                 * not accounted to NR_ISOLATED_*. They can be recognized
                 * as __PageMovable
                 */
                if (likely(!__PageMovable(page)))
                        mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON +
                                        page_is_file_lru(page), -thp_nr_pages(page));

                if (reason != MR_MEMORY_FAILURE)
                        /*
                         * We release the page in page_handle_poison.
                         */
                        put_page(page);
        } else {
                if (rc != -EAGAIN)
                        list_add_tail(&page->lru, ret);

                if (put_new_page)
                        put_new_page(newpage, private);
                else
                        put_page(newpage);
        }

        return rc;
}

/*
 * Counterpart of unmap_and_move_page() for hugepage migration.
 *
 * This function doesn't wait the completion of hugepage I/O
 * because there is no race between I/O and migration for hugepage.
 * Note that currently hugepage I/O occurs only in direct I/O
 * where no lock is held and PG_writeback is irrelevant,
 * and writeback status of all subpages are counted in the reference
 * count of the head page (i.e. if all subpages of a 2MB hugepage are
 * under direct I/O, the reference of the head page is 512 and a bit more.)
 * This means that when we try to migrate hugepage whose subpages are
 * doing direct I/O, some references remain after try_to_unmap() and
 * hugepage migration fails without data corruption.
 *
 * There is also no race when direct I/O is issued on the page under migration,
 * because then pte is replaced with migration swap entry and direct I/O code
 * will wait in the page fault for migration to complete.
 */
static int unmap_and_move_huge_page(new_page_t get_new_page,
                                free_page_t put_new_page, unsigned long private,
                                struct page *hpage, int force,
                                enum migrate_mode mode, int reason,
                                struct list_head *ret)
{
        int rc = -EAGAIN;
        int page_was_mapped = 0;
        struct page *new_hpage;
        struct anon_vma *anon_vma = NULL;
        struct address_space *mapping = NULL;

        /*
         * Migratability of hugepages depends on architectures and their size.
         * This check is necessary because some callers of hugepage migration
         * like soft offline and memory hotremove don't walk through page
         * tables or check whether the hugepage is pmd-based or not before
         * kicking migration.
         */
        if (!hugepage_migration_supported(page_hstate(hpage))) {
                list_move_tail(&hpage->lru, ret);
                return -ENOSYS;
        }

        if (page_count(hpage) == 1) {
                /* page was freed from under us. So we are done. */
                putback_active_hugepage(hpage);
                return MIGRATEPAGE_SUCCESS;
        }

        new_hpage = get_new_page(hpage, private);
        if (!new_hpage)
                return -ENOMEM;

        if (!trylock_page(hpage)) {
                if (!force)
                        goto out;
                switch (mode) {
                case MIGRATE_SYNC:
                case MIGRATE_SYNC_NO_COPY:
                        break;
                default:
                        goto out;
                }
                lock_page(hpage);
        }

        /*
         * Check for pages which are in the process of being freed.  Without
         * page_mapping() set, hugetlbfs specific move page routine will not
         * be called and we could leak usage counts for subpools.
         */
        if (hugetlb_page_subpool(hpage) && !page_mapping(hpage)) {
                rc = -EBUSY;
                goto out_unlock;
        }

        if (PageAnon(hpage))
                anon_vma = page_get_anon_vma(hpage);

        if (unlikely(!trylock_page(new_hpage)))
                goto put_anon;

        if (page_mapped(hpage)) {
                bool mapping_locked = false;
                enum ttu_flags ttu = 0;

                if (!PageAnon(hpage)) {
                        /*
                         * In shared mappings, try_to_unmap could potentially
                         * call huge_pmd_unshare.  Because of this, take
                         * semaphore in write mode here and set TTU_RMAP_LOCKED
                         * to let lower levels know we have taken the lock.
                         */
                        mapping = hugetlb_page_mapping_lock_write(hpage);
                        if (unlikely(!mapping))
                                goto unlock_put_anon;

                        mapping_locked = true;
                        ttu |= TTU_RMAP_LOCKED;
                }

                try_to_migrate(hpage, ttu);
                page_was_mapped = 1;

                if (mapping_locked)
                        i_mmap_unlock_write(mapping);
        }

        if (!page_mapped(hpage))
                rc = move_to_new_page(new_hpage, hpage, mode);

        if (page_was_mapped)
                remove_migration_ptes(hpage,
                        rc == MIGRATEPAGE_SUCCESS ? new_hpage : hpage, false);

unlock_put_anon:
        unlock_page(new_hpage);

put_anon:
        if (anon_vma)
                put_anon_vma(anon_vma);

        if (rc == MIGRATEPAGE_SUCCESS) {
                move_hugetlb_state(hpage, new_hpage, reason);
                put_new_page = NULL;
        }

out_unlock:
        unlock_page(hpage);
out:
        if (rc == MIGRATEPAGE_SUCCESS)
                putback_active_hugepage(hpage);
        else if (rc != -EAGAIN)
                list_move_tail(&hpage->lru, ret);

        /*
         * If migration was not successful and there's a freeing callback, use
         * it.  Otherwise, put_page() will drop the reference grabbed during
         * isolation.
         */
        if (put_new_page)
                put_new_page(new_hpage, private);
        else
                putback_active_hugepage(new_hpage);

        return rc;
}

static inline int try_split_thp(struct page *page, struct page **page2,
                                struct list_head *from)
{
        int rc = 0;

        lock_page(page);
        rc = split_huge_page_to_list(page, from);
        unlock_page(page);
        if (!rc)
                list_safe_reset_next(page, *page2, lru);

        return rc;
}

/*
 * migrate_pages - migrate the pages specified in a list, to the free pages
 *                 supplied as the target for the page migration
 *
 * @from:               The list of pages to be migrated.
 * @get_new_page:       The function used to allocate free pages to be used
 *                      as the target of the page migration.
 * @put_new_page:       The function used to free target pages if migration
 *                      fails, or NULL if no special handling is necessary.
 * @private:            Private data to be passed on to get_new_page()
 * @mode:               The migration mode that specifies the constraints for
 *                      page migration, if any.
 * @reason:             The reason for page migration.
 * @ret_succeeded:      Set to the number of normal pages migrated successfully if
 *                      the caller passes a non-NULL pointer.
 *
 * The function returns after 10 attempts or if no pages are movable any more
 * because the list has become empty or no retryable pages exist any more.
 * It is caller's responsibility to call putback_movable_pages() to return pages
 * to the LRU or free list only if ret != 0.
 *
 * Returns the number of {normal page, THP, hugetlb} that were not migrated, or
 * an error code. The number of THP splits will be considered as the number of
 * non-migrated THP, no matter how many subpages of the THP are migrated successfully.
 */
int migrate_pages(struct list_head *from, new_page_t get_new_page,
                free_page_t put_new_page, unsigned long private,
                enum migrate_mode mode, int reason, unsigned int *ret_succeeded)
{
        int retry = 1;
        int thp_retry = 1;
        int nr_failed = 0;
        int nr_failed_pages = 0;
        int nr_succeeded = 0;
        int nr_thp_succeeded = 0;
        int nr_thp_failed = 0;
        int nr_thp_split = 0;
        int pass = 0;
        bool is_thp = false;
        struct page *page;
        struct page *page2;
        int swapwrite = current->flags & PF_SWAPWRITE;
        int rc, nr_subpages;
        LIST_HEAD(ret_pages);
        LIST_HEAD(thp_split_pages);
        bool nosplit = (reason == MR_NUMA_MISPLACED);
        bool no_subpage_counting = false;

        trace_mm_migrate_pages_start(mode, reason);

        if (!swapwrite)
                current->flags |= PF_SWAPWRITE;

thp_subpage_migration:
        for (pass = 0; pass < 10 && (retry || thp_retry); pass++) {
                retry = 0;
                thp_retry = 0;

                list_for_each_entry_safe(page, page2, from, lru) {
retry:
                        /*
                         * THP statistics is based on the source huge page.
                         * Capture required information that might get lost
                         * during migration.
                         */
                        is_thp = PageTransHuge(page) && !PageHuge(page);
                        nr_subpages = compound_nr(page);
                        cond_resched();

                        if (PageHuge(page))
                                rc = unmap_and_move_huge_page(get_new_page,
                                                put_new_page, private, page,
                                                pass > 2, mode, reason,
                                                &ret_pages);
                        else
                                rc = unmap_and_move(get_new_page, put_new_page,
                                                private, page, pass > 2, mode,
                                                reason, &ret_pages);
                        /*
                         * The rules are:
                         *      Success: non hugetlb page will be freed, hugetlb
                         *               page will be put back
                         *      -EAGAIN: stay on the from list
                         *      -ENOMEM: stay on the from list
                         *      Other errno: put on ret_pages list then splice to
                         *                   from list
                         */
                        switch(rc) {
                        /*
                         * THP migration might be unsupported or the
                         * allocation could've failed so we should
                         * retry on the same page with the THP split
                         * to base pages.
                         *
                         * Head page is retried immediately and tail
                         * pages are added to the tail of the list so
                         * we encounter them after the rest of the list
                         * is processed.
                         */
                        case -ENOSYS:
                                /* THP migration is unsupported */
                                if (is_thp) {
                                        nr_thp_failed++;
                                        if (!try_split_thp(page, &page2, &thp_split_pages)) {
                                                nr_thp_split++;
                                                goto retry;
                                        }

                                        nr_failed_pages += nr_subpages;
                                        break;
                                }

                                /* Hugetlb migration is unsupported */
                                if (!no_subpage_counting)
                                        nr_failed++;
                                nr_failed_pages += nr_subpages;
                                break;
                        case -ENOMEM:
                                /*
                                 * When memory is low, don't bother to try to migrate
                                 * other pages, just exit.
                                 * THP NUMA faulting doesn't split THP to retry.
                                 */
                                if (is_thp && !nosplit) {
                                        nr_thp_failed++;
                                        if (!try_split_thp(page, &page2, &thp_split_pages)) {
                                                nr_thp_split++;
                                                goto retry;
                                        }

                                        nr_failed_pages += nr_subpages;
                                        goto out;
                                }

                                if (!no_subpage_counting)
                                        nr_failed++;
                                nr_failed_pages += nr_subpages;
                                goto out;
                        case -EAGAIN:
                                if (is_thp) {
                                        thp_retry++;
                                        break;
                                }
                                retry++;
                                break;
                        case MIGRATEPAGE_SUCCESS:
                                nr_succeeded += nr_subpages;
                                if (is_thp) {
                                        nr_thp_succeeded++;
                                        break;
                                }
                                break;
                        default:
                                /*
                                 * Permanent failure (-EBUSY, etc.):
                                 * unlike -EAGAIN case, the failed page is
                                 * removed from migration page list and not
                                 * retried in the next outer loop.
                                 */
                                if (is_thp) {
                                        nr_thp_failed++;
                                        nr_failed_pages += nr_subpages;
                                        break;
                                }

                                if (!no_subpage_counting)
                                        nr_failed++;
                                nr_failed_pages += nr_subpages;
                                break;
                        }
                }
        }
        nr_failed += retry;
        nr_thp_failed += thp_retry;
        /*
         * Try to migrate subpages of fail-to-migrate THPs, no nr_failed
         * counting in this round, since all subpages of a THP is counted
         * as 1 failure in the first round.
         */
        if (!list_empty(&thp_split_pages)) {
                /*
                 * Move non-migrated pages (after 10 retries) to ret_pages
                 * to avoid migrating them again.
                 */
                list_splice_init(from, &ret_pages);
                list_splice_init(&thp_split_pages, from);
                no_subpage_counting = true;
                retry = 1;
                goto thp_subpage_migration;
        }

        rc = nr_failed + nr_thp_failed;
out:
        /*
         * Put the permanent failure page back to migration list, they
         * will be put back to the right list by the caller.
         */
        list_splice(&ret_pages, from);

        count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
        count_vm_events(PGMIGRATE_FAIL, nr_failed_pages);
        count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);
        count_vm_events(THP_MIGRATION_FAIL, nr_thp_failed);
        count_vm_events(THP_MIGRATION_SPLIT, nr_thp_split);
        trace_mm_migrate_pages(nr_succeeded, nr_failed_pages, nr_thp_succeeded,
                               nr_thp_failed, nr_thp_split, mode, reason);

        if (!swapwrite)
                current->flags &= ~PF_SWAPWRITE;

        if (ret_succeeded)
                *ret_succeeded = nr_succeeded;

        return rc;
}

struct page *alloc_migration_target(struct page *page, unsigned long private)
{
        struct migration_target_control *mtc;
        gfp_t gfp_mask;
        unsigned int order = 0;
        struct page *new_page = NULL;
        int nid;
        int zidx;

        mtc = (struct migration_target_control *)private;
        gfp_mask = mtc->gfp_mask;
        nid = mtc->nid;
        if (nid == NUMA_NO_NODE)
                nid = page_to_nid(page);

        if (PageHuge(page)) {
                struct hstate *h = page_hstate(compound_head(page));

                gfp_mask = htlb_modify_alloc_mask(h, gfp_mask);
                return alloc_huge_page_nodemask(h, nid, mtc->nmask, gfp_mask);
        }

        if (PageTransHuge(page)) {
                /*
                 * clear __GFP_RECLAIM to make the migration callback
                 * consistent with regular THP allocations.
                 */
                gfp_mask &= ~__GFP_RECLAIM;
                gfp_mask |= GFP_TRANSHUGE;
                order = HPAGE_PMD_ORDER;
        }
        zidx = zone_idx(page_zone(page));
        if (is_highmem_idx(zidx) || zidx == ZONE_MOVABLE)
                gfp_mask |= __GFP_HIGHMEM;

        new_page = __alloc_pages(gfp_mask, order, nid, mtc->nmask);

        if (new_page && PageTransHuge(new_page))
                prep_transhuge_page(new_page);

        return new_page;
}

#ifdef CONFIG_NUMA

static int store_status(int __user *status, int start, int value, int nr)
{
        while (nr-- > 0) {
                if (put_user(value, status + start))
                        return -EFAULT;
                start++;
        }

        return 0;
}

static int do_move_pages_to_node(struct mm_struct *mm,
                struct list_head *pagelist, int node)
{
        int err;
        struct migration_target_control mtc = {
                .nid = node,
                .gfp_mask = GFP_HIGHUSER_MOVABLE | __GFP_THISNODE,
        };

        err = migrate_pages(pagelist, alloc_migration_target, NULL,
                (unsigned long)&mtc, MIGRATE_SYNC, MR_SYSCALL, NULL);
        if (err)
                putback_movable_pages(pagelist);
        return err;
}

/*
 * Resolves the given address to a struct page, isolates it from the LRU and
 * puts it to the given pagelist.
 * Returns:
 *     errno - if the page cannot be found/isolated
 *     0 - when it doesn't have to be migrated because it is already on the
 *         target node
 *     1 - when it has been queued
 */
static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
                int node, struct list_head *pagelist, bool migrate_all)
{
        struct vm_area_struct *vma;
        struct page *page;
        unsigned int follflags;
        int err;

        mmap_read_lock(mm);
        err = -EFAULT;
        vma = find_vma(mm, addr);
        if (!vma || addr < vma->vm_start || !vma_migratable(vma))
                goto out;

        /* FOLL_DUMP to ignore special (like zero) pages */
        follflags = FOLL_GET | FOLL_DUMP;
        page = follow_page(vma, addr, follflags);

        err = PTR_ERR(page);
        if (IS_ERR(page))
                goto out;

        err = -ENOENT;
        if (!page)
                goto out;

        err = 0;
        if (page_to_nid(page) == node)
                goto out_putpage;

        err = -EACCES;
        if (page_mapcount(page) > 1 && !migrate_all)
                goto out_putpage;

        if (PageHuge(page)) {
                if (PageHead(page)) {
                        isolate_huge_page(page, pagelist);
                        err = 1;
                }
        } else {
                struct page *head;

                head = compound_head(page);
                err = isolate_lru_page(head);
                if (err)
                        goto out_putpage;

                err = 1;
                list_add_tail(&head->lru, pagelist);
                mod_node_page_state(page_pgdat(head),
                        NR_ISOLATED_ANON + page_is_file_lru(head),
                        thp_nr_pages(head));
        }
out_putpage:
        /*
         * Either remove the duplicate refcount from
         * isolate_lru_page() or drop the page ref if it was
         * not isolated.
         */
        put_page(page);
out:
        mmap_read_unlock(mm);
        return err;
}

static int move_pages_and_store_status(struct mm_struct *mm, int node,
                struct list_head *pagelist, int __user *status,
                int start, int i, unsigned long nr_pages)
{
        int err;

        if (list_empty(pagelist))
                return 0;

        err = do_move_pages_to_node(mm, pagelist, node);
        if (err) {
                /*
                 * Positive err means the number of failed
                 * pages to migrate.  Since we are going to
                 * abort and return the number of non-migrated
                 * pages, so need to include the rest of the
                 * nr_pages that have not been attempted as
                 * well.
                 */
                if (err > 0)
                        err += nr_pages - i - 1;
                return err;
        }
        return store_status(status, start, node, i - start);
}

/*
 * Migrate an array of page address onto an array of nodes and fill
 * the corresponding array of status.
 */
static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
                         unsigned long nr_pages,
                         const void __user * __user *pages,
                         const int __user *nodes,
                         int __user *status, int flags)
{
        int current_node = NUMA_NO_NODE;
        LIST_HEAD(pagelist);
        int start, i;
        int err = 0, err1;

        lru_cache_disable();

        for (i = start = 0; i < nr_pages; i++) {
                const void __user *p;
                unsigned long addr;
                int node;

                err = -EFAULT;
                if (get_user(p, pages + i))
                        goto out_flush;
                if (get_user(node, nodes + i))
                        goto out_flush;
                addr = (unsigned long)untagged_addr(p);

                err = -ENODEV;
                if (node < 0 || node >= MAX_NUMNODES)
                        goto out_flush;
                if (!node_state(node, N_MEMORY))
                        goto out_flush;

                err = -EACCES;
                if (!node_isset(node, task_nodes))
                        goto out_flush;

                if (current_node == NUMA_NO_NODE) {
                        current_node = node;
                        start = i;
                } else if (node != current_node) {
                        err = move_pages_and_store_status(mm, current_node,
                                        &pagelist, status, start, i, nr_pages);
                        if (err)
                                goto out;
                        start = i;
                        current_node = node;
                }

                /*
                 * Errors in the page lookup or isolation are not fatal and we simply
                 * report them via status
                 */
                err = add_page_for_migration(mm, addr, current_node,
                                &pagelist, flags & MPOL_MF_MOVE_ALL);

                if (err > 0) {
                        /* The page is successfully queued for migration */
                        continue;
                }

                /*
                 * If the page is already on the target node (!err), store the
                 * node, otherwise, store the err.
                 */
                err = store_status(status, i, err ? : current_node, 1);
                if (err)
                        goto out_flush;

                err = move_pages_and_store_status(mm, current_node, &pagelist,
                                status, start, i, nr_pages);
                if (err)
                        goto out;
                current_node = NUMA_NO_NODE;
        }
out_flush:
        /* Make sure we do not overwrite the existing error */
        err1 = move_pages_and_store_status(mm, current_node, &pagelist,
                                status, start, i, nr_pages);
        if (err >= 0)
                err = err1;
out:
        lru_cache_enable();
        return err;
}

/*
 * Determine the nodes of an array of pages and store it in an array of status.
 */
static void do_pages_stat_array(struct mm_struct *mm, unsigned long nr_pages,
                                const void __user **pages, int *status)
{
        unsigned long i;

        mmap_read_lock(mm);

        for (i = 0; i < nr_pages; i++) {
                unsigned long addr = (unsigned long)(*pages);
                struct vm_area_struct *vma;
                struct page *page;
                int err = -EFAULT;

                vma = vma_lookup(mm, addr);
                if (!vma)
                        goto set_status;

                /* FOLL_DUMP to ignore special (like zero) pages */
                page = follow_page(vma, addr, FOLL_DUMP);

                err = PTR_ERR(page);
                if (IS_ERR(page))
                        goto set_status;

                err = page ? page_to_nid(page) : -ENOENT;
set_status:
                *status = err;

                pages++;
                status++;
        }

        mmap_read_unlock(mm);
}

static int get_compat_pages_array(const void __user *chunk_pages[],
                                  const void __user * __user *pages,
                                  unsigned long chunk_nr)
{
        compat_uptr_t __user *pages32 = (compat_uptr_t __user *)pages;
        compat_uptr_t p;
        int i;

        for (i = 0; i < chunk_nr; i++) {
                if (get_user(p, pages32 + i))
                        return -EFAULT;
                chunk_pages[i] = compat_ptr(p);
        }

        return 0;
}

/*
 * Determine the nodes of a user array of pages and store it in
 * a user array of status.
 */
static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
                         const void __user * __user *pages,
                         int __user *status)
{
#define DO_PAGES_STAT_CHUNK_NR 16
        const void __user *chunk_pages[DO_PAGES_STAT_CHUNK_NR];
        int chunk_status[DO_PAGES_STAT_CHUNK_NR];

        while (nr_pages) {
                unsigned long chunk_nr;

                chunk_nr = nr_pages;
                if (chunk_nr > DO_PAGES_STAT_CHUNK_NR)
                        chunk_nr = DO_PAGES_STAT_CHUNK_NR;

                if (in_compat_syscall()) {
                        if (get_compat_pages_array(chunk_pages, pages,
                                                   chunk_nr))
                                break;
                } else {
                        if (copy_from_user(chunk_pages, pages,
                                      chunk_nr * sizeof(*chunk_pages)))
                                break;
                }

                do_pages_stat_array(mm, chunk_nr, chunk_pages, chunk_status);

                if (copy_to_user(status, chunk_status, chunk_nr * sizeof(*status)))
                        break;

                pages += chunk_nr;
                status += chunk_nr;
                nr_pages -= chunk_nr;
        }
        return nr_pages ? -EFAULT : 0;
}

static struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes)
{
        struct task_struct *task;
        struct mm_struct *mm;

        /*
         * There is no need to check if current process has the right to modify
         * the specified process when they are same.
         */
        if (!pid) {
                mmget(current->mm);
                *mem_nodes = cpuset_mems_allowed(current);
                return current->mm;
        }

        /* Find the mm_struct */
        rcu_read_lock();
        task = find_task_by_vpid(pid);
        if (!task) {
                rcu_read_unlock();
                return ERR_PTR(-ESRCH);
        }
        get_task_struct(task);

        /*
         * Check if this process has the right to modify the specified
         * process. Use the regular "ptrace_may_access()" checks.
         */
        if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
                rcu_read_unlock();
                mm = ERR_PTR(-EPERM);
                goto out;
        }
        rcu_read_unlock();

        mm = ERR_PTR(security_task_movememory(task));
        if (IS_ERR(mm))
                goto out;
        *mem_nodes = cpuset_mems_allowed(task);
        mm = get_task_mm(task);
out:
        put_task_struct(task);
        if (!mm)
                mm = ERR_PTR(-EINVAL);
        return mm;
}

/*
 * Move a list of pages in the address space of the currently executing
 * process.
 */
static int kernel_move_pages(pid_t pid, unsigned long nr_pages,
                             const void __user * __user *pages,
                             const int __user *nodes,
                             int __user *status, int flags)
{
        struct mm_struct *mm;
        int err;
        nodemask_t task_nodes;

        /* Check flags */
        if (flags & ~(MPOL_MF_MOVE|MPOL_MF_MOVE_ALL))
                return -EINVAL;

        if ((flags & MPOL_MF_MOVE_ALL) && !capable(CAP_SYS_NICE))
                return -EPERM;

        mm = find_mm_struct(pid, &task_nodes);
        if (IS_ERR(mm))
                return PTR_ERR(mm);

        if (nodes)
                err = do_pages_move(mm, task_nodes, nr_pages, pages,
                                    nodes, status, flags);
        else
                err = do_pages_stat(mm, nr_pages, pages, status);

        mmput(mm);
        return err;
}

SYSCALL_DEFINE6(move_pages, pid_t, pid, unsigned long, nr_pages,
                const void __user * __user *, pages,
                const int __user *, nodes,
                int __user *, status, int, flags)
{
        return kernel_move_pages(pid, nr_pages, pages, nodes, status, flags);
}

#ifdef CONFIG_NUMA_BALANCING
/*
 * Returns true if this is a safe migration target node for misplaced NUMA
 * pages. Currently it only checks the watermarks which crude
 */
static bool migrate_balanced_pgdat(struct pglist_data *pgdat,
                                   unsigned long nr_migrate_pages)
{
        int z;

        for (z = pgdat->nr_zones - 1; z >= 0; z--) {
                struct zone *zone = pgdat->node_zones + z;

                if (!populated_zone(zone))
                        continue;

                /* Avoid waking kswapd by allocating pages_to_migrate pages. */
                if (!zone_watermark_ok(zone, 0,
                                       high_wmark_pages(zone) +
                                       nr_migrate_pages,
                                       ZONE_MOVABLE, 0))
                        continue;
                return true;
        }
        return false;
}

static struct page *alloc_misplaced_dst_page(struct page *page,
                                           unsigned long data)
{
        int nid = (int) data;
        struct page *newpage;

        newpage = __alloc_pages_node(nid,
                                         (GFP_HIGHUSER_MOVABLE |
                                          __GFP_THISNODE | __GFP_NOMEMALLOC |
                                          __GFP_NORETRY | __GFP_NOWARN) &
                                         ~__GFP_RECLAIM, 0);

        return newpage;
}

static struct page *alloc_misplaced_dst_page_thp(struct page *page,
                                                 unsigned long data)
{
        int nid = (int) data;
        struct page *newpage;

        newpage = alloc_pages_node(nid, (GFP_TRANSHUGE_LIGHT | __GFP_THISNODE),
                                   HPAGE_PMD_ORDER);
        if (!newpage)
                goto out;

        prep_transhuge_page(newpage);

out:
        return newpage;
}

static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
{
        int page_lru;
        int nr_pages = thp_nr_pages(page);

        VM_BUG_ON_PAGE(compound_order(page) && !PageTransHuge(page), page);

        /* Do not migrate THP mapped by multiple processes */
        if (PageTransHuge(page) && total_mapcount(page) > 1)
                return 0;

        /* Avoid migrating to a node that is nearly full */
        if (!migrate_balanced_pgdat(pgdat, nr_pages))
                return 0;

        if (isolate_lru_page(page))
                return 0;

        page_lru = page_is_file_lru(page);
        mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON + page_lru,
                            nr_pages);

        /*
         * Isolating the page has taken another reference, so the
         * caller's reference can be safely dropped without the page
         * disappearing underneath us during migration.
         */
        put_page(page);
        return 1;
}

/*
 * Attempt to migrate a misplaced page to the specified destination
 * node. Caller is expected to have an elevated reference count on
 * the page that will be dropped by this function before returning.
 */
int migrate_misplaced_page(struct page *page, struct vm_area_struct *vma,
                           int node)
{
        pg_data_t *pgdat = NODE_DATA(node);
        int isolated;
        int nr_remaining;
        LIST_HEAD(migratepages);
        new_page_t *new;
        bool compound;
        int nr_pages = thp_nr_pages(page);

        /*
         * PTE mapped THP or HugeTLB page can't reach here so the page could
         * be either base page or THP.  And it must be head page if it is
         * THP.
         */
        compound = PageTransHuge(page);

        if (compound)
                new = alloc_misplaced_dst_page_thp;
        else
                new = alloc_misplaced_dst_page;

        /*
         * Don't migrate file pages that are mapped in multiple processes
         * with execute permissions as they are probably shared libraries.
         */
        if (page_mapcount(page) != 1 && page_is_file_lru(page) &&
            (vma->vm_flags & VM_EXEC))
                goto out;

        /*
         * Also do not migrate dirty pages as not all filesystems can move
         * dirty pages in MIGRATE_ASYNC mode which is a waste of cycles.
         */
        if (page_is_file_lru(page) && PageDirty(page))
                goto out;

        isolated = numamigrate_isolate_page(pgdat, page);
        if (!isolated)
                goto out;

        list_add(&page->lru, &migratepages);
        nr_remaining = migrate_pages(&migratepages, *new, NULL, node,
                                     MIGRATE_ASYNC, MR_NUMA_MISPLACED, NULL);
        if (nr_remaining) {
                if (!list_empty(&migratepages)) {
                        list_del(&page->lru);
                        mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON +
                                        page_is_file_lru(page), -nr_pages);
                        putback_lru_page(page);
                }
                isolated = 0;
        } else
                count_vm_numa_events(NUMA_PAGE_MIGRATE, nr_pages);
        BUG_ON(!list_empty(&migratepages));
        return isolated;

out:
        put_page(page);
        return 0;
}
#endif /* CONFIG_NUMA_BALANCING */
#endif /* CONFIG_NUMA */

#ifdef CONFIG_DEVICE_PRIVATE
static int migrate_vma_collect_skip(unsigned long start,
                                    unsigned long end,
                                    struct mm_walk *walk)
{
        struct migrate_vma *migrate = walk->private;
        unsigned long addr;

        for (addr = start; addr < end; addr += PAGE_SIZE) {
                migrate->dst[migrate->npages] = 0;
                migrate->src[migrate->npages++] = 0;
        }

        return 0;
}

static int migrate_vma_collect_hole(unsigned long start,
                                    unsigned long end,
                                    __always_unused int depth,
                                    struct mm_walk *walk)
{
        struct migrate_vma *migrate = walk->private;
        unsigned long addr;

        /* Only allow populating anonymous memory. */
        if (!vma_is_anonymous(walk->vma))
                return migrate_vma_collect_skip(start, end, walk);

        for (addr = start; addr < end; addr += PAGE_SIZE) {
                migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
                migrate->dst[migrate->npages] = 0;
                migrate->npages++;
                migrate->cpages++;
        }

        return 0;
}

static int migrate_vma_collect_pmd(pmd_t *pmdp,
                                   unsigned long start,
                                   unsigned long end,
                                   struct mm_walk *walk)
{
        struct migrate_vma *migrate = walk->private;
        struct vm_area_struct *vma = walk->vma;
        struct mm_struct *mm = vma->vm_mm;
        unsigned long addr = start, unmapped = 0;
        spinlock_t *ptl;
        pte_t *ptep;

again:
        if (pmd_none(*pmdp))
                return migrate_vma_collect_hole(start, end, -1, walk);

        if (pmd_trans_huge(*pmdp)) {
                struct page *page;

                ptl = pmd_lock(mm, pmdp);
                if (unlikely(!pmd_trans_huge(*pmdp))) {
                        spin_unlock(ptl);
                        goto again;
                }

                page = pmd_page(*pmdp);
                if (is_huge_zero_page(page)) {
                        spin_unlock(ptl);
                        split_huge_pmd(vma, pmdp, addr);
                        if (pmd_trans_unstable(pmdp))
                                return migrate_vma_collect_skip(start, end,
                                                                walk);
                } else {
                        int ret;

                        get_page(page);
                        spin_unlock(ptl);
                        if (unlikely(!trylock_page(page)))
                                return migrate_vma_collect_skip(start, end,
                                                                walk);
                        ret = split_huge_page(page);
                        unlock_page(page);
                        put_page(page);
                        if (ret)
                                return migrate_vma_collect_skip(start, end,
                                                                walk);
                        if (pmd_none(*pmdp))
                                return migrate_vma_collect_hole(start, end, -1,
                                                                walk);
                }
        }

        if (unlikely(pmd_bad(*pmdp)))
                return migrate_vma_collect_skip(start, end, walk);

        ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
        arch_enter_lazy_mmu_mode();

        for (; addr < end; addr += PAGE_SIZE, ptep++) {
                unsigned long mpfn = 0, pfn;
                struct page *page;
                swp_entry_t entry;
                pte_t pte;

                pte = *ptep;

                if (pte_none(pte)) {
                        if (vma_is_anonymous(vma)) {
                                mpfn = MIGRATE_PFN_MIGRATE;
                                migrate->cpages++;
                        }
                        goto next;
                }

                if (!pte_present(pte)) {
                        /*
                         * Only care about unaddressable device page special
                         * page table entry. Other special swap entries are not
                         * migratable, and we ignore regular swapped page.
                         */
                        entry = pte_to_swp_entry(pte);
                        if (!is_device_private_entry(entry))
                                goto next;

                        page = pfn_swap_entry_to_page(entry);
                        if (!(migrate->flags &
                                MIGRATE_VMA_SELECT_DEVICE_PRIVATE) ||
                            page->pgmap->owner != migrate->pgmap_owner)
                                goto next;

                        mpfn = migrate_pfn(page_to_pfn(page)) |
                                        MIGRATE_PFN_MIGRATE;
                        if (is_writable_device_private_entry(entry))
                                mpfn |= MIGRATE_PFN_WRITE;
                } else {
                        if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM))
                                goto next;
                        pfn = pte_pfn(pte);
                        if (is_zero_pfn(pfn)) {
                                mpfn = MIGRATE_PFN_MIGRATE;
                                migrate->cpages++;
                                goto next;
                        }
                        page = vm_normal_page(migrate->vma, addr, pte);
                        mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
                        mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
                }

                /* FIXME support THP */
                if (!page || !page->mapping || PageTransCompound(page)) {
                        mpfn = 0;
                        goto next;
                }

                /*
                 * By getting a reference on the page we pin it and that blocks
                 * any kind of migration. Side effect is that it "freezes" the
                 * pte.
                 *
                 * We drop this reference after isolating the page from the lru
                 * for non device page (device page are not on the lru and thus
                 * can't be dropped from it).
                 */
                get_page(page);

                /*
                 * Optimize for the common case where page is only mapped once
                 * in one process. If we can lock the page, then we can safely
                 * set up a special migration page table entry now.
                 */
                if (trylock_page(page)) {
                        pte_t swp_pte;

                        migrate->cpages++;
                        ptep_get_and_clear(mm, addr, ptep);

                        /* Setup special migration page table entry */
                        if (mpfn & MIGRATE_PFN_WRITE)
                                entry = make_writable_migration_entry(
                                                        page_to_pfn(page));
                        else
                                entry = make_readable_migration_entry(
                                                        page_to_pfn(page));
                        swp_pte = swp_entry_to_pte(entry);
                        if (pte_present(pte)) {
                                if (pte_soft_dirty(pte))
                                        swp_pte = pte_swp_mksoft_dirty(swp_pte);
                                if (pte_uffd_wp(pte))
                                        swp_pte = pte_swp_mkuffd_wp(swp_pte);
                        } else {
                                if (pte_swp_soft_dirty(pte))
                                        swp_pte = pte_swp_mksoft_dirty(swp_pte);
                                if (pte_swp_uffd_wp(pte))
                                        swp_pte = pte_swp_mkuffd_wp(swp_pte);
                        }
                        set_pte_at(mm, addr, ptep, swp_pte);

                        /*
                         * This is like regular unmap: we remove the rmap and
                         * drop page refcount. Page won't be freed, as we took
                         * a reference just above.
                         */
                        page_remove_rmap(page, false);
                        put_page(page);

                        if (pte_present(pte))
                                unmapped++;
                } else {
                        put_page(page);
                        mpfn = 0;
                }

next:
                migrate->dst[migrate->npages] = 0;
                migrate->src[migrate->npages++] = mpfn;
        }
        arch_leave_lazy_mmu_mode();
        pte_unmap_unlock(ptep - 1, ptl);

        /* Only flush the TLB if we actually modified any entries */
        if (unmapped)
                flush_tlb_range(walk->vma, start, end);

        return 0;
}

static const struct mm_walk_ops migrate_vma_walk_ops = {
        .pmd_entry              = migrate_vma_collect_pmd,
        .pte_hole               = migrate_vma_collect_hole,
};

/*
 * migrate_vma_collect() - collect pages over a range of virtual addresses
 * @migrate: migrate struct containing all migration information
 *
 * This will walk the CPU page table. For each virtual address backed by a
 * valid page, it updates the src array and takes a reference on the page, in
 * order to pin the page until we lock it and unmap it.
 */
static void migrate_vma_collect(struct migrate_vma *migrate)
{
        struct mmu_notifier_range range;

        /*
         * Note that the pgmap_owner is passed to the mmu notifier callback so
         * that the registered device driver can skip invalidating device
         * private page mappings that won't be migrated.
         */
        mmu_notifier_range_init_owner(&range, MMU_NOTIFY_MIGRATE, 0,
                migrate->vma, migrate->vma->vm_mm, migrate->start, migrate->end,
                migrate->pgmap_owner);
        mmu_notifier_invalidate_range_start(&range);

        walk_page_range(migrate->vma->vm_mm, migrate->start, migrate->end,
                        &migrate_vma_walk_ops, migrate);

        mmu_notifier_invalidate_range_end(&range);
        migrate->end = migrate->start + (migrate->npages << PAGE_SHIFT);
}

/*
 * migrate_vma_check_page() - check if page is pinned or not
 * @page: struct page to check
 *
 * Pinned pages cannot be migrated. This is the same test as in
 * folio_migrate_mapping(), except that here we allow migration of a
 * ZONE_DEVICE page.
 */
static bool migrate_vma_check_page(struct page *page)
{
        /*
         * One extra ref because caller holds an extra reference, either from
         * isolate_lru_page() for a regular page, or migrate_vma_collect() for
         * a device page.
         */
        int extra = 1;

        /*
         * FIXME support THP (transparent huge page), it is bit more complex to
         * check them than regular pages, because they can be mapped with a pmd
         * or with a pte (split pte mapping).
         */
        if (PageCompound(page))
                return false;

        /* Page from ZONE_DEVICE have one extra reference */
        if (is_zone_device_page(page)) {
                /*
                 * Private page can never be pin as they have no valid pte and
                 * GUP will fail for those. Yet if there is a pending migration
                 * a thread might try to wait on the pte migration entry and
                 * will bump the page reference count. Sadly there is no way to
                 * differentiate a regular pin from migration wait. Hence to
                 * avoid 2 racing thread trying to migrate back to CPU to enter
                 * infinite loop (one stopping migration because the other is
                 * waiting on pte migration entry). We always return true here.
                 *
                 * FIXME proper solution is to rework migration_entry_wait() so
                 * it does not need to take a reference on page.
                 */
                return is_device_private_page(page);
        }

        /* For file back page */
        if (page_mapping(page))
                extra += 1 + page_has_private(page);

        if ((page_count(page) - extra) > page_mapcount(page))
                return false;

        return true;
}

/*
 * migrate_vma_unmap() - replace page mapping with special migration pte entry
 * @migrate: migrate struct containing all migration information
 *
 * Isolate pages from the LRU and replace mappings (CPU page table pte) with a
 * special migration pte entry and check if it has been pinned. Pinned pages are
 * restored because we cannot migrate them.
 *
 * This is the last step before we call the device driver callback to allocate
 * destination memory and copy contents of original page over to new page.
 */
static void migrate_vma_unmap(struct migrate_vma *migrate)
{
        const unsigned long npages = migrate->npages;
        const unsigned long start = migrate->start;
        unsigned long addr, i, restore = 0;
        bool allow_drain = true;

        lru_add_drain();

        for (i = 0; i < npages; i++) {
                struct page *page = migrate_pfn_to_page(migrate->src[i]);

                if (!page)
                        continue;

                /* ZONE_DEVICE pages are not on LRU */
                if (!is_zone_device_page(page)) {
                        if (!PageLRU(page) && allow_drain) {
                                /* Drain CPU's pagevec */
                                lru_add_drain_all();
                                allow_drain = false;
                        }

                        if (isolate_lru_page(page)) {
                                migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
                                migrate->cpages--;
                                restore++;
                                continue;
                        }

                        /* Drop the reference we took in collect */
                        put_page(page);
                }

                if (page_mapped(page))
                        try_to_migrate(page, 0);

                if (page_mapped(page) || !migrate_vma_check_page(page)) {
                        if (!is_zone_device_page(page)) {
                                get_page(page);
                                putback_lru_page(page);
                        }

                        migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
                        migrate->cpages--;
                        restore++;
                        continue;
                }
        }

        for (addr = start, i = 0; i < npages && restore; addr += PAGE_SIZE, i++) {
                struct page *page = migrate_pfn_to_page(migrate->src[i]);

                if (!page || (migrate->src[i] & MIGRATE_PFN_MIGRATE))
                        continue;

                remove_migration_ptes(page, page, false);

                migrate->src[i] = 0;
                unlock_page(page);
                put_page(page);
                restore--;
        }
}

/**
 * migrate_vma_setup() - prepare to migrate a range of memory
 * @args: contains the vma, start, and pfns arrays for the migration
 *
 * Returns: negative errno on failures, 0 when 0 or more pages were migrated
 * without an error.
 *
 * Prepare to migrate a range of memory virtual address range by collecting all
 * the pages backing each virtual address in the range, saving them inside the
 * src array.  Then lock those pages and unmap them. Once the pages are locked
 * and unmapped, check whether each page is pinned or not.  Pages that aren't
 * pinned have the MIGRATE_PFN_MIGRATE flag set (by this function) in the
 * corresponding src array entry.  Then restores any pages that are pinned, by
 * remapping and unlocking those pages.
 *
 * The caller should then allocate destination memory and copy source memory to
 * it for all those entries (ie with MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE
 * flag set).  Once these are allocated and copied, the caller must update each
 * corresponding entry in the dst array with the pfn value of the destination
 * page and with MIGRATE_PFN_VALID. Destination pages must be locked via
 * lock_page().
 *
 * Note that the caller does not have to migrate all the pages that are marked
 * with MIGRATE_PFN_MIGRATE flag in src array unless this is a migration from
 * device memory to system memory.  If the caller cannot migrate a device page
 * back to system memory, then it must return VM_FAULT_SIGBUS, which has severe
 * consequences for the userspace process, so it must be avoided if at all
 * possible.
 *
 * For empty entries inside CPU page table (pte_none() or pmd_none() is true) we
 * do set MIGRATE_PFN_MIGRATE flag inside the corresponding source array thus
 * allowing the caller to allocate device memory for those unbacked virtual
 * addresses.  For this the caller simply has to allocate device memory and
 * properly set the destination entry like for regular migration.  Note that
 * this can still fail, and thus inside the device driver you must check if the
 * migration was successful for those entries after calling migrate_vma_pages(),
 * just like for regular migration.
 *
 * After that, the callers must call migrate_vma_pages() to go over each entry
 * in the src array that has the MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag
 * set. If the corresponding entry in dst array has MIGRATE_PFN_VALID flag set,
 * then migrate_vma_pages() to migrate struct page information from the source
 * struct page to the destination struct page.  If it fails to migrate the
 * struct page information, then it clears the MIGRATE_PFN_MIGRATE flag in the
 * src array.
 *
 * At this point all successfully migrated pages have an entry in the src
 * array with MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag set and the dst
 * array entry with MIGRATE_PFN_VALID flag set.
 *
 * Once migrate_vma_pages() returns the caller may inspect which pages were
 * successfully migrated, and which were not.  Successfully migrated pages will
 * have the MIGRATE_PFN_MIGRATE flag set for their src array entry.
 *
 * It is safe to update device page table after migrate_vma_pages() because
 * both destination and source page are still locked, and the mmap_lock is held
 * in read mode (hence no one can unmap the range being migrated).
 *
 * Once the caller is done cleaning up things and updating its page table (if it
 * chose to do so, this is not an obligation) it finally calls
 * migrate_vma_finalize() to update the CPU page table to point to new pages
 * for successfully migrated pages or otherwise restore the CPU page table to
 * point to the original source pages.
 */
int migrate_vma_setup(struct migrate_vma *args)
{
        long nr_pages = (args->end - args->start) >> PAGE_SHIFT;

        args->start &= PAGE_MASK;
        args->end &= PAGE_MASK;
        if (!args->vma || is_vm_hugetlb_page(args->vma) ||
            (args->vma->vm_flags & VM_SPECIAL) || vma_is_dax(args->vma))
                return -EINVAL;
        if (nr_pages <= 0)
                return -EINVAL;
        if (args->start < args->vma->vm_start ||
            args->start >= args->vma->vm_end)
                return -EINVAL;
        if (args->end <= args->vma->vm_start || args->end > args->vma->vm_end)
                return -EINVAL;
        if (!args->src || !args->dst)
                return -EINVAL;

        memset(args->src, 0, sizeof(*args->src) * nr_pages);
        args->cpages = 0;
        args->npages = 0;

        migrate_vma_collect(args);

        if (args->cpages)
                migrate_vma_unmap(args);

        /*
         * At this point pages are locked and unmapped, and thus they have
         * stable content and can safely be copied to destination memory that
         * is allocated by the drivers.
         */
        return 0;

}
EXPORT_SYMBOL(migrate_vma_setup);

/*
 * This code closely matches the code in:
 *   __handle_mm_fault()
 *     handle_pte_fault()
 *       do_anonymous_page()
 * to map in an anonymous zero page but the struct page will be a ZONE_DEVICE
 * private page.
 */
static void migrate_vma_insert_page(struct migrate_vma *migrate,
                                    unsigned long addr,
                                    struct page *page,
                                    unsigned long *src)
{
        struct vm_area_struct *vma = migrate->vma;
        struct mm_struct *mm = vma->vm_mm;
        bool flush = false;
        spinlock_t *ptl;
        pte_t entry;
        pgd_t *pgdp;
        p4d_t *p4dp;
        pud_t *pudp;
        pmd_t *pmdp;
        pte_t *ptep;

        /* Only allow populating anonymous memory */
        if (!vma_is_anonymous(vma))
                goto abort;

        pgdp = pgd_offset(mm, addr);
        p4dp = p4d_alloc(mm, pgdp, addr);
        if (!p4dp)
                goto abort;
        pudp = pud_alloc(mm, p4dp, addr);
        if (!pudp)
                goto abort;
        pmdp = pmd_alloc(mm, pudp, addr);
        if (!pmdp)
                goto abort;

        if (pmd_trans_huge(*pmdp) || pmd_devmap(*pmdp))
                goto abort;

        /*
         * Use pte_alloc() instead of pte_alloc_map().  We can't run
         * pte_offset_map() on pmds where a huge pmd might be created
         * from a different thread.
         *
         * pte_alloc_map() is safe to use under mmap_write_lock(mm) or when
         * parallel threads are excluded by other means.
         *
         * Here we only have mmap_read_lock(mm).
         */
        if (pte_alloc(mm, pmdp))
                goto abort;

        /* See the comment in pte_alloc_one_map() */
        if (unlikely(pmd_trans_unstable(pmdp)))
                goto abort;

        if (unlikely(anon_vma_prepare(vma)))
                goto abort;
        if (mem_cgroup_charge(page_folio(page), vma->vm_mm, GFP_KERNEL))
                goto abort;

        /*
         * The memory barrier inside __SetPageUptodate makes sure that
         * preceding stores to the page contents become visible before
         * the set_pte_at() write.
         */
        __SetPageUptodate(page);

        if (is_zone_device_page(page)) {
                if (is_device_private_page(page)) {
                        swp_entry_t swp_entry;

                        if (vma->vm_flags & VM_WRITE)
                                swp_entry = make_writable_device_private_entry(
                                                        page_to_pfn(page));
                        else
                                swp_entry = make_readable_device_private_entry(
                                                        page_to_pfn(page));
                        entry = swp_entry_to_pte(swp_entry);
                } else {
                        /*
                         * For now we only support migrating to un-addressable
                         * device memory.
                         */
                        pr_warn_once("Unsupported ZONE_DEVICE page type.\n");
                        goto abort;
                }
        } else {
                entry = mk_pte(page, vma->vm_page_prot);
                if (vma->vm_flags & VM_WRITE)
                        entry = pte_mkwrite(pte_mkdirty(entry));
        }

        ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);

        if (check_stable_address_space(mm))
                goto unlock_abort;

        if (pte_present(*ptep)) {
                unsigned long pfn = pte_pfn(*ptep);

                if (!is_zero_pfn(pfn))
                        goto unlock_abort;
                flush = true;
        } else if (!pte_none(*ptep))
                goto unlock_abort;

        /*
         * Check for userfaultfd but do not deliver the fault. Instead,
         * just back off.
         */
        if (userfaultfd_missing(vma))
                goto unlock_abort;

        inc_mm_counter(mm, MM_ANONPAGES);
        page_add_new_anon_rmap(page, vma, addr, false);
        if (!is_zone_device_page(page))
                lru_cache_add_inactive_or_unevictable(page, vma);
        get_page(page);

        if (flush) {
                flush_cache_page(vma, addr, pte_pfn(*ptep));
                ptep_clear_flush_notify(vma, addr, ptep);
                set_pte_at_notify(mm, addr, ptep, entry);
                update_mmu_cache(vma, addr, ptep);
        } else {
                /* No need to invalidate - it was non-present before */
                set_pte_at(mm, addr, ptep, entry);
                update_mmu_cache(vma, addr, ptep);
        }

        pte_unmap_unlock(ptep, ptl);
        *src = MIGRATE_PFN_MIGRATE;
        return;

unlock_abort:
        pte_unmap_unlock(ptep, ptl);
abort:
        *src &= ~MIGRATE_PFN_MIGRATE;
}

/**
 * migrate_vma_pages() - migrate meta-data from src page to dst page
 * @migrate: migrate struct containing all migration information
 *
 * This migrates struct page meta-data from source struct page to destination
 * struct page. This effectively finishes the migration from source page to the
 * destination page.
 */
void migrate_vma_pages(struct migrate_vma *migrate)
{
        const unsigned long npages = migrate->npages;
        const unsigned long start = migrate->start;
        struct mmu_notifier_range range;
        unsigned long addr, i;
        bool notified = false;

        for (i = 0, addr = start; i < npages; addr += PAGE_SIZE, i++) {
                struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
                struct page *page = migrate_pfn_to_page(migrate->src[i]);
                struct address_space *mapping;
                int r;

                if (!newpage) {
                        migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
                        continue;
                }

                if (!page) {
                        if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE))
                                continue;
                        if (!notified) {
                                notified = true;

                                mmu_notifier_range_init_owner(&range,
                                        MMU_NOTIFY_MIGRATE, 0, migrate->vma,
                                        migrate->vma->vm_mm, addr, migrate->end,
                                        migrate->pgmap_owner);
                                mmu_notifier_invalidate_range_start(&range);
                        }
                        migrate_vma_insert_page(migrate, addr, newpage,
                                                &migrate->src[i]);
                        continue;
                }

                mapping = page_mapping(page);

                if (is_zone_device_page(newpage)) {
                        if (is_device_private_page(newpage)) {
                                /*
                                 * For now only support private anonymous when
                                 * migrating to un-addressable device memory.
                                 */
                                if (mapping) {
                                        migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
                                        continue;
                                }
                        } else {
                                /*
                                 * Other types of ZONE_DEVICE page are not
                                 * supported.
                                 */
                                migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
                                continue;
                        }
                }

                r = migrate_page(mapping, newpage, page, MIGRATE_SYNC_NO_COPY);
                if (r != MIGRATEPAGE_SUCCESS)
                        migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
        }

        /*
         * No need to double call mmu_notifier->invalidate_range() callback as
         * the above ptep_clear_flush_notify() inside migrate_vma_insert_page()
         * did already call it.
         */
        if (notified)
                mmu_notifier_invalidate_range_only_end(&range);
}
EXPORT_SYMBOL(migrate_vma_pages);

/**
 * migrate_vma_finalize() - restore CPU page table entry
 * @migrate: migrate struct containing all migration information
 *
 * This replaces the special migration pte entry with either a mapping to the
 * new page if migration was successful for that page, or to the original page
 * otherwise.
 *
 * This also unlocks the pages and puts them back on the lru, or drops the extra
 * refcount, for device pages.
 */
void migrate_vma_finalize(struct migrate_vma *migrate)
{
        const unsigned long npages = migrate->npages;
        unsigned long i;

        for (i = 0; i < npages; i++) {
                struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
                struct page *page = migrate_pfn_to_page(migrate->src[i]);

                if (!page) {
                        if (newpage) {
                                unlock_page(newpage);
                                put_page(newpage);
                        }
                        continue;
                }

                if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE) || !newpage) {
                        if (newpage) {
                                unlock_page(newpage);
                                put_page(newpage);
                        }
                        newpage = page;
                }

                remove_migration_ptes(page, newpage, false);
                unlock_page(page);

                if (is_zone_device_page(page))
                        put_page(page);
                else
                        putback_lru_page(page);

                if (newpage != page) {
                        unlock_page(newpage);
                        if (is_zone_device_page(newpage))
                                put_page(newpage);
                        else
                                putback_lru_page(newpage);
                }
        }
}
EXPORT_SYMBOL(migrate_vma_finalize);
#endif /* CONFIG_DEVICE_PRIVATE */

#if defined(CONFIG_HOTPLUG_CPU)
/* Disable reclaim-based migration. */
static void __disable_all_migrate_targets(void)
{
        int node, i;

        if (!node_demotion)
                return;

        for_each_online_node(node) {
                node_demotion[node].nr = 0;
                for (i = 0; i < DEMOTION_TARGET_NODES; i++)
                        node_demotion[node].nodes[i] = NUMA_NO_NODE;
        }
}

static void disable_all_migrate_targets(void)
{
        __disable_all_migrate_targets();

        /*
         * Ensure that the "disable" is visible across the system.
         * Readers will see either a combination of before+disable
         * state or disable+after.  They will never see before and
         * after state together.
         *
         * The before+after state together might have cycles and
         * could cause readers to do things like loop until this
         * function finishes.  This ensures they can only see a
         * single "bad" read and would, for instance, only loop
         * once.
         */
        synchronize_rcu();
}

/*
 * Find an automatic demotion target for 'node'.
 * Failing here is OK.  It might just indicate
 * being at the end of a chain.
 */
static int establish_migrate_target(int node, nodemask_t *used,
                                    int best_distance)
{
        int migration_target, index, val;
        struct demotion_nodes *nd;

        if (!node_demotion)
                return NUMA_NO_NODE;

        nd = &node_demotion[node];

        migration_target = find_next_best_node(node, used);
        if (migration_target == NUMA_NO_NODE)
                return NUMA_NO_NODE;

        /*
         * If the node has been set a migration target node before,
         * which means it's the best distance between them. Still
         * check if this node can be demoted to other target nodes
         * if they have a same best distance.
         */
        if (best_distance != -1) {
                val = node_distance(node, migration_target);
                if (val > best_distance)
                        return NUMA_NO_NODE;
        }

        index = nd->nr;
        if (WARN_ONCE(index >= DEMOTION_TARGET_NODES,
                      "Exceeds maximum demotion target nodes\n"))
                return NUMA_NO_NODE;

        nd->nodes[index] = migration_target;
        nd->nr++;

        return migration_target;
}

/*
 * When memory fills up on a node, memory contents can be
 * automatically migrated to another node instead of
 * discarded at reclaim.
 *
 * Establish a "migration path" which will start at nodes
 * with CPUs and will follow the priorities used to build the
 * page allocator zonelists.
 *
 * The difference here is that cycles must be avoided.  If
 * node0 migrates to node1, then neither node1, nor anything
 * node1 migrates to can migrate to node0. Also one node can
 * be migrated to multiple nodes if the target nodes all have
 * a same best-distance against the source node.
 *
 * This function can run simultaneously with readers of
 * node_demotion[].  However, it can not run simultaneously
 * with itself.  Exclusion is provided by memory hotplug events
 * being single-threaded.
 */
static void __set_migration_target_nodes(void)
{
        nodemask_t next_pass    = NODE_MASK_NONE;
        nodemask_t this_pass    = NODE_MASK_NONE;
        nodemask_t used_targets = NODE_MASK_NONE;
        int node, best_distance;

        /*
         * Avoid any oddities like cycles that could occur
         * from changes in the topology.  This will leave
         * a momentary gap when migration is disabled.
         */
        disable_all_migrate_targets();

        /*
         * Allocations go close to CPUs, first.  Assume that
         * the migration path starts at the nodes with CPUs.
         */
        next_pass = node_states[N_CPU];
again:
        this_pass = next_pass;
        next_pass = NODE_MASK_NONE;
        /*
         * To avoid cycles in the migration "graph", ensure
         * that migration sources are not future targets by
         * setting them in 'used_targets'.  Do this only
         * once per pass so that multiple source nodes can
         * share a target node.
         *
         * 'used_targets' will become unavailable in future
         * passes.  This limits some opportunities for
         * multiple source nodes to share a destination.
         */
        nodes_or(used_targets, used_targets, this_pass);

        for_each_node_mask(node, this_pass) {
                best_distance = -1;

                /*
                 * Try to set up the migration path for the node, and the target
                 * migration nodes can be multiple, so doing a loop to find all
                 * the target nodes if they all have a best node distance.
                 */
                do {
                        int target_node =
                                establish_migrate_target(node, &used_targets,
                                                         best_distance);

                        if (target_node == NUMA_NO_NODE)
                                break;

                        if (best_distance == -1)
                                best_distance = node_distance(node, target_node);

                        /*
                         * Visit targets from this pass in the next pass.
                         * Eventually, every node will have been part of
                         * a pass, and will become set in 'used_targets'.
                         */
                        node_set(target_node, next_pass);
                } while (1);
        }
        /*
         * 'next_pass' contains nodes which became migration
         * targets in this pass.  Make additional passes until
         * no more migrations targets are available.
         */
        if (!nodes_empty(next_pass))
                goto again;
}

/*
 * For callers that do not hold get_online_mems() already.
 */
static void set_migration_target_nodes(void)
{
        get_online_mems();
        __set_migration_target_nodes();
        put_online_mems();
}

/*
 * This leaves migrate-on-reclaim transiently disabled between
 * the MEM_GOING_OFFLINE and MEM_OFFLINE events.  This runs
 * whether reclaim-based migration is enabled or not, which
 * ensures that the user can turn reclaim-based migration at
 * any time without needing to recalculate migration targets.
 *
 * These callbacks already hold get_online_mems().  That is why
 * __set_migration_target_nodes() can be used as opposed to
 * set_migration_target_nodes().
 */
static int __meminit migrate_on_reclaim_callback(struct notifier_block *self,
                                                 unsigned long action, void *_arg)
{
        struct memory_notify *arg = _arg;

        /*
         * Only update the node migration order when a node is
         * changing status, like online->offline.  This avoids
         * the overhead of synchronize_rcu() in most cases.
         */
        if (arg->status_change_nid < 0)
                return notifier_from_errno(0);

        switch (action) {
        case MEM_GOING_OFFLINE:
                /*
                 * Make sure there are not transient states where
                 * an offline node is a migration target.  This
                 * will leave migration disabled until the offline
                 * completes and the MEM_OFFLINE case below runs.
                 */
                disable_all_migrate_targets();
                break;
        case MEM_OFFLINE:
        case MEM_ONLINE:
                /*
                 * Recalculate the target nodes once the node
                 * reaches its final state (online or offline).
                 */
                __set_migration_target_nodes();
                break;
        case MEM_CANCEL_OFFLINE:
                /*
                 * MEM_GOING_OFFLINE disabled all the migration
                 * targets.  Reenable them.
                 */
                __set_migration_target_nodes();
                break;
        case MEM_GOING_ONLINE:
        case MEM_CANCEL_ONLINE:
                break;
        }

        return notifier_from_errno(0);
}

/*
 * React to hotplug events that might affect the migration targets
 * like events that online or offline NUMA nodes.
 *
 * The ordering is also currently dependent on which nodes have
 * CPUs.  That means we need CPU on/offline notification too.
 */
static int migration_online_cpu(unsigned int cpu)
{
        set_migration_target_nodes();
        return 0;
}

static int migration_offline_cpu(unsigned int cpu)
{
        set_migration_target_nodes();
        return 0;
}

static int __init migrate_on_reclaim_init(void)
{
        int ret;

        node_demotion = kmalloc_array(nr_node_ids,
                                      sizeof(struct demotion_nodes),
                                      GFP_KERNEL);
        WARN_ON(!node_demotion);

        ret = cpuhp_setup_state_nocalls(CPUHP_MM_DEMOTION_DEAD, "mm/demotion:offline",
                                        NULL, migration_offline_cpu);
        /*
         * In the unlikely case that this fails, the automatic
         * migration targets may become suboptimal for nodes
         * where N_CPU changes.  With such a small impact in a
         * rare case, do not bother trying to do anything special.
         */
        WARN_ON(ret < 0);
        ret = cpuhp_setup_state(CPUHP_AP_MM_DEMOTION_ONLINE, "mm/demotion:online",
                                migration_online_cpu, NULL);
        WARN_ON(ret < 0);

        hotplug_memory_notifier(migrate_on_reclaim_callback, 100);
        return 0;
}
late_initcall(migrate_on_reclaim_init);
#endif /* CONFIG_HOTPLUG_CPU */

bool numa_demotion_enabled = false;

#ifdef CONFIG_SYSFS
static ssize_t numa_demotion_enabled_show(struct kobject *kobj,
                                          struct kobj_attribute *attr, char *buf)
{
        return sysfs_emit(buf, "%s\n",
                          numa_demotion_enabled ? "true" : "false");
}

static ssize_t numa_demotion_enabled_store(struct kobject *kobj,
                                           struct kobj_attribute *attr,
                                           const char *buf, size_t count)
{
        if (!strncmp(buf, "true", 4) || !strncmp(buf, "1", 1))
                numa_demotion_enabled = true;
        else if (!strncmp(buf, "false", 5) || !strncmp(buf, "0", 1))
                numa_demotion_enabled = false;
        else
                return -EINVAL;

        return count;
}

static struct kobj_attribute numa_demotion_enabled_attr =
        __ATTR(demotion_enabled, 0644, numa_demotion_enabled_show,
               numa_demotion_enabled_store);

static struct attribute *numa_attrs[] = {
        &numa_demotion_enabled_attr.attr,
        NULL,
};

static const struct attribute_group numa_attr_group = {
        .attrs = numa_attrs,
};

static int __init numa_init_sysfs(void)
{
        int err;
        struct kobject *numa_kobj;

        numa_kobj = kobject_create_and_add("numa", mm_kobj);
        if (!numa_kobj) {
                pr_err("failed to create numa kobject\n");
                return -ENOMEM;
        }
        err = sysfs_create_group(numa_kobj, &numa_attr_group);
        if (err) {
                pr_err("failed to register numa group\n");
                goto delete_obj;
        }
        return 0;

delete_obj:
        kobject_put(numa_kobj);
        return err;
}
subsys_initcall(numa_init_sysfs);
#endif