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

// SPDX-License-Identifier: GPL-2.0
/*
 *      mm/mremap.c
 *
 *      (C) Copyright 1996 Linus Torvalds
 *
 *      Address space accounting code   <alan@lxorguk.ukuu.org.uk>
 *      (C) Copyright 2002 Red Hat Inc, All Rights Reserved
 */

#include <linux/mm.h>
#include <linux/mm_inline.h>
#include <linux/hugetlb.h>
#include <linux/shm.h>
#include <linux/ksm.h>
#include <linux/mman.h>
#include <linux/swap.h>
#include <linux/capability.h>
#include <linux/fs.h>
#include <linux/swapops.h>
#include <linux/highmem.h>
#include <linux/security.h>
#include <linux/syscalls.h>
#include <linux/mmu_notifier.h>
#include <linux/uaccess.h>
#include <linux/userfaultfd_k.h>
#include <linux/mempolicy.h>

#include <asm/cacheflush.h>
#include <asm/tlb.h>
#include <asm/pgalloc.h>

#include "internal.h"

/* Classify the kind of remap operation being performed. */
enum mremap_type {
        MREMAP_INVALID,         /* Initial state. */
        MREMAP_NO_RESIZE,       /* old_len == new_len, if not moved, do nothing. */
        MREMAP_SHRINK,          /* old_len > new_len. */
        MREMAP_EXPAND,          /* old_len < new_len. */
};

/*
 * Describes a VMA mremap() operation and is threaded throughout it.
 *
 * Any of the fields may be mutated by the operation, however these values will
 * always accurately reflect the remap (for instance, we may adjust lengths and
 * delta to account for hugetlb alignment).
 */
struct vma_remap_struct {
        /* User-provided state. */
        unsigned long addr;     /* User-specified address from which we remap. */
        unsigned long old_len;  /* Length of range being remapped. */
        unsigned long new_len;  /* Desired new length of mapping. */
        unsigned long flags;    /* user-specified MREMAP_* flags. */
        unsigned long new_addr; /* Optionally, desired new address. */

        /* uffd state. */
        struct vm_userfaultfd_ctx *uf;
        struct list_head *uf_unmap_early;
        struct list_head *uf_unmap;

        /* VMA state, determined in do_mremap(). */
        struct vm_area_struct *vma;

        /* Internal state, determined in do_mremap(). */
        unsigned long delta;            /* Absolute delta of old_len,new_len. */
        bool mlocked;                   /* Was the VMA mlock()'d? */
        enum mremap_type remap_type;    /* expand, shrink, etc. */
        bool mmap_locked;               /* Is mm currently write-locked? */
        unsigned long charged;          /* If VM_ACCOUNT, # pages to account. */
};

static pud_t *get_old_pud(struct mm_struct *mm, unsigned long addr)
{
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;

        pgd = pgd_offset(mm, addr);
        if (pgd_none_or_clear_bad(pgd))
                return NULL;

        p4d = p4d_offset(pgd, addr);
        if (p4d_none_or_clear_bad(p4d))
                return NULL;

        pud = pud_offset(p4d, addr);
        if (pud_none_or_clear_bad(pud))
                return NULL;

        return pud;
}

static pmd_t *get_old_pmd(struct mm_struct *mm, unsigned long addr)
{
        pud_t *pud;
        pmd_t *pmd;

        pud = get_old_pud(mm, addr);
        if (!pud)
                return NULL;

        pmd = pmd_offset(pud, addr);
        if (pmd_none(*pmd))
                return NULL;

        return pmd;
}

static pud_t *alloc_new_pud(struct mm_struct *mm, unsigned long addr)
{
        pgd_t *pgd;
        p4d_t *p4d;

        pgd = pgd_offset(mm, addr);
        p4d = p4d_alloc(mm, pgd, addr);
        if (!p4d)
                return NULL;

        return pud_alloc(mm, p4d, addr);
}

static pmd_t *alloc_new_pmd(struct mm_struct *mm, unsigned long addr)
{
        pud_t *pud;
        pmd_t *pmd;

        pud = alloc_new_pud(mm, addr);
        if (!pud)
                return NULL;

        pmd = pmd_alloc(mm, pud, addr);
        if (!pmd)
                return NULL;

        VM_BUG_ON(pmd_trans_huge(*pmd));

        return pmd;
}

static void take_rmap_locks(struct vm_area_struct *vma)
{
        if (vma->vm_file)
                i_mmap_lock_write(vma->vm_file->f_mapping);
        if (vma->anon_vma)
                anon_vma_lock_write(vma->anon_vma);
}

static void drop_rmap_locks(struct vm_area_struct *vma)
{
        if (vma->anon_vma)
                anon_vma_unlock_write(vma->anon_vma);
        if (vma->vm_file)
                i_mmap_unlock_write(vma->vm_file->f_mapping);
}

static pte_t move_soft_dirty_pte(pte_t pte)
{
        /*
         * Set soft dirty bit so we can notice
         * in userspace the ptes were moved.
         */
#ifdef CONFIG_MEM_SOFT_DIRTY
        if (pte_present(pte))
                pte = pte_mksoft_dirty(pte);
        else if (is_swap_pte(pte))
                pte = pte_swp_mksoft_dirty(pte);
#endif
        return pte;
}

static int move_ptes(struct pagetable_move_control *pmc,
                unsigned long extent, pmd_t *old_pmd, pmd_t *new_pmd)
{
        struct vm_area_struct *vma = pmc->old;
        bool need_clear_uffd_wp = vma_has_uffd_without_event_remap(vma);
        struct mm_struct *mm = vma->vm_mm;
        pte_t *old_pte, *new_pte, pte;
        pmd_t dummy_pmdval;
        spinlock_t *old_ptl, *new_ptl;
        bool force_flush = false;
        unsigned long old_addr = pmc->old_addr;
        unsigned long new_addr = pmc->new_addr;
        unsigned long old_end = old_addr + extent;
        unsigned long len = old_end - old_addr;
        int err = 0;

        /*
         * When need_rmap_locks is true, we take the i_mmap_rwsem and anon_vma
         * locks to ensure that rmap will always observe either the old or the
         * new ptes. This is the easiest way to avoid races with
         * truncate_pagecache(), page migration, etc...
         *
         * When need_rmap_locks is false, we use other ways to avoid
         * such races:
         *
         * - During exec() shift_arg_pages(), we use a specially tagged vma
         *   which rmap call sites look for using vma_is_temporary_stack().
         *
         * - During mremap(), new_vma is often known to be placed after vma
         *   in rmap traversal order. This ensures rmap will always observe
         *   either the old pte, or the new pte, or both (the page table locks
         *   serialize access to individual ptes, but only rmap traversal
         *   order guarantees that we won't miss both the old and new ptes).
         */
        if (pmc->need_rmap_locks)
                take_rmap_locks(vma);

        /*
         * We don't have to worry about the ordering of src and dst
         * pte locks because exclusive mmap_lock prevents deadlock.
         */
        old_pte = pte_offset_map_lock(mm, old_pmd, old_addr, &old_ptl);
        if (!old_pte) {
                err = -EAGAIN;
                goto out;
        }
        /*
         * Now new_pte is none, so hpage_collapse_scan_file() path can not find
         * this by traversing file->f_mapping, so there is no concurrency with
         * retract_page_tables(). In addition, we already hold the exclusive
         * mmap_lock, so this new_pte page is stable, so there is no need to get
         * pmdval and do pmd_same() check.
         */
        new_pte = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &dummy_pmdval,
                                           &new_ptl);
        if (!new_pte) {
                pte_unmap_unlock(old_pte, old_ptl);
                err = -EAGAIN;
                goto out;
        }
        if (new_ptl != old_ptl)
                spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
        flush_tlb_batched_pending(vma->vm_mm);
        arch_enter_lazy_mmu_mode();

        for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
                                   new_pte++, new_addr += PAGE_SIZE) {
                VM_WARN_ON_ONCE(!pte_none(*new_pte));

                if (pte_none(ptep_get(old_pte)))
                        continue;

                pte = ptep_get_and_clear(mm, old_addr, old_pte);
                /*
                 * If we are remapping a valid PTE, make sure
                 * to flush TLB before we drop the PTL for the
                 * PTE.
                 *
                 * NOTE! Both old and new PTL matter: the old one
                 * for racing with folio_mkclean(), the new one to
                 * make sure the physical page stays valid until
                 * the TLB entry for the old mapping has been
                 * flushed.
                 */
                if (pte_present(pte))
                        force_flush = true;
                pte = move_pte(pte, old_addr, new_addr);
                pte = move_soft_dirty_pte(pte);

                if (need_clear_uffd_wp && pte_marker_uffd_wp(pte))
                        pte_clear(mm, new_addr, new_pte);
                else {
                        if (need_clear_uffd_wp) {
                                if (pte_present(pte))
                                        pte = pte_clear_uffd_wp(pte);
                                else if (is_swap_pte(pte))
                                        pte = pte_swp_clear_uffd_wp(pte);
                        }
                        set_pte_at(mm, new_addr, new_pte, pte);
                }
        }

        arch_leave_lazy_mmu_mode();
        if (force_flush)
                flush_tlb_range(vma, old_end - len, old_end);
        if (new_ptl != old_ptl)
                spin_unlock(new_ptl);
        pte_unmap(new_pte - 1);
        pte_unmap_unlock(old_pte - 1, old_ptl);
out:
        if (pmc->need_rmap_locks)
                drop_rmap_locks(vma);
        return err;
}

#ifndef arch_supports_page_table_move
#define arch_supports_page_table_move arch_supports_page_table_move
static inline bool arch_supports_page_table_move(void)
{
        return IS_ENABLED(CONFIG_HAVE_MOVE_PMD) ||
                IS_ENABLED(CONFIG_HAVE_MOVE_PUD);
}
#endif

#ifdef CONFIG_HAVE_MOVE_PMD
static bool move_normal_pmd(struct pagetable_move_control *pmc,
                        pmd_t *old_pmd, pmd_t *new_pmd)
{
        spinlock_t *old_ptl, *new_ptl;
        struct vm_area_struct *vma = pmc->old;
        struct mm_struct *mm = vma->vm_mm;
        bool res = false;
        pmd_t pmd;

        if (!arch_supports_page_table_move())
                return false;
        /*
         * The destination pmd shouldn't be established, free_pgtables()
         * should have released it.
         *
         * However, there's a case during execve() where we use mremap
         * to move the initial stack, and in that case the target area
         * may overlap the source area (always moving down).
         *
         * If everything is PMD-aligned, that works fine, as moving
         * each pmd down will clear the source pmd. But if we first
         * have a few 4kB-only pages that get moved down, and then
         * hit the "now the rest is PMD-aligned, let's do everything
         * one pmd at a time", we will still have the old (now empty
         * of any 4kB pages, but still there) PMD in the page table
         * tree.
         *
         * Warn on it once - because we really should try to figure
         * out how to do this better - but then say "I won't move
         * this pmd".
         *
         * One alternative might be to just unmap the target pmd at
         * this point, and verify that it really is empty. We'll see.
         */
        if (WARN_ON_ONCE(!pmd_none(*new_pmd)))
                return false;

        /* If this pmd belongs to a uffd vma with remap events disabled, we need
         * to ensure that the uffd-wp state is cleared from all pgtables. This
         * means recursing into lower page tables in move_page_tables(), and we
         * can reuse the existing code if we simply treat the entry as "not
         * moved".
         */
        if (vma_has_uffd_without_event_remap(vma))
                return false;

        /*
         * We don't have to worry about the ordering of src and dst
         * ptlocks because exclusive mmap_lock prevents deadlock.
         */
        old_ptl = pmd_lock(mm, old_pmd);
        new_ptl = pmd_lockptr(mm, new_pmd);
        if (new_ptl != old_ptl)
                spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);

        pmd = *old_pmd;

        /* Racing with collapse? */
        if (unlikely(!pmd_present(pmd) || pmd_leaf(pmd)))
                goto out_unlock;
        /* Clear the pmd */
        pmd_clear(old_pmd);
        res = true;

        VM_BUG_ON(!pmd_none(*new_pmd));

        pmd_populate(mm, new_pmd, pmd_pgtable(pmd));
        flush_tlb_range(vma, pmc->old_addr, pmc->old_addr + PMD_SIZE);
out_unlock:
        if (new_ptl != old_ptl)
                spin_unlock(new_ptl);
        spin_unlock(old_ptl);

        return res;
}
#else
static inline bool move_normal_pmd(struct pagetable_move_control *pmc,
                pmd_t *old_pmd, pmd_t *new_pmd)
{
        return false;
}
#endif

#if CONFIG_PGTABLE_LEVELS > 2 && defined(CONFIG_HAVE_MOVE_PUD)
static bool move_normal_pud(struct pagetable_move_control *pmc,
                pud_t *old_pud, pud_t *new_pud)
{
        spinlock_t *old_ptl, *new_ptl;
        struct vm_area_struct *vma = pmc->old;
        struct mm_struct *mm = vma->vm_mm;
        pud_t pud;

        if (!arch_supports_page_table_move())
                return false;
        /*
         * The destination pud shouldn't be established, free_pgtables()
         * should have released it.
         */
        if (WARN_ON_ONCE(!pud_none(*new_pud)))
                return false;

        /* If this pud belongs to a uffd vma with remap events disabled, we need
         * to ensure that the uffd-wp state is cleared from all pgtables. This
         * means recursing into lower page tables in move_page_tables(), and we
         * can reuse the existing code if we simply treat the entry as "not
         * moved".
         */
        if (vma_has_uffd_without_event_remap(vma))
                return false;

        /*
         * We don't have to worry about the ordering of src and dst
         * ptlocks because exclusive mmap_lock prevents deadlock.
         */
        old_ptl = pud_lock(mm, old_pud);
        new_ptl = pud_lockptr(mm, new_pud);
        if (new_ptl != old_ptl)
                spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);

        /* Clear the pud */
        pud = *old_pud;
        pud_clear(old_pud);

        VM_BUG_ON(!pud_none(*new_pud));

        pud_populate(mm, new_pud, pud_pgtable(pud));
        flush_tlb_range(vma, pmc->old_addr, pmc->old_addr + PUD_SIZE);
        if (new_ptl != old_ptl)
                spin_unlock(new_ptl);
        spin_unlock(old_ptl);

        return true;
}
#else
static inline bool move_normal_pud(struct pagetable_move_control *pmc,
                pud_t *old_pud, pud_t *new_pud)
{
        return false;
}
#endif

#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
static bool move_huge_pud(struct pagetable_move_control *pmc,
                pud_t *old_pud, pud_t *new_pud)
{
        spinlock_t *old_ptl, *new_ptl;
        struct vm_area_struct *vma = pmc->old;
        struct mm_struct *mm = vma->vm_mm;
        pud_t pud;

        /*
         * The destination pud shouldn't be established, free_pgtables()
         * should have released it.
         */
        if (WARN_ON_ONCE(!pud_none(*new_pud)))
                return false;

        /*
         * We don't have to worry about the ordering of src and dst
         * ptlocks because exclusive mmap_lock prevents deadlock.
         */
        old_ptl = pud_lock(mm, old_pud);
        new_ptl = pud_lockptr(mm, new_pud);
        if (new_ptl != old_ptl)
                spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);

        /* Clear the pud */
        pud = *old_pud;
        pud_clear(old_pud);

        VM_BUG_ON(!pud_none(*new_pud));

        /* Set the new pud */
        /* mark soft_ditry when we add pud level soft dirty support */
        set_pud_at(mm, pmc->new_addr, new_pud, pud);
        flush_pud_tlb_range(vma, pmc->old_addr, pmc->old_addr + HPAGE_PUD_SIZE);
        if (new_ptl != old_ptl)
                spin_unlock(new_ptl);
        spin_unlock(old_ptl);

        return true;
}
#else
static bool move_huge_pud(struct pagetable_move_control *pmc,
                pud_t *old_pud, pud_t *new_pud)

{
        WARN_ON_ONCE(1);
        return false;

}
#endif

enum pgt_entry {
        NORMAL_PMD,
        HPAGE_PMD,
        NORMAL_PUD,
        HPAGE_PUD,
};

/*
 * Returns an extent of the corresponding size for the pgt_entry specified if
 * valid. Else returns a smaller extent bounded by the end of the source and
 * destination pgt_entry.
 */
static __always_inline unsigned long get_extent(enum pgt_entry entry,
                                                struct pagetable_move_control *pmc)
{
        unsigned long next, extent, mask, size;
        unsigned long old_addr = pmc->old_addr;
        unsigned long old_end = pmc->old_end;
        unsigned long new_addr = pmc->new_addr;

        switch (entry) {
        case HPAGE_PMD:
        case NORMAL_PMD:
                mask = PMD_MASK;
                size = PMD_SIZE;
                break;
        case HPAGE_PUD:
        case NORMAL_PUD:
                mask = PUD_MASK;
                size = PUD_SIZE;
                break;
        default:
                BUILD_BUG();
                break;
        }

        next = (old_addr + size) & mask;
        /* even if next overflowed, extent below will be ok */
        extent = next - old_addr;
        if (extent > old_end - old_addr)
                extent = old_end - old_addr;
        next = (new_addr + size) & mask;
        if (extent > next - new_addr)
                extent = next - new_addr;
        return extent;
}

/*
 * Should move_pgt_entry() acquire the rmap locks? This is either expressed in
 * the PMC, or overridden in the case of normal, larger page tables.
 */
static bool should_take_rmap_locks(struct pagetable_move_control *pmc,
                                   enum pgt_entry entry)
{
        switch (entry) {
        case NORMAL_PMD:
        case NORMAL_PUD:
                return true;
        default:
                return pmc->need_rmap_locks;
        }
}

/*
 * Attempts to speedup the move by moving entry at the level corresponding to
 * pgt_entry. Returns true if the move was successful, else false.
 */
static bool move_pgt_entry(struct pagetable_move_control *pmc,
                           enum pgt_entry entry, void *old_entry, void *new_entry)
{
        bool moved = false;
        bool need_rmap_locks = should_take_rmap_locks(pmc, entry);

        /* See comment in move_ptes() */
        if (need_rmap_locks)
                take_rmap_locks(pmc->old);

        switch (entry) {
        case NORMAL_PMD:
                moved = move_normal_pmd(pmc, old_entry, new_entry);
                break;
        case NORMAL_PUD:
                moved = move_normal_pud(pmc, old_entry, new_entry);
                break;
        case HPAGE_PMD:
                moved = IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
                        move_huge_pmd(pmc->old, pmc->old_addr, pmc->new_addr, old_entry,
                                      new_entry);
                break;
        case HPAGE_PUD:
                moved = IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
                        move_huge_pud(pmc, old_entry, new_entry);
                break;

        default:
                WARN_ON_ONCE(1);
                break;
        }

        if (need_rmap_locks)
                drop_rmap_locks(pmc->old);

        return moved;
}

/*
 * A helper to check if aligning down is OK. The aligned address should fall
 * on *no mapping*. For the stack moving down, that's a special move within
 * the VMA that is created to span the source and destination of the move,
 * so we make an exception for it.
 */
static bool can_align_down(struct pagetable_move_control *pmc,
                           struct vm_area_struct *vma, unsigned long addr_to_align,
                           unsigned long mask)
{
        unsigned long addr_masked = addr_to_align & mask;

        /*
         * If @addr_to_align of either source or destination is not the beginning
         * of the corresponding VMA, we can't align down or we will destroy part
         * of the current mapping.
         */
        if (!pmc->for_stack && vma->vm_start != addr_to_align)
                return false;

        /* In the stack case we explicitly permit in-VMA alignment. */
        if (pmc->for_stack && addr_masked >= vma->vm_start)
                return true;

        /*
         * Make sure the realignment doesn't cause the address to fall on an
         * existing mapping.
         */
        return find_vma_intersection(vma->vm_mm, addr_masked, vma->vm_start) == NULL;
}

/*
 * Determine if are in fact able to realign for efficiency to a higher page
 * table boundary.
 */
static bool can_realign_addr(struct pagetable_move_control *pmc,
                             unsigned long pagetable_mask)
{
        unsigned long align_mask = ~pagetable_mask;
        unsigned long old_align = pmc->old_addr & align_mask;
        unsigned long new_align = pmc->new_addr & align_mask;
        unsigned long pagetable_size = align_mask + 1;
        unsigned long old_align_next = pagetable_size - old_align;

        /*
         * We don't want to have to go hunting for VMAs from the end of the old
         * VMA to the next page table boundary, also we want to make sure the
         * operation is wortwhile.
         *
         * So ensure that we only perform this realignment if the end of the
         * range being copied reaches or crosses the page table boundary.
         *
         * boundary                        boundary
         *    .<- old_align ->                .
         *    .              |----------------.-----------|
         *    .              |          vma   .           |
         *    .              |----------------.-----------|
         *    .              <----------------.----------->
         *    .                          len_in
         *    <------------------------------->
         *    .         pagetable_size        .
         *    .              <---------------->
         *    .                old_align_next .
         */
        if (pmc->len_in < old_align_next)
                return false;

        /* Skip if the addresses are already aligned. */
        if (old_align == 0)
                return false;

        /* Only realign if the new and old addresses are mutually aligned. */
        if (old_align != new_align)
                return false;

        /* Ensure realignment doesn't cause overlap with existing mappings. */
        if (!can_align_down(pmc, pmc->old, pmc->old_addr, pagetable_mask) ||
            !can_align_down(pmc, pmc->new, pmc->new_addr, pagetable_mask))
                return false;

        return true;
}

/*
 * Opportunistically realign to specified boundary for faster copy.
 *
 * Consider an mremap() of a VMA with page table boundaries as below, and no
 * preceding VMAs from the lower page table boundary to the start of the VMA,
 * with the end of the range reaching or crossing the page table boundary.
 *
 *   boundary                        boundary
 *      .              |----------------.-----------|
 *      .              |          vma   .           |
 *      .              |----------------.-----------|
 *      .         pmc->old_addr         .      pmc->old_end
 *      .              <---------------------------->
 *      .                  move these page tables
 *
 * If we proceed with moving page tables in this scenario, we will have a lot of
 * work to do traversing old page tables and establishing new ones in the
 * destination across multiple lower level page tables.
 *
 * The idea here is simply to align pmc->old_addr, pmc->new_addr down to the
 * page table boundary, so we can simply copy a single page table entry for the
 * aligned portion of the VMA instead:
 *
 *   boundary                        boundary
 *      .              |----------------.-----------|
 *      .              |          vma   .           |
 *      .              |----------------.-----------|
 * pmc->old_addr                        .      pmc->old_end
 *      <------------------------------------------->
 *      .           move these page tables
 */
static void try_realign_addr(struct pagetable_move_control *pmc,
                             unsigned long pagetable_mask)
{

        if (!can_realign_addr(pmc, pagetable_mask))
                return;

        /*
         * Simply align to page table boundaries. Note that we do NOT update the
         * pmc->old_end value, and since the move_page_tables() operation spans
         * from [old_addr, old_end) (offsetting new_addr as it is performed),
         * this simply changes the start of the copy, not the end.
         */
        pmc->old_addr &= pagetable_mask;
        pmc->new_addr &= pagetable_mask;
}

/* Is the page table move operation done? */
static bool pmc_done(struct pagetable_move_control *pmc)
{
        return pmc->old_addr >= pmc->old_end;
}

/* Advance to the next page table, offset by extent bytes. */
static void pmc_next(struct pagetable_move_control *pmc, unsigned long extent)
{
        pmc->old_addr += extent;
        pmc->new_addr += extent;
}

/*
 * Determine how many bytes in the specified input range have had their page
 * tables moved so far.
 */
static unsigned long pmc_progress(struct pagetable_move_control *pmc)
{
        unsigned long orig_old_addr = pmc->old_end - pmc->len_in;
        unsigned long old_addr = pmc->old_addr;

        /*
         * Prevent negative return values when {old,new}_addr was realigned but
         * we broke out of the loop in move_page_tables() for the first PMD
         * itself.
         */
        return old_addr < orig_old_addr ? 0 : old_addr - orig_old_addr;
}

unsigned long move_page_tables(struct pagetable_move_control *pmc)
{
        unsigned long extent;
        struct mmu_notifier_range range;
        pmd_t *old_pmd, *new_pmd;
        pud_t *old_pud, *new_pud;
        struct mm_struct *mm = pmc->old->vm_mm;

        if (!pmc->len_in)
                return 0;

        if (is_vm_hugetlb_page(pmc->old))
                return move_hugetlb_page_tables(pmc->old, pmc->new, pmc->old_addr,
                                                pmc->new_addr, pmc->len_in);

        /*
         * If possible, realign addresses to PMD boundary for faster copy.
         * Only realign if the mremap copying hits a PMD boundary.
         */
        try_realign_addr(pmc, PMD_MASK);

        flush_cache_range(pmc->old, pmc->old_addr, pmc->old_end);
        mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, mm,
                                pmc->old_addr, pmc->old_end);
        mmu_notifier_invalidate_range_start(&range);

        for (; !pmc_done(pmc); pmc_next(pmc, extent)) {
                cond_resched();
                /*
                 * If extent is PUD-sized try to speed up the move by moving at the
                 * PUD level if possible.
                 */
                extent = get_extent(NORMAL_PUD, pmc);

                old_pud = get_old_pud(mm, pmc->old_addr);
                if (!old_pud)
                        continue;
                new_pud = alloc_new_pud(mm, pmc->new_addr);
                if (!new_pud)
                        break;
                if (pud_trans_huge(*old_pud) || pud_devmap(*old_pud)) {
                        if (extent == HPAGE_PUD_SIZE) {
                                move_pgt_entry(pmc, HPAGE_PUD, old_pud, new_pud);
                                /* We ignore and continue on error? */
                                continue;
                        }
                } else if (IS_ENABLED(CONFIG_HAVE_MOVE_PUD) && extent == PUD_SIZE) {
                        if (move_pgt_entry(pmc, NORMAL_PUD, old_pud, new_pud))
                                continue;
                }

                extent = get_extent(NORMAL_PMD, pmc);
                old_pmd = get_old_pmd(mm, pmc->old_addr);
                if (!old_pmd)
                        continue;
                new_pmd = alloc_new_pmd(mm, pmc->new_addr);
                if (!new_pmd)
                        break;
again:
                if (is_swap_pmd(*old_pmd) || pmd_trans_huge(*old_pmd) ||
                    pmd_devmap(*old_pmd)) {
                        if (extent == HPAGE_PMD_SIZE &&
                            move_pgt_entry(pmc, HPAGE_PMD, old_pmd, new_pmd))
                                continue;
                        split_huge_pmd(pmc->old, old_pmd, pmc->old_addr);
                } else if (IS_ENABLED(CONFIG_HAVE_MOVE_PMD) &&
                           extent == PMD_SIZE) {
                        /*
                         * If the extent is PMD-sized, try to speed the move by
                         * moving at the PMD level if possible.
                         */
                        if (move_pgt_entry(pmc, NORMAL_PMD, old_pmd, new_pmd))
                                continue;
                }
                if (pmd_none(*old_pmd))
                        continue;
                if (pte_alloc(pmc->new->vm_mm, new_pmd))
                        break;
                if (move_ptes(pmc, extent, old_pmd, new_pmd) < 0)
                        goto again;
        }

        mmu_notifier_invalidate_range_end(&range);

        return pmc_progress(pmc);
}

/* Set vrm->delta to the difference in VMA size specified by user. */
static void vrm_set_delta(struct vma_remap_struct *vrm)
{
        vrm->delta = abs_diff(vrm->old_len, vrm->new_len);
}

/* Determine what kind of remap this is - shrink, expand or no resize at all. */
static enum mremap_type vrm_remap_type(struct vma_remap_struct *vrm)
{
        if (vrm->delta == 0)
                return MREMAP_NO_RESIZE;

        if (vrm->old_len > vrm->new_len)
                return MREMAP_SHRINK;

        return MREMAP_EXPAND;
}

/*
 * When moving a VMA to vrm->new_adr, does this result in the new and old VMAs
 * overlapping?
 */
static bool vrm_overlaps(struct vma_remap_struct *vrm)
{
        unsigned long start_old = vrm->addr;
        unsigned long start_new = vrm->new_addr;
        unsigned long end_old = vrm->addr + vrm->old_len;
        unsigned long end_new = vrm->new_addr + vrm->new_len;

        /*
         * start_old    end_old
         *     |-----------|
         *     |           |
         *     |-----------|
         *             |-------------|
         *             |             |
         *             |-------------|
         *         start_new      end_new
         */
        if (end_old > start_new && end_new > start_old)
                return true;

        return false;
}

/* Do the mremap() flags require that the new_addr parameter be specified? */
static bool vrm_implies_new_addr(struct vma_remap_struct *vrm)
{
        return vrm->flags & (MREMAP_FIXED | MREMAP_DONTUNMAP);
}

/*
 * Find an unmapped area for the requested vrm->new_addr.
 *
 * If MREMAP_FIXED then this is equivalent to a MAP_FIXED mmap() call. If only
 * MREMAP_DONTUNMAP is set, then this is equivalent to providing a hint to
 * mmap(), otherwise this is equivalent to mmap() specifying a NULL address.
 *
 * Returns 0 on success (with vrm->new_addr updated), or an error code upon
 * failure.
 */
static unsigned long vrm_set_new_addr(struct vma_remap_struct *vrm)
{
        struct vm_area_struct *vma = vrm->vma;
        unsigned long map_flags = 0;
        /* Page Offset _into_ the VMA. */
        pgoff_t internal_pgoff = (vrm->addr - vma->vm_start) >> PAGE_SHIFT;
        pgoff_t pgoff = vma->vm_pgoff + internal_pgoff;
        unsigned long new_addr = vrm_implies_new_addr(vrm) ? vrm->new_addr : 0;
        unsigned long res;

        if (vrm->flags & MREMAP_FIXED)
                map_flags |= MAP_FIXED;
        if (vma->vm_flags & VM_MAYSHARE)
                map_flags |= MAP_SHARED;

        res = get_unmapped_area(vma->vm_file, new_addr, vrm->new_len, pgoff,
                                map_flags);
        if (IS_ERR_VALUE(res))
                return res;

        vrm->new_addr = res;
        return 0;
}

/*
 * Keep track of pages which have been added to the memory mapping. If the VMA
 * is accounted, also check to see if there is sufficient memory.
 *
 * Returns true on success, false if insufficient memory to charge.
 */
static bool vrm_charge(struct vma_remap_struct *vrm)
{
        unsigned long charged;

        if (!(vrm->vma->vm_flags & VM_ACCOUNT))
                return true;

        /*
         * If we don't unmap the old mapping, then we account the entirety of
         * the length of the new one. Otherwise it's just the delta in size.
         */
        if (vrm->flags & MREMAP_DONTUNMAP)
                charged = vrm->new_len >> PAGE_SHIFT;
        else
                charged = vrm->delta >> PAGE_SHIFT;


        /* This accounts 'charged' pages of memory. */
        if (security_vm_enough_memory_mm(current->mm, charged))
                return false;

        vrm->charged = charged;
        return true;
}

/*
 * an error has occurred so we will not be using vrm->charged memory. Unaccount
 * this memory if the VMA is accounted.
 */
static void vrm_uncharge(struct vma_remap_struct *vrm)
{
        if (!(vrm->vma->vm_flags & VM_ACCOUNT))
                return;

        vm_unacct_memory(vrm->charged);
        vrm->charged = 0;
}

/*
 * Update mm exec_vm, stack_vm, data_vm, and locked_vm fields as needed to
 * account for 'bytes' memory used, and if locked, indicate this in the VRM so
 * we can handle this correctly later.
 */
static void vrm_stat_account(struct vma_remap_struct *vrm,
                             unsigned long bytes)
{
        unsigned long pages = bytes >> PAGE_SHIFT;
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma = vrm->vma;

        vm_stat_account(mm, vma->vm_flags, pages);
        if (vma->vm_flags & VM_LOCKED) {
                mm->locked_vm += pages;
                vrm->mlocked = true;
        }
}

/*
 * Perform checks before attempting to write a VMA prior to it being
 * moved.
 */
static unsigned long prep_move_vma(struct vma_remap_struct *vrm)
{
        unsigned long err = 0;
        struct vm_area_struct *vma = vrm->vma;
        unsigned long old_addr = vrm->addr;
        unsigned long old_len = vrm->old_len;
        unsigned long dummy = vma->vm_flags;

        /*
         * We'd prefer to avoid failure later on in do_munmap:
         * which may split one vma into three before unmapping.
         */
        if (current->mm->map_count >= sysctl_max_map_count - 3)
                return -ENOMEM;

        if (vma->vm_ops && vma->vm_ops->may_split) {
                if (vma->vm_start != old_addr)
                        err = vma->vm_ops->may_split(vma, old_addr);
                if (!err && vma->vm_end != old_addr + old_len)
                        err = vma->vm_ops->may_split(vma, old_addr + old_len);
                if (err)
                        return err;
        }

        /*
         * Advise KSM to break any KSM pages in the area to be moved:
         * it would be confusing if they were to turn up at the new
         * location, where they happen to coincide with different KSM
         * pages recently unmapped.  But leave vma->vm_flags as it was,
         * so KSM can come around to merge on vma and new_vma afterwards.
         */
        err = ksm_madvise(vma, old_addr, old_addr + old_len,
                          MADV_UNMERGEABLE, &dummy);
        if (err)
                return err;

        return 0;
}

/*
 * Unmap source VMA for VMA move, turning it from a copy to a move, being
 * careful to ensure we do not underflow memory account while doing so if an
 * accountable move.
 *
 * This is best effort, if we fail to unmap then we simply try to correct
 * accounting and exit.
 */
static void unmap_source_vma(struct vma_remap_struct *vrm)
{
        struct mm_struct *mm = current->mm;
        unsigned long addr = vrm->addr;
        unsigned long len = vrm->old_len;
        struct vm_area_struct *vma = vrm->vma;
        VMA_ITERATOR(vmi, mm, addr);
        int err;
        unsigned long vm_start;
        unsigned long vm_end;
        /*
         * It might seem odd that we check for MREMAP_DONTUNMAP here, given this
         * function implies that we unmap the original VMA, which seems
         * contradictory.
         *
         * However, this occurs when this operation was attempted and an error
         * arose, in which case we _do_ wish to unmap the _new_ VMA, which means
         * we actually _do_ want it be unaccounted.
         */
        bool accountable_move = (vma->vm_flags & VM_ACCOUNT) &&
                !(vrm->flags & MREMAP_DONTUNMAP);

        /*
         * So we perform a trick here to prevent incorrect accounting. Any merge
         * or new VMA allocation performed in copy_vma() does not adjust
         * accounting, it is expected that callers handle this.
         *
         * And indeed we already have, accounting appropriately in the case of
         * both in vrm_charge().
         *
         * However, when we unmap the existing VMA (to effect the move), this
         * code will, if the VMA has VM_ACCOUNT set, attempt to unaccount
         * removed pages.
         *
         * To avoid this we temporarily clear this flag, reinstating on any
         * portions of the original VMA that remain.
         */
        if (accountable_move) {
                vm_flags_clear(vma, VM_ACCOUNT);
                /* We are about to split vma, so store the start/end. */
                vm_start = vma->vm_start;
                vm_end = vma->vm_end;
        }

        err = do_vmi_munmap(&vmi, mm, addr, len, vrm->uf_unmap, /* unlock= */false);
        vrm->vma = NULL; /* Invalidated. */
        if (err) {
                /* OOM: unable to split vma, just get accounts right */
                vm_acct_memory(len >> PAGE_SHIFT);
                return;
        }

        /*
         * If we mremap() from a VMA like this:
         *
         *    addr  end
         *     |     |
         *     v     v
         * |-------------|
         * |             |
         * |-------------|
         *
         * Having cleared VM_ACCOUNT from the whole VMA, after we unmap above
         * we'll end up with:
         *
         *    addr  end
         *     |     |
         *     v     v
         * |---|     |---|
         * | A |     | B |
         * |---|     |---|
         *
         * The VMI is still pointing at addr, so vma_prev() will give us A, and
         * a subsequent or lone vma_next() will give as B.
         *
         * do_vmi_munmap() will have restored the VMI back to addr.
         */
        if (accountable_move) {
                unsigned long end = addr + len;

                if (vm_start < addr) {
                        struct vm_area_struct *prev = vma_prev(&vmi);

                        vm_flags_set(prev, VM_ACCOUNT); /* Acquires VMA lock. */
                }

                if (vm_end > end) {
                        struct vm_area_struct *next = vma_next(&vmi);

                        vm_flags_set(next, VM_ACCOUNT); /* Acquires VMA lock. */
                }
        }
}

/*
 * Copy vrm->vma over to vrm->new_addr possibly adjusting size as part of the
 * process. Additionally handle an error occurring on moving of page tables,
 * where we reset vrm state to cause unmapping of the new VMA.
 *
 * Outputs the newly installed VMA to new_vma_ptr. Returns 0 on success or an
 * error code.
 */
static int copy_vma_and_data(struct vma_remap_struct *vrm,
                             struct vm_area_struct **new_vma_ptr)
{
        unsigned long internal_offset = vrm->addr - vrm->vma->vm_start;
        unsigned long internal_pgoff = internal_offset >> PAGE_SHIFT;
        unsigned long new_pgoff = vrm->vma->vm_pgoff + internal_pgoff;
        unsigned long moved_len;
        struct vm_area_struct *vma = vrm->vma;
        struct vm_area_struct *new_vma;
        int err = 0;
        PAGETABLE_MOVE(pmc, NULL, NULL, vrm->addr, vrm->new_addr, vrm->old_len);

        new_vma = copy_vma(&vma, vrm->new_addr, vrm->new_len, new_pgoff,
                           &pmc.need_rmap_locks);
        if (!new_vma) {
                vrm_uncharge(vrm);
                *new_vma_ptr = NULL;
                return -ENOMEM;
        }
        vrm->vma = vma;
        pmc.old = vma;
        pmc.new = new_vma;

        moved_len = move_page_tables(&pmc);
        if (moved_len < vrm->old_len)
                err = -ENOMEM;
        else if (vma->vm_ops && vma->vm_ops->mremap)
                err = vma->vm_ops->mremap(new_vma);

        if (unlikely(err)) {
                PAGETABLE_MOVE(pmc_revert, new_vma, vma, vrm->new_addr,
                               vrm->addr, moved_len);

                /*
                 * On error, move entries back from new area to old,
                 * which will succeed since page tables still there,
                 * and then proceed to unmap new area instead of old.
                 */
                pmc_revert.need_rmap_locks = true;
                move_page_tables(&pmc_revert);

                vrm->vma = new_vma;
                vrm->old_len = vrm->new_len;
                vrm->addr = vrm->new_addr;
        } else {
                mremap_userfaultfd_prep(new_vma, vrm->uf);
        }

        fixup_hugetlb_reservations(vma);

        *new_vma_ptr = new_vma;
        return err;
}

/*
 * Perform final tasks for MADV_DONTUNMAP operation, clearing mlock() and
 * account flags on remaining VMA by convention (it cannot be mlock()'d any
 * longer, as pages in range are no longer mapped), and removing anon_vma_chain
 * links from it (if the entire VMA was copied over).
 */
static void dontunmap_complete(struct vma_remap_struct *vrm,
                               struct vm_area_struct *new_vma)
{
        unsigned long start = vrm->addr;
        unsigned long end = vrm->addr + vrm->old_len;
        unsigned long old_start = vrm->vma->vm_start;
        unsigned long old_end = vrm->vma->vm_end;

        /*
         * We always clear VM_LOCKED[ONFAULT] | VM_ACCOUNT on the old
         * vma.
         */
        vm_flags_clear(vrm->vma, VM_LOCKED_MASK | VM_ACCOUNT);

        /*
         * anon_vma links of the old vma is no longer needed after its page
         * table has been moved.
         */
        if (new_vma != vrm->vma && start == old_start && end == old_end)
                unlink_anon_vmas(vrm->vma);

        /* Because we won't unmap we don't need to touch locked_vm. */
}

static unsigned long move_vma(struct vma_remap_struct *vrm)
{
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *new_vma;
        unsigned long hiwater_vm;
        int err;

        err = prep_move_vma(vrm);
        if (err)
                return err;

        /* If accounted, charge the number of bytes the operation will use. */
        if (!vrm_charge(vrm))
                return -ENOMEM;

        /* We don't want racing faults. */
        vma_start_write(vrm->vma);

        /* Perform copy step. */
        err = copy_vma_and_data(vrm, &new_vma);
        /*
         * If we established the copied-to VMA, we attempt to recover from the
         * error by setting the destination VMA to the source VMA and unmapping
         * it below.
         */
        if (err && !new_vma)
                return err;

        /*
         * If we failed to move page tables we still do total_vm increment
         * since do_munmap() will decrement it by old_len == new_len.
         *
         * Since total_vm is about to be raised artificially high for a
         * moment, we need to restore high watermark afterwards: if stats
         * are taken meanwhile, total_vm and hiwater_vm appear too high.
         * If this were a serious issue, we'd add a flag to do_munmap().
         */
        hiwater_vm = mm->hiwater_vm;

        vrm_stat_account(vrm, vrm->new_len);
        if (unlikely(!err && (vrm->flags & MREMAP_DONTUNMAP)))
                dontunmap_complete(vrm, new_vma);
        else
                unmap_source_vma(vrm);

        mm->hiwater_vm = hiwater_vm;

        return err ? (unsigned long)err : vrm->new_addr;
}

/*
 * resize_is_valid() - Ensure the vma can be resized to the new length at the give
 * address.
 *
 * Return 0 on success, error otherwise.
 */
static int resize_is_valid(struct vma_remap_struct *vrm)
{
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma = vrm->vma;
        unsigned long addr = vrm->addr;
        unsigned long old_len = vrm->old_len;
        unsigned long new_len = vrm->new_len;
        unsigned long pgoff;

        /*
         * !old_len is a special case where an attempt is made to 'duplicate'
         * a mapping.  This makes no sense for private mappings as it will
         * instead create a fresh/new mapping unrelated to the original.  This
         * is contrary to the basic idea of mremap which creates new mappings
         * based on the original.  There are no known use cases for this
         * behavior.  As a result, fail such attempts.
         */
        if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE))) {
                pr_warn_once("%s (%d): attempted to duplicate a private mapping with mremap.  This is not supported.\n",
                             current->comm, current->pid);
                return -EINVAL;
        }

        if ((vrm->flags & MREMAP_DONTUNMAP) &&
                        (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP)))
                return -EINVAL;

        /* We can't remap across vm area boundaries */
        if (old_len > vma->vm_end - addr)
                return -EFAULT;

        if (new_len == old_len)
                return 0;

        /* Need to be careful about a growing mapping */
        pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
        pgoff += vma->vm_pgoff;
        if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
                return -EINVAL;

        if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
                return -EFAULT;

        if (!mlock_future_ok(mm, vma->vm_flags, vrm->delta))
                return -EAGAIN;

        if (!may_expand_vm(mm, vma->vm_flags, vrm->delta >> PAGE_SHIFT))
                return -ENOMEM;

        return 0;
}

/*
 * The user has requested that the VMA be shrunk (i.e., old_len > new_len), so
 * execute this, optionally dropping the mmap lock when we do so.
 *
 * In both cases this invalidates the VMA, however if we don't drop the lock,
 * then load the correct VMA into vrm->vma afterwards.
 */
static unsigned long shrink_vma(struct vma_remap_struct *vrm,
                                bool drop_lock)
{
        struct mm_struct *mm = current->mm;
        unsigned long unmap_start = vrm->addr + vrm->new_len;
        unsigned long unmap_bytes = vrm->delta;
        unsigned long res;
        VMA_ITERATOR(vmi, mm, unmap_start);

        VM_BUG_ON(vrm->remap_type != MREMAP_SHRINK);

        res = do_vmi_munmap(&vmi, mm, unmap_start, unmap_bytes,
                            vrm->uf_unmap, drop_lock);
        vrm->vma = NULL; /* Invalidated. */
        if (res)
                return res;

        /*
         * If we've not dropped the lock, then we should reload the VMA to
         * replace the invalidated VMA with the one that may have now been
         * split.
         */
        if (drop_lock) {
                vrm->mmap_locked = false;
        } else {
                vrm->vma = vma_lookup(mm, vrm->addr);
                if (!vrm->vma)
                        return -EFAULT;
        }

        return 0;
}

/*
 * mremap_to() - remap a vma to a new location.
 * Returns: The new address of the vma or an error.
 */
static unsigned long mremap_to(struct vma_remap_struct *vrm)
{
        struct mm_struct *mm = current->mm;
        unsigned long err;

        /* Is the new length or address silly? */
        if (vrm->new_len > TASK_SIZE ||
            vrm->new_addr > TASK_SIZE - vrm->new_len)
                return -EINVAL;

        if (vrm_overlaps(vrm))
                return -EINVAL;

        if (vrm->flags & MREMAP_FIXED) {
                /*
                 * In mremap_to().
                 * VMA is moved to dst address, and munmap dst first.
                 * do_munmap will check if dst is sealed.
                 */
                err = do_munmap(mm, vrm->new_addr, vrm->new_len,
                                vrm->uf_unmap_early);
                vrm->vma = NULL; /* Invalidated. */
                if (err)
                        return err;

                /*
                 * If we remap a portion of a VMA elsewhere in the same VMA,
                 * this can invalidate the old VMA. Reset.
                 */
                vrm->vma = vma_lookup(mm, vrm->addr);
                if (!vrm->vma)
                        return -EFAULT;
        }

        if (vrm->remap_type == MREMAP_SHRINK) {
                err = shrink_vma(vrm, /* drop_lock= */false);
                if (err)
                        return err;

                /* Set up for the move now shrink has been executed. */
                vrm->old_len = vrm->new_len;
        }

        err = resize_is_valid(vrm);
        if (err)
                return err;

        /* MREMAP_DONTUNMAP expands by old_len since old_len == new_len */
        if (vrm->flags & MREMAP_DONTUNMAP) {
                vm_flags_t vm_flags = vrm->vma->vm_flags;
                unsigned long pages = vrm->old_len >> PAGE_SHIFT;

                if (!may_expand_vm(mm, vm_flags, pages))
                        return -ENOMEM;
        }

        err = vrm_set_new_addr(vrm);
        if (err)
                return err;

        return move_vma(vrm);
}

static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
{
        unsigned long end = vma->vm_end + delta;

        if (end < vma->vm_end) /* overflow */
                return 0;
        if (find_vma_intersection(vma->vm_mm, vma->vm_end, end))
                return 0;
        if (get_unmapped_area(NULL, vma->vm_start, end - vma->vm_start,
                              0, MAP_FIXED) & ~PAGE_MASK)
                return 0;
        return 1;
}

/* Determine whether we are actually able to execute an in-place expansion. */
static bool vrm_can_expand_in_place(struct vma_remap_struct *vrm)
{
        /* Number of bytes from vrm->addr to end of VMA. */
        unsigned long suffix_bytes = vrm->vma->vm_end - vrm->addr;

        /* If end of range aligns to end of VMA, we can just expand in-place. */
        if (suffix_bytes != vrm->old_len)
                return false;

        /* Check whether this is feasible. */
        if (!vma_expandable(vrm->vma, vrm->delta))
                return false;

        return true;
}

/*
 * Are the parameters passed to mremap() valid? If so return 0, otherwise return
 * error.
 */
static unsigned long check_mremap_params(struct vma_remap_struct *vrm)

{
        unsigned long addr = vrm->addr;
        unsigned long flags = vrm->flags;

        /* Ensure no unexpected flag values. */
        if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP))
                return -EINVAL;

        /* Start address must be page-aligned. */
        if (offset_in_page(addr))
                return -EINVAL;

        /*
         * We allow a zero old-len as a special case
         * for DOS-emu "duplicate shm area" thing. But
         * a zero new-len is nonsensical.
         */
        if (!PAGE_ALIGN(vrm->new_len))
                return -EINVAL;

        /* Remainder of checks are for cases with specific new_addr. */
        if (!vrm_implies_new_addr(vrm))
                return 0;

        /* The new address must be page-aligned. */
        if (offset_in_page(vrm->new_addr))
                return -EINVAL;

        /* A fixed address implies a move. */
        if (!(flags & MREMAP_MAYMOVE))
                return -EINVAL;

        /* MREMAP_DONTUNMAP does not allow resizing in the process. */
        if (flags & MREMAP_DONTUNMAP && vrm->old_len != vrm->new_len)
                return -EINVAL;

        /*
         * move_vma() need us to stay 4 maps below the threshold, otherwise
         * it will bail out at the very beginning.
         * That is a problem if we have already unmaped the regions here
         * (new_addr, and old_addr), because userspace will not know the
         * state of the vma's after it gets -ENOMEM.
         * So, to avoid such scenario we can pre-compute if the whole
         * operation has high chances to success map-wise.
         * Worst-scenario case is when both vma's (new_addr and old_addr) get
         * split in 3 before unmapping it.
         * That means 2 more maps (1 for each) to the ones we already hold.
         * Check whether current map count plus 2 still leads us to 4 maps below
         * the threshold, otherwise return -ENOMEM here to be more safe.
         */
        if ((current->mm->map_count + 2) >= sysctl_max_map_count - 3)
                return -ENOMEM;

        return 0;
}

/*
 * We know we can expand the VMA in-place by delta pages, so do so.
 *
 * If we discover the VMA is locked, update mm_struct statistics accordingly and
 * indicate so to the caller.
 */
static unsigned long expand_vma_in_place(struct vma_remap_struct *vrm)
{
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma = vrm->vma;
        VMA_ITERATOR(vmi, mm, vma->vm_end);

        if (!vrm_charge(vrm))
                return -ENOMEM;

        /*
         * Function vma_merge_extend() is called on the
         * extension we are adding to the already existing vma,
         * vma_merge_extend() will merge this extension with the
         * already existing vma (expand operation itself) and
         * possibly also with the next vma if it becomes
         * adjacent to the expanded vma and otherwise
         * compatible.
         */
        vma = vma_merge_extend(&vmi, vma, vrm->delta);
        if (!vma) {
                vrm_uncharge(vrm);
                return -ENOMEM;
        }
        vrm->vma = vma;

        vrm_stat_account(vrm, vrm->delta);

        return 0;
}

static bool align_hugetlb(struct vma_remap_struct *vrm)
{
        struct hstate *h __maybe_unused = hstate_vma(vrm->vma);

        vrm->old_len = ALIGN(vrm->old_len, huge_page_size(h));
        vrm->new_len = ALIGN(vrm->new_len, huge_page_size(h));

        /* addrs must be huge page aligned */
        if (vrm->addr & ~huge_page_mask(h))
                return false;
        if (vrm->new_addr & ~huge_page_mask(h))
                return false;

        /*
         * Don't allow remap expansion, because the underlying hugetlb
         * reservation is not yet capable to handle split reservation.
         */
        if (vrm->new_len > vrm->old_len)
                return false;

        vrm_set_delta(vrm);

        return true;
}

/*
 * We are mremap()'ing without specifying a fixed address to move to, but are
 * requesting that the VMA's size be increased.
 *
 * Try to do so in-place, if this fails, then move the VMA to a new location to
 * action the change.
 */
static unsigned long expand_vma(struct vma_remap_struct *vrm)
{
        unsigned long err;
        unsigned long addr = vrm->addr;

        err = resize_is_valid(vrm);
        if (err)
                return err;

        /*
         * [addr, old_len) spans precisely to the end of the VMA, so try to
         * expand it in-place.
         */
        if (vrm_can_expand_in_place(vrm)) {
                err = expand_vma_in_place(vrm);
                if (err)
                        return err;

                /*
                 * We want to populate the newly expanded portion of the VMA to
                 * satisfy the expectation that mlock()'ing a VMA maintains all
                 * of its pages in memory.
                 */
                if (vrm->mlocked)
                        vrm->new_addr = addr;

                /* OK we're done! */
                return addr;
        }

        /*
         * We weren't able to just expand or shrink the area,
         * we need to create a new one and move it.
         */

        /* We're not allowed to move the VMA, so error out. */
        if (!(vrm->flags & MREMAP_MAYMOVE))
                return -ENOMEM;

        /* Find a new location to move the VMA to. */
        err = vrm_set_new_addr(vrm);
        if (err)
                return err;

        return move_vma(vrm);
}

/*
 * Attempt to resize the VMA in-place, if we cannot, then move the VMA to the
 * first available address to perform the operation.
 */
static unsigned long mremap_at(struct vma_remap_struct *vrm)
{
        unsigned long res;

        switch (vrm->remap_type) {
        case MREMAP_INVALID:
                break;
        case MREMAP_NO_RESIZE:
                /* NO-OP CASE - resizing to the same size. */
                return vrm->addr;
        case MREMAP_SHRINK:
                /*
                 * SHRINK CASE. Can always be done in-place.
                 *
                 * Simply unmap the shrunken portion of the VMA. This does all
                 * the needed commit accounting, and we indicate that the mmap
                 * lock should be dropped.
                 */
                res = shrink_vma(vrm, /* drop_lock= */true);
                if (res)
                        return res;

                return vrm->addr;
        case MREMAP_EXPAND:
                return expand_vma(vrm);
        }

        BUG();
}

static unsigned long do_mremap(struct vma_remap_struct *vrm)
{
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma;
        unsigned long ret;

        ret = check_mremap_params(vrm);
        if (ret)
                return ret;

        vrm->old_len = PAGE_ALIGN(vrm->old_len);
        vrm->new_len = PAGE_ALIGN(vrm->new_len);
        vrm_set_delta(vrm);

        if (mmap_write_lock_killable(mm))
                return -EINTR;
        vrm->mmap_locked = true;

        vma = vrm->vma = vma_lookup(mm, vrm->addr);
        if (!vma) {
                ret = -EFAULT;
                goto out;
        }

        /* If mseal()'d, mremap() is prohibited. */
        if (!can_modify_vma(vma)) {
                ret = -EPERM;
                goto out;
        }

        /* Align to hugetlb page size, if required. */
        if (is_vm_hugetlb_page(vma) && !align_hugetlb(vrm)) {
                ret = -EINVAL;
                goto out;
        }

        vrm->remap_type = vrm_remap_type(vrm);

        /* Actually execute mremap. */
        ret = vrm_implies_new_addr(vrm) ? mremap_to(vrm) : mremap_at(vrm);

out:
        if (vrm->mmap_locked) {
                mmap_write_unlock(mm);
                vrm->mmap_locked = false;

                if (!offset_in_page(ret) && vrm->mlocked && vrm->new_len > vrm->old_len)
                        mm_populate(vrm->new_addr + vrm->old_len, vrm->delta);
        }

        userfaultfd_unmap_complete(mm, vrm->uf_unmap_early);
        mremap_userfaultfd_complete(vrm->uf, vrm->addr, ret, vrm->old_len);
        userfaultfd_unmap_complete(mm, vrm->uf_unmap);

        return ret;
}

/*
 * Expand (or shrink) an existing mapping, potentially moving it at the
 * same time (controlled by the MREMAP_MAYMOVE flag and available VM space)
 *
 * MREMAP_FIXED option added 5-Dec-1999 by Benjamin LaHaise
 * This option implies MREMAP_MAYMOVE.
 */
SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
                unsigned long, new_len, unsigned long, flags,
                unsigned long, new_addr)
{
        struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
        LIST_HEAD(uf_unmap_early);
        LIST_HEAD(uf_unmap);
        /*
         * There is a deliberate asymmetry here: we strip the pointer tag
         * from the old address but leave the new address alone. This is
         * for consistency with mmap(), where we prevent the creation of
         * aliasing mappings in userspace by leaving the tag bits of the
         * mapping address intact. A non-zero tag will cause the subsequent
         * range checks to reject the address as invalid.
         *
         * See Documentation/arch/arm64/tagged-address-abi.rst for more
         * information.
         */
        struct vma_remap_struct vrm = {
                .addr = untagged_addr(addr),
                .old_len = old_len,
                .new_len = new_len,
                .flags = flags,
                .new_addr = new_addr,

                .uf = &uf,
                .uf_unmap_early = &uf_unmap_early,
                .uf_unmap = &uf_unmap,

                .remap_type = MREMAP_INVALID, /* We set later. */
        };

        return do_mremap(&vrm);
}