mm: rmap: move the cache flushing to the correct place for hugetlb PMD sharing

[linux-block.git] / mm / memory-failure.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright (C) 2008, 2009 Intel Corporation
 * Authors: Andi Kleen, Fengguang Wu
 *
 * High level machine check handler. Handles pages reported by the
 * hardware as being corrupted usually due to a multi-bit ECC memory or cache
 * failure.
 * 
 * In addition there is a "soft offline" entry point that allows stop using
 * not-yet-corrupted-by-suspicious pages without killing anything.
 *
 * Handles page cache pages in various states.  The tricky part
 * here is that we can access any page asynchronously in respect to 
 * other VM users, because memory failures could happen anytime and 
 * anywhere. This could violate some of their assumptions. This is why 
 * this code has to be extremely careful. Generally it tries to use 
 * normal locking rules, as in get the standard locks, even if that means 
 * the error handling takes potentially a long time.
 *
 * It can be very tempting to add handling for obscure cases here.
 * In general any code for handling new cases should only be added iff:
 * - You know how to test it.
 * - You have a test that can be added to mce-test
 *   https://git.kernel.org/cgit/utils/cpu/mce/mce-test.git/
 * - The case actually shows up as a frequent (top 10) page state in
 *   tools/vm/page-types when running a real workload.
 * 
 * There are several operations here with exponential complexity because
 * of unsuitable VM data structures. For example the operation to map back 
 * from RMAP chains to processes has to walk the complete process list and 
 * has non linear complexity with the number. But since memory corruptions
 * are rare we hope to get away with this. This avoids impacting the core 
 * VM.
 */
#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/page-flags.h>
#include <linux/kernel-page-flags.h>
#include <linux/sched/signal.h>
#include <linux/sched/task.h>
#include <linux/dax.h>
#include <linux/ksm.h>
#include <linux/rmap.h>
#include <linux/export.h>
#include <linux/pagemap.h>
#include <linux/swap.h>
#include <linux/backing-dev.h>
#include <linux/migrate.h>
#include <linux/suspend.h>
#include <linux/slab.h>
#include <linux/swapops.h>
#include <linux/hugetlb.h>
#include <linux/memory_hotplug.h>
#include <linux/mm_inline.h>
#include <linux/memremap.h>
#include <linux/kfifo.h>
#include <linux/ratelimit.h>
#include <linux/page-isolation.h>
#include <linux/pagewalk.h>
#include <linux/shmem_fs.h>
#include "swap.h"
#include "internal.h"
#include "ras/ras_event.h"

int sysctl_memory_failure_early_kill __read_mostly = 0;

int sysctl_memory_failure_recovery __read_mostly = 1;

atomic_long_t num_poisoned_pages __read_mostly = ATOMIC_LONG_INIT(0);

static bool __page_handle_poison(struct page *page)
{
        int ret;

        zone_pcp_disable(page_zone(page));
        ret = dissolve_free_huge_page(page);
        if (!ret)
                ret = take_page_off_buddy(page);
        zone_pcp_enable(page_zone(page));

        return ret > 0;
}

static bool page_handle_poison(struct page *page, bool hugepage_or_freepage, bool release)
{
        if (hugepage_or_freepage) {
                /*
                 * Doing this check for free pages is also fine since dissolve_free_huge_page
                 * returns 0 for non-hugetlb pages as well.
                 */
                if (!__page_handle_poison(page))
                        /*
                         * We could fail to take off the target page from buddy
                         * for example due to racy page allocation, but that's
                         * acceptable because soft-offlined page is not broken
                         * and if someone really want to use it, they should
                         * take it.
                         */
                        return false;
        }

        SetPageHWPoison(page);
        if (release)
                put_page(page);
        page_ref_inc(page);
        num_poisoned_pages_inc();

        return true;
}

#if defined(CONFIG_HWPOISON_INJECT) || defined(CONFIG_HWPOISON_INJECT_MODULE)

u32 hwpoison_filter_enable = 0;
u32 hwpoison_filter_dev_major = ~0U;
u32 hwpoison_filter_dev_minor = ~0U;
u64 hwpoison_filter_flags_mask;
u64 hwpoison_filter_flags_value;
EXPORT_SYMBOL_GPL(hwpoison_filter_enable);
EXPORT_SYMBOL_GPL(hwpoison_filter_dev_major);
EXPORT_SYMBOL_GPL(hwpoison_filter_dev_minor);
EXPORT_SYMBOL_GPL(hwpoison_filter_flags_mask);
EXPORT_SYMBOL_GPL(hwpoison_filter_flags_value);

static int hwpoison_filter_dev(struct page *p)
{
        struct address_space *mapping;
        dev_t dev;

        if (hwpoison_filter_dev_major == ~0U &&
            hwpoison_filter_dev_minor == ~0U)
                return 0;

        mapping = page_mapping(p);
        if (mapping == NULL || mapping->host == NULL)
                return -EINVAL;

        dev = mapping->host->i_sb->s_dev;
        if (hwpoison_filter_dev_major != ~0U &&
            hwpoison_filter_dev_major != MAJOR(dev))
                return -EINVAL;
        if (hwpoison_filter_dev_minor != ~0U &&
            hwpoison_filter_dev_minor != MINOR(dev))
                return -EINVAL;

        return 0;
}

static int hwpoison_filter_flags(struct page *p)
{
        if (!hwpoison_filter_flags_mask)
                return 0;

        if ((stable_page_flags(p) & hwpoison_filter_flags_mask) ==
                                    hwpoison_filter_flags_value)
                return 0;
        else
                return -EINVAL;
}

/*
 * This allows stress tests to limit test scope to a collection of tasks
 * by putting them under some memcg. This prevents killing unrelated/important
 * processes such as /sbin/init. Note that the target task may share clean
 * pages with init (eg. libc text), which is harmless. If the target task
 * share _dirty_ pages with another task B, the test scheme must make sure B
 * is also included in the memcg. At last, due to race conditions this filter
 * can only guarantee that the page either belongs to the memcg tasks, or is
 * a freed page.
 */
#ifdef CONFIG_MEMCG
u64 hwpoison_filter_memcg;
EXPORT_SYMBOL_GPL(hwpoison_filter_memcg);
static int hwpoison_filter_task(struct page *p)
{
        if (!hwpoison_filter_memcg)
                return 0;

        if (page_cgroup_ino(p) != hwpoison_filter_memcg)
                return -EINVAL;

        return 0;
}
#else
static int hwpoison_filter_task(struct page *p) { return 0; }
#endif

int hwpoison_filter(struct page *p)
{
        if (!hwpoison_filter_enable)
                return 0;

        if (hwpoison_filter_dev(p))
                return -EINVAL;

        if (hwpoison_filter_flags(p))
                return -EINVAL;

        if (hwpoison_filter_task(p))
                return -EINVAL;

        return 0;
}
#else
int hwpoison_filter(struct page *p)
{
        return 0;
}
#endif

EXPORT_SYMBOL_GPL(hwpoison_filter);

/*
 * Kill all processes that have a poisoned page mapped and then isolate
 * the page.
 *
 * General strategy:
 * Find all processes having the page mapped and kill them.
 * But we keep a page reference around so that the page is not
 * actually freed yet.
 * Then stash the page away
 *
 * There's no convenient way to get back to mapped processes
 * from the VMAs. So do a brute-force search over all
 * running processes.
 *
 * Remember that machine checks are not common (or rather
 * if they are common you have other problems), so this shouldn't
 * be a performance issue.
 *
 * Also there are some races possible while we get from the
 * error detection to actually handle it.
 */

struct to_kill {
        struct list_head nd;
        struct task_struct *tsk;
        unsigned long addr;
        short size_shift;
};

/*
 * Send all the processes who have the page mapped a signal.
 * ``action optional'' if they are not immediately affected by the error
 * ``action required'' if error happened in current execution context
 */
static int kill_proc(struct to_kill *tk, unsigned long pfn, int flags)
{
        struct task_struct *t = tk->tsk;
        short addr_lsb = tk->size_shift;
        int ret = 0;

        pr_err("Memory failure: %#lx: Sending SIGBUS to %s:%d due to hardware memory corruption\n",
                        pfn, t->comm, t->pid);

        if ((flags & MF_ACTION_REQUIRED) && (t == current))
                ret = force_sig_mceerr(BUS_MCEERR_AR,
                                 (void __user *)tk->addr, addr_lsb);
        else
                /*
                 * Signal other processes sharing the page if they have
                 * PF_MCE_EARLY set.
                 * Don't use force here, it's convenient if the signal
                 * can be temporarily blocked.
                 * This could cause a loop when the user sets SIGBUS
                 * to SIG_IGN, but hopefully no one will do that?
                 */
                ret = send_sig_mceerr(BUS_MCEERR_AO, (void __user *)tk->addr,
                                      addr_lsb, t);  /* synchronous? */
        if (ret < 0)
                pr_info("Memory failure: Error sending signal to %s:%d: %d\n",
                        t->comm, t->pid, ret);
        return ret;
}

/*
 * Unknown page type encountered. Try to check whether it can turn PageLRU by
 * lru_add_drain_all.
 */
void shake_page(struct page *p)
{
        if (PageHuge(p))
                return;

        if (!PageSlab(p)) {
                lru_add_drain_all();
                if (PageLRU(p) || is_free_buddy_page(p))
                        return;
        }

        /*
         * TODO: Could shrink slab caches here if a lightweight range-based
         * shrinker will be available.
         */
}
EXPORT_SYMBOL_GPL(shake_page);

static unsigned long dev_pagemap_mapping_shift(struct page *page,
                struct vm_area_struct *vma)
{
        unsigned long address = vma_address(page, vma);
        unsigned long ret = 0;
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;
        pmd_t *pmd;
        pte_t *pte;

        VM_BUG_ON_VMA(address == -EFAULT, vma);
        pgd = pgd_offset(vma->vm_mm, address);
        if (!pgd_present(*pgd))
                return 0;
        p4d = p4d_offset(pgd, address);
        if (!p4d_present(*p4d))
                return 0;
        pud = pud_offset(p4d, address);
        if (!pud_present(*pud))
                return 0;
        if (pud_devmap(*pud))
                return PUD_SHIFT;
        pmd = pmd_offset(pud, address);
        if (!pmd_present(*pmd))
                return 0;
        if (pmd_devmap(*pmd))
                return PMD_SHIFT;
        pte = pte_offset_map(pmd, address);
        if (pte_present(*pte) && pte_devmap(*pte))
                ret = PAGE_SHIFT;
        pte_unmap(pte);
        return ret;
}

/*
 * Failure handling: if we can't find or can't kill a process there's
 * not much we can do.  We just print a message and ignore otherwise.
 */

/*
 * Schedule a process for later kill.
 * Uses GFP_ATOMIC allocations to avoid potential recursions in the VM.
 */
static void add_to_kill(struct task_struct *tsk, struct page *p,
                       struct vm_area_struct *vma,
                       struct list_head *to_kill)
{
        struct to_kill *tk;

        tk = kmalloc(sizeof(struct to_kill), GFP_ATOMIC);
        if (!tk) {
                pr_err("Memory failure: Out of memory while machine check handling\n");
                return;
        }

        tk->addr = page_address_in_vma(p, vma);
        if (is_zone_device_page(p))
                tk->size_shift = dev_pagemap_mapping_shift(p, vma);
        else
                tk->size_shift = page_shift(compound_head(p));

        /*
         * Send SIGKILL if "tk->addr == -EFAULT". Also, as
         * "tk->size_shift" is always non-zero for !is_zone_device_page(),
         * so "tk->size_shift == 0" effectively checks no mapping on
         * ZONE_DEVICE. Indeed, when a devdax page is mmapped N times
         * to a process' address space, it's possible not all N VMAs
         * contain mappings for the page, but at least one VMA does.
         * Only deliver SIGBUS with payload derived from the VMA that
         * has a mapping for the page.
         */
        if (tk->addr == -EFAULT) {
                pr_info("Memory failure: Unable to find user space address %lx in %s\n",
                        page_to_pfn(p), tsk->comm);
        } else if (tk->size_shift == 0) {
                kfree(tk);
                return;
        }

        get_task_struct(tsk);
        tk->tsk = tsk;
        list_add_tail(&tk->nd, to_kill);
}

/*
 * Kill the processes that have been collected earlier.
 *
 * Only do anything when FORCEKILL is set, otherwise just free the
 * list (this is used for clean pages which do not need killing)
 * Also when FAIL is set do a force kill because something went
 * wrong earlier.
 */
static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
                unsigned long pfn, int flags)
{
        struct to_kill *tk, *next;

        list_for_each_entry_safe (tk, next, to_kill, nd) {
                if (forcekill) {
                        /*
                         * In case something went wrong with munmapping
                         * make sure the process doesn't catch the
                         * signal and then access the memory. Just kill it.
                         */
                        if (fail || tk->addr == -EFAULT) {
                                pr_err("Memory failure: %#lx: forcibly killing %s:%d because of failure to unmap corrupted page\n",
                                       pfn, tk->tsk->comm, tk->tsk->pid);
                                do_send_sig_info(SIGKILL, SEND_SIG_PRIV,
                                                 tk->tsk, PIDTYPE_PID);
                        }

                        /*
                         * In theory the process could have mapped
                         * something else on the address in-between. We could
                         * check for that, but we need to tell the
                         * process anyways.
                         */
                        else if (kill_proc(tk, pfn, flags) < 0)
                                pr_err("Memory failure: %#lx: Cannot send advisory machine check signal to %s:%d\n",
                                       pfn, tk->tsk->comm, tk->tsk->pid);
                }
                put_task_struct(tk->tsk);
                kfree(tk);
        }
}

/*
 * Find a dedicated thread which is supposed to handle SIGBUS(BUS_MCEERR_AO)
 * on behalf of the thread group. Return task_struct of the (first found)
 * dedicated thread if found, and return NULL otherwise.
 *
 * We already hold read_lock(&tasklist_lock) in the caller, so we don't
 * have to call rcu_read_lock/unlock() in this function.
 */
static struct task_struct *find_early_kill_thread(struct task_struct *tsk)
{
        struct task_struct *t;

        for_each_thread(tsk, t) {
                if (t->flags & PF_MCE_PROCESS) {
                        if (t->flags & PF_MCE_EARLY)
                                return t;
                } else {
                        if (sysctl_memory_failure_early_kill)
                                return t;
                }
        }
        return NULL;
}

/*
 * Determine whether a given process is "early kill" process which expects
 * to be signaled when some page under the process is hwpoisoned.
 * Return task_struct of the dedicated thread (main thread unless explicitly
 * specified) if the process is "early kill" and otherwise returns NULL.
 *
 * Note that the above is true for Action Optional case. For Action Required
 * case, it's only meaningful to the current thread which need to be signaled
 * with SIGBUS, this error is Action Optional for other non current
 * processes sharing the same error page,if the process is "early kill", the
 * task_struct of the dedicated thread will also be returned.
 */
static struct task_struct *task_early_kill(struct task_struct *tsk,
                                           int force_early)
{
        if (!tsk->mm)
                return NULL;
        /*
         * Comparing ->mm here because current task might represent
         * a subthread, while tsk always points to the main thread.
         */
        if (force_early && tsk->mm == current->mm)
                return current;

        return find_early_kill_thread(tsk);
}

/*
 * Collect processes when the error hit an anonymous page.
 */
static void collect_procs_anon(struct page *page, struct list_head *to_kill,
                                int force_early)
{
        struct folio *folio = page_folio(page);
        struct vm_area_struct *vma;
        struct task_struct *tsk;
        struct anon_vma *av;
        pgoff_t pgoff;

        av = folio_lock_anon_vma_read(folio);
        if (av == NULL) /* Not actually mapped anymore */
                return;

        pgoff = page_to_pgoff(page);
        read_lock(&tasklist_lock);
        for_each_process (tsk) {
                struct anon_vma_chain *vmac;
                struct task_struct *t = task_early_kill(tsk, force_early);

                if (!t)
                        continue;
                anon_vma_interval_tree_foreach(vmac, &av->rb_root,
                                               pgoff, pgoff) {
                        vma = vmac->vma;
                        if (!page_mapped_in_vma(page, vma))
                                continue;
                        if (vma->vm_mm == t->mm)
                                add_to_kill(t, page, vma, to_kill);
                }
        }
        read_unlock(&tasklist_lock);
        page_unlock_anon_vma_read(av);
}

/*
 * Collect processes when the error hit a file mapped page.
 */
static void collect_procs_file(struct page *page, struct list_head *to_kill,
                                int force_early)
{
        struct vm_area_struct *vma;
        struct task_struct *tsk;
        struct address_space *mapping = page->mapping;
        pgoff_t pgoff;

        i_mmap_lock_read(mapping);
        read_lock(&tasklist_lock);
        pgoff = page_to_pgoff(page);
        for_each_process(tsk) {
                struct task_struct *t = task_early_kill(tsk, force_early);

                if (!t)
                        continue;
                vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff,
                                      pgoff) {
                        /*
                         * Send early kill signal to tasks where a vma covers
                         * the page but the corrupted page is not necessarily
                         * mapped it in its pte.
                         * Assume applications who requested early kill want
                         * to be informed of all such data corruptions.
                         */
                        if (vma->vm_mm == t->mm)
                                add_to_kill(t, page, vma, to_kill);
                }
        }
        read_unlock(&tasklist_lock);
        i_mmap_unlock_read(mapping);
}

/*
 * Collect the processes who have the corrupted page mapped to kill.
 */
static void collect_procs(struct page *page, struct list_head *tokill,
                                int force_early)
{
        if (!page->mapping)
                return;

        if (PageAnon(page))
                collect_procs_anon(page, tokill, force_early);
        else
                collect_procs_file(page, tokill, force_early);
}

struct hwp_walk {
        struct to_kill tk;
        unsigned long pfn;
        int flags;
};

static void set_to_kill(struct to_kill *tk, unsigned long addr, short shift)
{
        tk->addr = addr;
        tk->size_shift = shift;
}

static int check_hwpoisoned_entry(pte_t pte, unsigned long addr, short shift,
                                unsigned long poisoned_pfn, struct to_kill *tk)
{
        unsigned long pfn = 0;

        if (pte_present(pte)) {
                pfn = pte_pfn(pte);
        } else {
                swp_entry_t swp = pte_to_swp_entry(pte);

                if (is_hwpoison_entry(swp))
                        pfn = hwpoison_entry_to_pfn(swp);
        }

        if (!pfn || pfn != poisoned_pfn)
                return 0;

        set_to_kill(tk, addr, shift);
        return 1;
}

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
static int check_hwpoisoned_pmd_entry(pmd_t *pmdp, unsigned long addr,
                                      struct hwp_walk *hwp)
{
        pmd_t pmd = *pmdp;
        unsigned long pfn;
        unsigned long hwpoison_vaddr;

        if (!pmd_present(pmd))
                return 0;
        pfn = pmd_pfn(pmd);
        if (pfn <= hwp->pfn && hwp->pfn < pfn + HPAGE_PMD_NR) {
                hwpoison_vaddr = addr + ((hwp->pfn - pfn) << PAGE_SHIFT);
                set_to_kill(&hwp->tk, hwpoison_vaddr, PAGE_SHIFT);
                return 1;
        }
        return 0;
}
#else
static int check_hwpoisoned_pmd_entry(pmd_t *pmdp, unsigned long addr,
                                      struct hwp_walk *hwp)
{
        return 0;
}
#endif

static int hwpoison_pte_range(pmd_t *pmdp, unsigned long addr,
                              unsigned long end, struct mm_walk *walk)
{
        struct hwp_walk *hwp = walk->private;
        int ret = 0;
        pte_t *ptep, *mapped_pte;
        spinlock_t *ptl;

        ptl = pmd_trans_huge_lock(pmdp, walk->vma);
        if (ptl) {
                ret = check_hwpoisoned_pmd_entry(pmdp, addr, hwp);
                spin_unlock(ptl);
                goto out;
        }

        if (pmd_trans_unstable(pmdp))
                goto out;

        mapped_pte = ptep = pte_offset_map_lock(walk->vma->vm_mm, pmdp,
                                                addr, &ptl);
        for (; addr != end; ptep++, addr += PAGE_SIZE) {
                ret = check_hwpoisoned_entry(*ptep, addr, PAGE_SHIFT,
                                             hwp->pfn, &hwp->tk);
                if (ret == 1)
                        break;
        }
        pte_unmap_unlock(mapped_pte, ptl);
out:
        cond_resched();
        return ret;
}

#ifdef CONFIG_HUGETLB_PAGE
static int hwpoison_hugetlb_range(pte_t *ptep, unsigned long hmask,
                            unsigned long addr, unsigned long end,
                            struct mm_walk *walk)
{
        struct hwp_walk *hwp = walk->private;
        pte_t pte = huge_ptep_get(ptep);
        struct hstate *h = hstate_vma(walk->vma);

        return check_hwpoisoned_entry(pte, addr, huge_page_shift(h),
                                      hwp->pfn, &hwp->tk);
}
#else
#define hwpoison_hugetlb_range  NULL
#endif

static const struct mm_walk_ops hwp_walk_ops = {
        .pmd_entry = hwpoison_pte_range,
        .hugetlb_entry = hwpoison_hugetlb_range,
};

/*
 * Sends SIGBUS to the current process with error info.
 *
 * This function is intended to handle "Action Required" MCEs on already
 * hardware poisoned pages. They could happen, for example, when
 * memory_failure() failed to unmap the error page at the first call, or
 * when multiple local machine checks happened on different CPUs.
 *
 * MCE handler currently has no easy access to the error virtual address,
 * so this function walks page table to find it. The returned virtual address
 * is proper in most cases, but it could be wrong when the application
 * process has multiple entries mapping the error page.
 */
static int kill_accessing_process(struct task_struct *p, unsigned long pfn,
                                  int flags)
{
        int ret;
        struct hwp_walk priv = {
                .pfn = pfn,
        };
        priv.tk.tsk = p;

        mmap_read_lock(p->mm);
        ret = walk_page_range(p->mm, 0, TASK_SIZE, &hwp_walk_ops,
                              (void *)&priv);
        if (ret == 1 && priv.tk.addr)
                kill_proc(&priv.tk, pfn, flags);
        else
                ret = 0;
        mmap_read_unlock(p->mm);
        return ret > 0 ? -EHWPOISON : -EFAULT;
}

static const char *action_name[] = {
        [MF_IGNORED] = "Ignored",
        [MF_FAILED] = "Failed",
        [MF_DELAYED] = "Delayed",
        [MF_RECOVERED] = "Recovered",
};

static const char * const action_page_types[] = {
        [MF_MSG_KERNEL]                 = "reserved kernel page",
        [MF_MSG_KERNEL_HIGH_ORDER]      = "high-order kernel page",
        [MF_MSG_SLAB]                   = "kernel slab page",
        [MF_MSG_DIFFERENT_COMPOUND]     = "different compound page after locking",
        [MF_MSG_HUGE]                   = "huge page",
        [MF_MSG_FREE_HUGE]              = "free huge page",
        [MF_MSG_NON_PMD_HUGE]           = "non-pmd-sized huge page",
        [MF_MSG_UNMAP_FAILED]           = "unmapping failed page",
        [MF_MSG_DIRTY_SWAPCACHE]        = "dirty swapcache page",
        [MF_MSG_CLEAN_SWAPCACHE]        = "clean swapcache page",
        [MF_MSG_DIRTY_MLOCKED_LRU]      = "dirty mlocked LRU page",
        [MF_MSG_CLEAN_MLOCKED_LRU]      = "clean mlocked LRU page",
        [MF_MSG_DIRTY_UNEVICTABLE_LRU]  = "dirty unevictable LRU page",
        [MF_MSG_CLEAN_UNEVICTABLE_LRU]  = "clean unevictable LRU page",
        [MF_MSG_DIRTY_LRU]              = "dirty LRU page",
        [MF_MSG_CLEAN_LRU]              = "clean LRU page",
        [MF_MSG_TRUNCATED_LRU]          = "already truncated LRU page",
        [MF_MSG_BUDDY]                  = "free buddy page",
        [MF_MSG_DAX]                    = "dax page",
        [MF_MSG_UNSPLIT_THP]            = "unsplit thp",
        [MF_MSG_UNKNOWN]                = "unknown page",
};

/*
 * XXX: It is possible that a page is isolated from LRU cache,
 * and then kept in swap cache or failed to remove from page cache.
 * The page count will stop it from being freed by unpoison.
 * Stress tests should be aware of this memory leak problem.
 */
static int delete_from_lru_cache(struct page *p)
{
        if (!isolate_lru_page(p)) {
                /*
                 * Clear sensible page flags, so that the buddy system won't
                 * complain when the page is unpoison-and-freed.
                 */
                ClearPageActive(p);
                ClearPageUnevictable(p);

                /*
                 * Poisoned page might never drop its ref count to 0 so we have
                 * to uncharge it manually from its memcg.
                 */
                mem_cgroup_uncharge(page_folio(p));

                /*
                 * drop the page count elevated by isolate_lru_page()
                 */
                put_page(p);
                return 0;
        }
        return -EIO;
}

static int truncate_error_page(struct page *p, unsigned long pfn,
                                struct address_space *mapping)
{
        int ret = MF_FAILED;

        if (mapping->a_ops->error_remove_page) {
                int err = mapping->a_ops->error_remove_page(mapping, p);

                if (err != 0) {
                        pr_info("Memory failure: %#lx: Failed to punch page: %d\n",
                                pfn, err);
                } else if (page_has_private(p) &&
                           !try_to_release_page(p, GFP_NOIO)) {
                        pr_info("Memory failure: %#lx: failed to release buffers\n",
                                pfn);
                } else {
                        ret = MF_RECOVERED;
                }
        } else {
                /*
                 * If the file system doesn't support it just invalidate
                 * This fails on dirty or anything with private pages
                 */
                if (invalidate_inode_page(p))
                        ret = MF_RECOVERED;
                else
                        pr_info("Memory failure: %#lx: Failed to invalidate\n",
                                pfn);
        }

        return ret;
}

struct page_state {
        unsigned long mask;
        unsigned long res;
        enum mf_action_page_type type;

        /* Callback ->action() has to unlock the relevant page inside it. */
        int (*action)(struct page_state *ps, struct page *p);
};

/*
 * Return true if page is still referenced by others, otherwise return
 * false.
 *
 * The extra_pins is true when one extra refcount is expected.
 */
static bool has_extra_refcount(struct page_state *ps, struct page *p,
                               bool extra_pins)
{
        int count = page_count(p) - 1;

        if (extra_pins)
                count -= 1;

        if (count > 0) {
                pr_err("Memory failure: %#lx: %s still referenced by %d users\n",
                       page_to_pfn(p), action_page_types[ps->type], count);
                return true;
        }

        return false;
}

/*
 * Error hit kernel page.
 * Do nothing, try to be lucky and not touch this instead. For a few cases we
 * could be more sophisticated.
 */
static int me_kernel(struct page_state *ps, struct page *p)
{
        unlock_page(p);
        return MF_IGNORED;
}

/*
 * Page in unknown state. Do nothing.
 */
static int me_unknown(struct page_state *ps, struct page *p)
{
        pr_err("Memory failure: %#lx: Unknown page state\n", page_to_pfn(p));
        unlock_page(p);
        return MF_FAILED;
}

/*
 * Clean (or cleaned) page cache page.
 */
static int me_pagecache_clean(struct page_state *ps, struct page *p)
{
        int ret;
        struct address_space *mapping;
        bool extra_pins;

        delete_from_lru_cache(p);

        /*
         * For anonymous pages we're done the only reference left
         * should be the one m_f() holds.
         */
        if (PageAnon(p)) {
                ret = MF_RECOVERED;
                goto out;
        }

        /*
         * Now truncate the page in the page cache. This is really
         * more like a "temporary hole punch"
         * Don't do this for block devices when someone else
         * has a reference, because it could be file system metadata
         * and that's not safe to truncate.
         */
        mapping = page_mapping(p);
        if (!mapping) {
                /*
                 * Page has been teared down in the meanwhile
                 */
                ret = MF_FAILED;
                goto out;
        }

        /*
         * The shmem page is kept in page cache instead of truncating
         * so is expected to have an extra refcount after error-handling.
         */
        extra_pins = shmem_mapping(mapping);

        /*
         * Truncation is a bit tricky. Enable it per file system for now.
         *
         * Open: to take i_rwsem or not for this? Right now we don't.
         */
        ret = truncate_error_page(p, page_to_pfn(p), mapping);
        if (has_extra_refcount(ps, p, extra_pins))
                ret = MF_FAILED;

out:
        unlock_page(p);

        return ret;
}

/*
 * Dirty pagecache page
 * Issues: when the error hit a hole page the error is not properly
 * propagated.
 */
static int me_pagecache_dirty(struct page_state *ps, struct page *p)
{
        struct address_space *mapping = page_mapping(p);

        SetPageError(p);
        /* TBD: print more information about the file. */
        if (mapping) {
                /*
                 * IO error will be reported by write(), fsync(), etc.
                 * who check the mapping.
                 * This way the application knows that something went
                 * wrong with its dirty file data.
                 *
                 * There's one open issue:
                 *
                 * The EIO will be only reported on the next IO
                 * operation and then cleared through the IO map.
                 * Normally Linux has two mechanisms to pass IO error
                 * first through the AS_EIO flag in the address space
                 * and then through the PageError flag in the page.
                 * Since we drop pages on memory failure handling the
                 * only mechanism open to use is through AS_AIO.
                 *
                 * This has the disadvantage that it gets cleared on
                 * the first operation that returns an error, while
                 * the PageError bit is more sticky and only cleared
                 * when the page is reread or dropped.  If an
                 * application assumes it will always get error on
                 * fsync, but does other operations on the fd before
                 * and the page is dropped between then the error
                 * will not be properly reported.
                 *
                 * This can already happen even without hwpoisoned
                 * pages: first on metadata IO errors (which only
                 * report through AS_EIO) or when the page is dropped
                 * at the wrong time.
                 *
                 * So right now we assume that the application DTRT on
                 * the first EIO, but we're not worse than other parts
                 * of the kernel.
                 */
                mapping_set_error(mapping, -EIO);
        }

        return me_pagecache_clean(ps, p);
}

/*
 * Clean and dirty swap cache.
 *
 * Dirty swap cache page is tricky to handle. The page could live both in page
 * cache and swap cache(ie. page is freshly swapped in). So it could be
 * referenced concurrently by 2 types of PTEs:
 * normal PTEs and swap PTEs. We try to handle them consistently by calling
 * try_to_unmap(TTU_IGNORE_HWPOISON) to convert the normal PTEs to swap PTEs,
 * and then
 *      - clear dirty bit to prevent IO
 *      - remove from LRU
 *      - but keep in the swap cache, so that when we return to it on
 *        a later page fault, we know the application is accessing
 *        corrupted data and shall be killed (we installed simple
 *        interception code in do_swap_page to catch it).
 *
 * Clean swap cache pages can be directly isolated. A later page fault will
 * bring in the known good data from disk.
 */
static int me_swapcache_dirty(struct page_state *ps, struct page *p)
{
        int ret;
        bool extra_pins = false;

        ClearPageDirty(p);
        /* Trigger EIO in shmem: */
        ClearPageUptodate(p);

        ret = delete_from_lru_cache(p) ? MF_FAILED : MF_DELAYED;
        unlock_page(p);

        if (ret == MF_DELAYED)
                extra_pins = true;

        if (has_extra_refcount(ps, p, extra_pins))
                ret = MF_FAILED;

        return ret;
}

static int me_swapcache_clean(struct page_state *ps, struct page *p)
{
        int ret;

        delete_from_swap_cache(p);

        ret = delete_from_lru_cache(p) ? MF_FAILED : MF_RECOVERED;
        unlock_page(p);

        if (has_extra_refcount(ps, p, false))
                ret = MF_FAILED;

        return ret;
}

/*
 * Huge pages. Needs work.
 * Issues:
 * - Error on hugepage is contained in hugepage unit (not in raw page unit.)
 *   To narrow down kill region to one page, we need to break up pmd.
 */
static int me_huge_page(struct page_state *ps, struct page *p)
{
        int res;
        struct page *hpage = compound_head(p);
        struct address_space *mapping;

        if (!PageHuge(hpage))
                return MF_DELAYED;

        mapping = page_mapping(hpage);
        if (mapping) {
                res = truncate_error_page(hpage, page_to_pfn(p), mapping);
                unlock_page(hpage);
        } else {
                res = MF_FAILED;
                unlock_page(hpage);
                /*
                 * migration entry prevents later access on error hugepage,
                 * so we can free and dissolve it into buddy to save healthy
                 * subpages.
                 */
                put_page(hpage);
                if (__page_handle_poison(p)) {
                        page_ref_inc(p);
                        res = MF_RECOVERED;
                }
        }

        if (has_extra_refcount(ps, p, false))
                res = MF_FAILED;

        return res;
}

/*
 * Various page states we can handle.
 *
 * A page state is defined by its current page->flags bits.
 * The table matches them in order and calls the right handler.
 *
 * This is quite tricky because we can access page at any time
 * in its live cycle, so all accesses have to be extremely careful.
 *
 * This is not complete. More states could be added.
 * For any missing state don't attempt recovery.
 */

#define dirty           (1UL << PG_dirty)
#define sc              ((1UL << PG_swapcache) | (1UL << PG_swapbacked))
#define unevict         (1UL << PG_unevictable)
#define mlock           (1UL << PG_mlocked)
#define lru             (1UL << PG_lru)
#define head            (1UL << PG_head)
#define slab            (1UL << PG_slab)
#define reserved        (1UL << PG_reserved)

static struct page_state error_states[] = {
        { reserved,     reserved,       MF_MSG_KERNEL,  me_kernel },
        /*
         * free pages are specially detected outside this table:
         * PG_buddy pages only make a small fraction of all free pages.
         */

        /*
         * Could in theory check if slab page is free or if we can drop
         * currently unused objects without touching them. But just
         * treat it as standard kernel for now.
         */
        { slab,         slab,           MF_MSG_SLAB,    me_kernel },

        { head,         head,           MF_MSG_HUGE,            me_huge_page },

        { sc|dirty,     sc|dirty,       MF_MSG_DIRTY_SWAPCACHE, me_swapcache_dirty },
        { sc|dirty,     sc,             MF_MSG_CLEAN_SWAPCACHE, me_swapcache_clean },

        { mlock|dirty,  mlock|dirty,    MF_MSG_DIRTY_MLOCKED_LRU,       me_pagecache_dirty },
        { mlock|dirty,  mlock,          MF_MSG_CLEAN_MLOCKED_LRU,       me_pagecache_clean },

        { unevict|dirty, unevict|dirty, MF_MSG_DIRTY_UNEVICTABLE_LRU,   me_pagecache_dirty },
        { unevict|dirty, unevict,       MF_MSG_CLEAN_UNEVICTABLE_LRU,   me_pagecache_clean },

        { lru|dirty,    lru|dirty,      MF_MSG_DIRTY_LRU,       me_pagecache_dirty },
        { lru|dirty,    lru,            MF_MSG_CLEAN_LRU,       me_pagecache_clean },

        /*
         * Catchall entry: must be at end.
         */
        { 0,            0,              MF_MSG_UNKNOWN, me_unknown },
};

#undef dirty
#undef sc
#undef unevict
#undef mlock
#undef lru
#undef head
#undef slab
#undef reserved

/*
 * "Dirty/Clean" indication is not 100% accurate due to the possibility of
 * setting PG_dirty outside page lock. See also comment above set_page_dirty().
 */
static void action_result(unsigned long pfn, enum mf_action_page_type type,
                          enum mf_result result)
{
        trace_memory_failure_event(pfn, type, result);

        pr_err("Memory failure: %#lx: recovery action for %s: %s\n",
                pfn, action_page_types[type], action_name[result]);
}

static int page_action(struct page_state *ps, struct page *p,
                        unsigned long pfn)
{
        int result;

        /* page p should be unlocked after returning from ps->action().  */
        result = ps->action(ps, p);

        action_result(pfn, ps->type, result);

        /* Could do more checks here if page looks ok */
        /*
         * Could adjust zone counters here to correct for the missing page.
         */

        return (result == MF_RECOVERED || result == MF_DELAYED) ? 0 : -EBUSY;
}

static inline bool PageHWPoisonTakenOff(struct page *page)
{
        return PageHWPoison(page) && page_private(page) == MAGIC_HWPOISON;
}

void SetPageHWPoisonTakenOff(struct page *page)
{
        set_page_private(page, MAGIC_HWPOISON);
}

void ClearPageHWPoisonTakenOff(struct page *page)
{
        if (PageHWPoison(page))
                set_page_private(page, 0);
}

/*
 * Return true if a page type of a given page is supported by hwpoison
 * mechanism (while handling could fail), otherwise false.  This function
 * does not return true for hugetlb or device memory pages, so it's assumed
 * to be called only in the context where we never have such pages.
 */
static inline bool HWPoisonHandlable(struct page *page, unsigned long flags)
{
        /* Soft offline could migrate non-LRU movable pages */
        if ((flags & MF_SOFT_OFFLINE) && __PageMovable(page))
                return true;

        return PageLRU(page) || is_free_buddy_page(page);
}

static int __get_hwpoison_page(struct page *page, unsigned long flags)
{
        struct page *head = compound_head(page);
        int ret = 0;
        bool hugetlb = false;

        ret = get_hwpoison_huge_page(head, &hugetlb);
        if (hugetlb)
                return ret;

        /*
         * This check prevents from calling get_hwpoison_unless_zero()
         * for any unsupported type of page in order to reduce the risk of
         * unexpected races caused by taking a page refcount.
         */
        if (!HWPoisonHandlable(head, flags))
                return -EBUSY;

        if (get_page_unless_zero(head)) {
                if (head == compound_head(page))
                        return 1;

                pr_info("Memory failure: %#lx cannot catch tail\n",
                        page_to_pfn(page));
                put_page(head);
        }

        return 0;
}

static int get_any_page(struct page *p, unsigned long flags)
{
        int ret = 0, pass = 0;
        bool count_increased = false;

        if (flags & MF_COUNT_INCREASED)
                count_increased = true;

try_again:
        if (!count_increased) {
                ret = __get_hwpoison_page(p, flags);
                if (!ret) {
                        if (page_count(p)) {
                                /* We raced with an allocation, retry. */
                                if (pass++ < 3)
                                        goto try_again;
                                ret = -EBUSY;
                        } else if (!PageHuge(p) && !is_free_buddy_page(p)) {
                                /* We raced with put_page, retry. */
                                if (pass++ < 3)
                                        goto try_again;
                                ret = -EIO;
                        }
                        goto out;
                } else if (ret == -EBUSY) {
                        /*
                         * We raced with (possibly temporary) unhandlable
                         * page, retry.
                         */
                        if (pass++ < 3) {
                                shake_page(p);
                                goto try_again;
                        }
                        ret = -EIO;
                        goto out;
                }
        }

        if (PageHuge(p) || HWPoisonHandlable(p, flags)) {
                ret = 1;
        } else {
                /*
                 * A page we cannot handle. Check whether we can turn
                 * it into something we can handle.
                 */
                if (pass++ < 3) {
                        put_page(p);
                        shake_page(p);
                        count_increased = false;
                        goto try_again;
                }
                put_page(p);
                ret = -EIO;
        }
out:
        if (ret == -EIO)
                dump_page(p, "hwpoison: unhandlable page");

        return ret;
}

static int __get_unpoison_page(struct page *page)
{
        struct page *head = compound_head(page);
        int ret = 0;
        bool hugetlb = false;

        ret = get_hwpoison_huge_page(head, &hugetlb);
        if (hugetlb)
                return ret;

        /*
         * PageHWPoisonTakenOff pages are not only marked as PG_hwpoison,
         * but also isolated from buddy freelist, so need to identify the
         * state and have to cancel both operations to unpoison.
         */
        if (PageHWPoisonTakenOff(page))
                return -EHWPOISON;

        return get_page_unless_zero(page) ? 1 : 0;
}

/**
 * get_hwpoison_page() - Get refcount for memory error handling
 * @p:          Raw error page (hit by memory error)
 * @flags:      Flags controlling behavior of error handling
 *
 * get_hwpoison_page() takes a page refcount of an error page to handle memory
 * error on it, after checking that the error page is in a well-defined state
 * (defined as a page-type we can successfully handle the memory error on it,
 * such as LRU page and hugetlb page).
 *
 * Memory error handling could be triggered at any time on any type of page,
 * so it's prone to race with typical memory management lifecycle (like
 * allocation and free).  So to avoid such races, get_hwpoison_page() takes
 * extra care for the error page's state (as done in __get_hwpoison_page()),
 * and has some retry logic in get_any_page().
 *
 * When called from unpoison_memory(), the caller should already ensure that
 * the given page has PG_hwpoison. So it's never reused for other page
 * allocations, and __get_unpoison_page() never races with them.
 *
 * Return: 0 on failure,
 *         1 on success for in-use pages in a well-defined state,
 *         -EIO for pages on which we can not handle memory errors,
 *         -EBUSY when get_hwpoison_page() has raced with page lifecycle
 *         operations like allocation and free,
 *         -EHWPOISON when the page is hwpoisoned and taken off from buddy.
 */
static int get_hwpoison_page(struct page *p, unsigned long flags)
{
        int ret;

        zone_pcp_disable(page_zone(p));
        if (flags & MF_UNPOISON)
                ret = __get_unpoison_page(p);
        else
                ret = get_any_page(p, flags);
        zone_pcp_enable(page_zone(p));

        return ret;
}

/*
 * Do all that is necessary to remove user space mappings. Unmap
 * the pages and send SIGBUS to the processes if the data was dirty.
 */
static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
                                  int flags, struct page *hpage)
{
        struct folio *folio = page_folio(hpage);
        enum ttu_flags ttu = TTU_IGNORE_MLOCK | TTU_SYNC;
        struct address_space *mapping;
        LIST_HEAD(tokill);
        bool unmap_success;
        int kill = 1, forcekill;
        bool mlocked = PageMlocked(hpage);

        /*
         * Here we are interested only in user-mapped pages, so skip any
         * other types of pages.
         */
        if (PageReserved(p) || PageSlab(p))
                return true;
        if (!(PageLRU(hpage) || PageHuge(p)))
                return true;

        /*
         * This check implies we don't kill processes if their pages
         * are in the swap cache early. Those are always late kills.
         */
        if (!page_mapped(hpage))
                return true;

        if (PageKsm(p)) {
                pr_err("Memory failure: %#lx: can't handle KSM pages.\n", pfn);
                return false;
        }

        if (PageSwapCache(p)) {
                pr_err("Memory failure: %#lx: keeping poisoned page in swap cache\n",
                        pfn);
                ttu |= TTU_IGNORE_HWPOISON;
        }

        /*
         * Propagate the dirty bit from PTEs to struct page first, because we
         * need this to decide if we should kill or just drop the page.
         * XXX: the dirty test could be racy: set_page_dirty() may not always
         * be called inside page lock (it's recommended but not enforced).
         */
        mapping = page_mapping(hpage);
        if (!(flags & MF_MUST_KILL) && !PageDirty(hpage) && mapping &&
            mapping_can_writeback(mapping)) {
                if (page_mkclean(hpage)) {
                        SetPageDirty(hpage);
                } else {
                        kill = 0;
                        ttu |= TTU_IGNORE_HWPOISON;
                        pr_info("Memory failure: %#lx: corrupted page was clean: dropped without side effects\n",
                                pfn);
                }
        }

        /*
         * First collect all the processes that have the page
         * mapped in dirty form.  This has to be done before try_to_unmap,
         * because ttu takes the rmap data structures down.
         *
         * Error handling: We ignore errors here because
         * there's nothing that can be done.
         */
        if (kill)
                collect_procs(hpage, &tokill, flags & MF_ACTION_REQUIRED);

        if (PageHuge(hpage) && !PageAnon(hpage)) {
                /*
                 * For hugetlb pages 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 indicate we have taken the lock
                 * at this higher level.
                 */
                mapping = hugetlb_page_mapping_lock_write(hpage);
                if (mapping) {
                        try_to_unmap(folio, ttu|TTU_RMAP_LOCKED);
                        i_mmap_unlock_write(mapping);
                } else
                        pr_info("Memory failure: %#lx: could not lock mapping for mapped huge page\n", pfn);
        } else {
                try_to_unmap(folio, ttu);
        }

        unmap_success = !page_mapped(hpage);
        if (!unmap_success)
                pr_err("Memory failure: %#lx: failed to unmap page (mapcount=%d)\n",
                       pfn, page_mapcount(hpage));

        /*
         * try_to_unmap() might put mlocked page in lru cache, so call
         * shake_page() again to ensure that it's flushed.
         */
        if (mlocked)
                shake_page(hpage);

        /*
         * Now that the dirty bit has been propagated to the
         * struct page and all unmaps done we can decide if
         * killing is needed or not.  Only kill when the page
         * was dirty or the process is not restartable,
         * otherwise the tokill list is merely
         * freed.  When there was a problem unmapping earlier
         * use a more force-full uncatchable kill to prevent
         * any accesses to the poisoned memory.
         */
        forcekill = PageDirty(hpage) || (flags & MF_MUST_KILL);
        kill_procs(&tokill, forcekill, !unmap_success, pfn, flags);

        return unmap_success;
}

static int identify_page_state(unsigned long pfn, struct page *p,
                                unsigned long page_flags)
{
        struct page_state *ps;

        /*
         * The first check uses the current page flags which may not have any
         * relevant information. The second check with the saved page flags is
         * carried out only if the first check can't determine the page status.
         */
        for (ps = error_states;; ps++)
                if ((p->flags & ps->mask) == ps->res)
                        break;

        page_flags |= (p->flags & (1UL << PG_dirty));

        if (!ps->mask)
                for (ps = error_states;; ps++)
                        if ((page_flags & ps->mask) == ps->res)
                                break;
        return page_action(ps, p, pfn);
}

static int try_to_split_thp_page(struct page *page, const char *msg)
{
        lock_page(page);
        if (unlikely(split_huge_page(page))) {
                unsigned long pfn = page_to_pfn(page);

                unlock_page(page);
                pr_info("%s: %#lx: thp split failed\n", msg, pfn);
                put_page(page);
                return -EBUSY;
        }
        unlock_page(page);

        return 0;
}

/*
 * Called from hugetlb code with hugetlb_lock held.
 *
 * Return values:
 *   0             - free hugepage
 *   1             - in-use hugepage
 *   2             - not a hugepage
 *   -EBUSY        - the hugepage is busy (try to retry)
 *   -EHWPOISON    - the hugepage is already hwpoisoned
 */
int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
{
        struct page *page = pfn_to_page(pfn);
        struct page *head = compound_head(page);
        int ret = 2;    /* fallback to normal page handling */
        bool count_increased = false;

        if (!PageHeadHuge(head))
                goto out;

        if (flags & MF_COUNT_INCREASED) {
                ret = 1;
                count_increased = true;
        } else if (HPageFreed(head)) {
                ret = 0;
        } else if (HPageMigratable(head)) {
                ret = get_page_unless_zero(head);
                if (ret)
                        count_increased = true;
        } else {
                ret = -EBUSY;
                goto out;
        }

        if (TestSetPageHWPoison(head)) {
                ret = -EHWPOISON;
                goto out;
        }

        return ret;
out:
        if (count_increased)
                put_page(head);
        return ret;
}

#ifdef CONFIG_HUGETLB_PAGE
/*
 * Taking refcount of hugetlb pages needs extra care about race conditions
 * with basic operations like hugepage allocation/free/demotion.
 * So some of prechecks for hwpoison (pinning, and testing/setting
 * PageHWPoison) should be done in single hugetlb_lock range.
 */
static int try_memory_failure_hugetlb(unsigned long pfn, int flags, int *hugetlb)
{
        int res;
        struct page *p = pfn_to_page(pfn);
        struct page *head;
        unsigned long page_flags;
        bool retry = true;

        *hugetlb = 1;
retry:
        res = get_huge_page_for_hwpoison(pfn, flags);
        if (res == 2) { /* fallback to normal page handling */
                *hugetlb = 0;
                return 0;
        } else if (res == -EHWPOISON) {
                pr_err("Memory failure: %#lx: already hardware poisoned\n", pfn);
                if (flags & MF_ACTION_REQUIRED) {
                        head = compound_head(p);
                        res = kill_accessing_process(current, page_to_pfn(head), flags);
                }
                return res;
        } else if (res == -EBUSY) {
                if (retry) {
                        retry = false;
                        goto retry;
                }
                action_result(pfn, MF_MSG_UNKNOWN, MF_IGNORED);
                return res;
        }

        head = compound_head(p);
        lock_page(head);

        if (hwpoison_filter(p)) {
                ClearPageHWPoison(head);
                res = -EOPNOTSUPP;
                goto out;
        }

        num_poisoned_pages_inc();

        /*
         * Handling free hugepage.  The possible race with hugepage allocation
         * or demotion can be prevented by PageHWPoison flag.
         */
        if (res == 0) {
                unlock_page(head);
                res = MF_FAILED;
                if (__page_handle_poison(p)) {
                        page_ref_inc(p);
                        res = MF_RECOVERED;
                }
                action_result(pfn, MF_MSG_FREE_HUGE, res);
                return res == MF_RECOVERED ? 0 : -EBUSY;
        }

        page_flags = head->flags;

        /*
         * TODO: hwpoison for pud-sized hugetlb doesn't work right now, so
         * simply disable it. In order to make it work properly, we need
         * make sure that:
         *  - conversion of a pud that maps an error hugetlb into hwpoison
         *    entry properly works, and
         *  - other mm code walking over page table is aware of pud-aligned
         *    hwpoison entries.
         */
        if (huge_page_size(page_hstate(head)) > PMD_SIZE) {
                action_result(pfn, MF_MSG_NON_PMD_HUGE, MF_IGNORED);
                res = -EBUSY;
                goto out;
        }

        if (!hwpoison_user_mappings(p, pfn, flags, head)) {
                action_result(pfn, MF_MSG_UNMAP_FAILED, MF_IGNORED);
                res = -EBUSY;
                goto out;
        }

        return identify_page_state(pfn, p, page_flags);
out:
        unlock_page(head);
        return res;
}
#else
static inline int try_memory_failure_hugetlb(unsigned long pfn, int flags, int *hugetlb)
{
        return 0;
}
#endif

static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
                struct dev_pagemap *pgmap)
{
        struct page *page = pfn_to_page(pfn);
        unsigned long size = 0;
        struct to_kill *tk;
        LIST_HEAD(tokill);
        int rc = -EBUSY;
        loff_t start;
        dax_entry_t cookie;

        if (flags & MF_COUNT_INCREASED)
                /*
                 * Drop the extra refcount in case we come from madvise().
                 */
                put_page(page);

        /* device metadata space is not recoverable */
        if (!pgmap_pfn_valid(pgmap, pfn)) {
                rc = -ENXIO;
                goto out;
        }

        /*
         * Pages instantiated by device-dax (not filesystem-dax)
         * may be compound pages.
         */
        page = compound_head(page);

        /*
         * Prevent the inode from being freed while we are interrogating
         * the address_space, typically this would be handled by
         * lock_page(), but dax pages do not use the page lock. This
         * also prevents changes to the mapping of this pfn until
         * poison signaling is complete.
         */
        cookie = dax_lock_page(page);
        if (!cookie)
                goto out;

        if (hwpoison_filter(page)) {
                rc = -EOPNOTSUPP;
                goto unlock;
        }

        if (pgmap->type == MEMORY_DEVICE_PRIVATE) {
                /*
                 * TODO: Handle HMM pages which may need coordination
                 * with device-side memory.
                 */
                goto unlock;
        }

        /*
         * Use this flag as an indication that the dax page has been
         * remapped UC to prevent speculative consumption of poison.
         */
        SetPageHWPoison(page);

        /*
         * Unlike System-RAM there is no possibility to swap in a
         * different physical page at a given virtual address, so all
         * userspace consumption of ZONE_DEVICE memory necessitates
         * SIGBUS (i.e. MF_MUST_KILL)
         */
        flags |= MF_ACTION_REQUIRED | MF_MUST_KILL;
        collect_procs(page, &tokill, true);

        list_for_each_entry(tk, &tokill, nd)
                if (tk->size_shift)
                        size = max(size, 1UL << tk->size_shift);
        if (size) {
                /*
                 * Unmap the largest mapping to avoid breaking up
                 * device-dax mappings which are constant size. The
                 * actual size of the mapping being torn down is
                 * communicated in siginfo, see kill_proc()
                 */
                start = (page->index << PAGE_SHIFT) & ~(size - 1);
                unmap_mapping_range(page->mapping, start, size, 0);
        }
        kill_procs(&tokill, true, false, pfn, flags);
        rc = 0;
unlock:
        dax_unlock_page(page, cookie);
out:
        /* drop pgmap ref acquired in caller */
        put_dev_pagemap(pgmap);
        action_result(pfn, MF_MSG_DAX, rc ? MF_FAILED : MF_RECOVERED);
        return rc;
}

static DEFINE_MUTEX(mf_mutex);

/**
 * memory_failure - Handle memory failure of a page.
 * @pfn: Page Number of the corrupted page
 * @flags: fine tune action taken
 *
 * This function is called by the low level machine check code
 * of an architecture when it detects hardware memory corruption
 * of a page. It tries its best to recover, which includes
 * dropping pages, killing processes etc.
 *
 * The function is primarily of use for corruptions that
 * happen outside the current execution context (e.g. when
 * detected by a background scrubber)
 *
 * Must run in process context (e.g. a work queue) with interrupts
 * enabled and no spinlocks hold.
 *
 * Return: 0 for successfully handled the memory error,
 *         -EOPNOTSUPP for memory_filter() filtered the error event,
 *         < 0(except -EOPNOTSUPP) on failure.
 */
int memory_failure(unsigned long pfn, int flags)
{
        struct page *p;
        struct page *hpage;
        struct dev_pagemap *pgmap;
        int res = 0;
        unsigned long page_flags;
        bool retry = true;
        int hugetlb = 0;

        if (!sysctl_memory_failure_recovery)
                panic("Memory failure on page %lx", pfn);

        mutex_lock(&mf_mutex);

        p = pfn_to_online_page(pfn);
        if (!p) {
                res = arch_memory_failure(pfn, flags);
                if (res == 0)
                        goto unlock_mutex;

                if (pfn_valid(pfn)) {
                        pgmap = get_dev_pagemap(pfn, NULL);
                        if (pgmap) {
                                res = memory_failure_dev_pagemap(pfn, flags,
                                                                 pgmap);
                                goto unlock_mutex;
                        }
                }
                pr_err("Memory failure: %#lx: memory outside kernel control\n",
                        pfn);
                res = -ENXIO;
                goto unlock_mutex;
        }

try_again:
        res = try_memory_failure_hugetlb(pfn, flags, &hugetlb);
        if (hugetlb)
                goto unlock_mutex;

        if (TestSetPageHWPoison(p)) {
                pr_err("Memory failure: %#lx: already hardware poisoned\n",
                        pfn);
                res = -EHWPOISON;
                if (flags & MF_ACTION_REQUIRED)
                        res = kill_accessing_process(current, pfn, flags);
                if (flags & MF_COUNT_INCREASED)
                        put_page(p);
                goto unlock_mutex;
        }

        hpage = compound_head(p);
        num_poisoned_pages_inc();

        /*
         * We need/can do nothing about count=0 pages.
         * 1) it's a free page, and therefore in safe hand:
         *    prep_new_page() will be the gate keeper.
         * 2) it's part of a non-compound high order page.
         *    Implies some kernel user: cannot stop them from
         *    R/W the page; let's pray that the page has been
         *    used and will be freed some time later.
         * In fact it's dangerous to directly bump up page count from 0,
         * that may make page_ref_freeze()/page_ref_unfreeze() mismatch.
         */
        if (!(flags & MF_COUNT_INCREASED)) {
                res = get_hwpoison_page(p, flags);
                if (!res) {
                        if (is_free_buddy_page(p)) {
                                if (take_page_off_buddy(p)) {
                                        page_ref_inc(p);
                                        res = MF_RECOVERED;
                                } else {
                                        /* We lost the race, try again */
                                        if (retry) {
                                                ClearPageHWPoison(p);
                                                num_poisoned_pages_dec();
                                                retry = false;
                                                goto try_again;
                                        }
                                        res = MF_FAILED;
                                }
                                action_result(pfn, MF_MSG_BUDDY, res);
                                res = res == MF_RECOVERED ? 0 : -EBUSY;
                        } else {
                                action_result(pfn, MF_MSG_KERNEL_HIGH_ORDER, MF_IGNORED);
                                res = -EBUSY;
                        }
                        goto unlock_mutex;
                } else if (res < 0) {
                        action_result(pfn, MF_MSG_UNKNOWN, MF_IGNORED);
                        res = -EBUSY;
                        goto unlock_mutex;
                }
        }

        if (PageTransHuge(hpage)) {
                /*
                 * Bail out before SetPageHasHWPoisoned() if hpage is
                 * huge_zero_page, although PG_has_hwpoisoned is not
                 * checked in set_huge_zero_page().
                 *
                 * TODO: Handle memory failure of huge_zero_page thoroughly.
                 */
                if (is_huge_zero_page(hpage)) {
                        action_result(pfn, MF_MSG_UNSPLIT_THP, MF_IGNORED);
                        res = -EBUSY;
                        goto unlock_mutex;
                }

                /*
                 * The flag must be set after the refcount is bumped
                 * otherwise it may race with THP split.
                 * And the flag can't be set in get_hwpoison_page() since
                 * it is called by soft offline too and it is just called
                 * for !MF_COUNT_INCREASE.  So here seems to be the best
                 * place.
                 *
                 * Don't need care about the above error handling paths for
                 * get_hwpoison_page() since they handle either free page
                 * or unhandlable page.  The refcount is bumped iff the
                 * page is a valid handlable page.
                 */
                SetPageHasHWPoisoned(hpage);
                if (try_to_split_thp_page(p, "Memory Failure") < 0) {
                        action_result(pfn, MF_MSG_UNSPLIT_THP, MF_IGNORED);
                        res = -EBUSY;
                        goto unlock_mutex;
                }
                VM_BUG_ON_PAGE(!page_count(p), p);
        }

        /*
         * We ignore non-LRU pages for good reasons.
         * - PG_locked is only well defined for LRU pages and a few others
         * - to avoid races with __SetPageLocked()
         * - to avoid races with __SetPageSlab*() (and more non-atomic ops)
         * The check (unnecessarily) ignores LRU pages being isolated and
         * walked by the page reclaim code, however that's not a big loss.
         */
        shake_page(p);

        lock_page(p);

        /*
         * We're only intended to deal with the non-Compound page here.
         * However, the page could have changed compound pages due to
         * race window. If this happens, we could try again to hopefully
         * handle the page next round.
         */
        if (PageCompound(p)) {
                if (retry) {
                        if (TestClearPageHWPoison(p))
                                num_poisoned_pages_dec();
                        unlock_page(p);
                        put_page(p);
                        flags &= ~MF_COUNT_INCREASED;
                        retry = false;
                        goto try_again;
                }
                action_result(pfn, MF_MSG_DIFFERENT_COMPOUND, MF_IGNORED);
                res = -EBUSY;
                goto unlock_page;
        }

        /*
         * We use page flags to determine what action should be taken, but
         * the flags can be modified by the error containment action.  One
         * example is an mlocked page, where PG_mlocked is cleared by
         * page_remove_rmap() in try_to_unmap_one(). So to determine page status
         * correctly, we save a copy of the page flags at this time.
         */
        page_flags = p->flags;

        if (hwpoison_filter(p)) {
                if (TestClearPageHWPoison(p))
                        num_poisoned_pages_dec();
                unlock_page(p);
                put_page(p);
                res = -EOPNOTSUPP;
                goto unlock_mutex;
        }

        /*
         * __munlock_pagevec may clear a writeback page's LRU flag without
         * page_lock. We need wait writeback completion for this page or it
         * may trigger vfs BUG while evict inode.
         */
        if (!PageLRU(p) && !PageWriteback(p))
                goto identify_page_state;

        /*
         * It's very difficult to mess with pages currently under IO
         * and in many cases impossible, so we just avoid it here.
         */
        wait_on_page_writeback(p);

        /*
         * Now take care of user space mappings.
         * Abort on fail: __delete_from_page_cache() assumes unmapped page.
         */
        if (!hwpoison_user_mappings(p, pfn, flags, p)) {
                action_result(pfn, MF_MSG_UNMAP_FAILED, MF_IGNORED);
                res = -EBUSY;
                goto unlock_page;
        }

        /*
         * Torn down by someone else?
         */
        if (PageLRU(p) && !PageSwapCache(p) && p->mapping == NULL) {
                action_result(pfn, MF_MSG_TRUNCATED_LRU, MF_IGNORED);
                res = -EBUSY;
                goto unlock_page;
        }

identify_page_state:
        res = identify_page_state(pfn, p, page_flags);
        mutex_unlock(&mf_mutex);
        return res;
unlock_page:
        unlock_page(p);
unlock_mutex:
        mutex_unlock(&mf_mutex);
        return res;
}
EXPORT_SYMBOL_GPL(memory_failure);

#define MEMORY_FAILURE_FIFO_ORDER       4
#define MEMORY_FAILURE_FIFO_SIZE        (1 << MEMORY_FAILURE_FIFO_ORDER)

struct memory_failure_entry {
        unsigned long pfn;
        int flags;
};

struct memory_failure_cpu {
        DECLARE_KFIFO(fifo, struct memory_failure_entry,
                      MEMORY_FAILURE_FIFO_SIZE);
        spinlock_t lock;
        struct work_struct work;
};

static DEFINE_PER_CPU(struct memory_failure_cpu, memory_failure_cpu);

/**
 * memory_failure_queue - Schedule handling memory failure of a page.
 * @pfn: Page Number of the corrupted page
 * @flags: Flags for memory failure handling
 *
 * This function is called by the low level hardware error handler
 * when it detects hardware memory corruption of a page. It schedules
 * the recovering of error page, including dropping pages, killing
 * processes etc.
 *
 * The function is primarily of use for corruptions that
 * happen outside the current execution context (e.g. when
 * detected by a background scrubber)
 *
 * Can run in IRQ context.
 */
void memory_failure_queue(unsigned long pfn, int flags)
{
        struct memory_failure_cpu *mf_cpu;
        unsigned long proc_flags;
        struct memory_failure_entry entry = {
                .pfn =          pfn,
                .flags =        flags,
        };

        mf_cpu = &get_cpu_var(memory_failure_cpu);
        spin_lock_irqsave(&mf_cpu->lock, proc_flags);
        if (kfifo_put(&mf_cpu->fifo, entry))
                schedule_work_on(smp_processor_id(), &mf_cpu->work);
        else
                pr_err("Memory failure: buffer overflow when queuing memory failure at %#lx\n",
                       pfn);
        spin_unlock_irqrestore(&mf_cpu->lock, proc_flags);
        put_cpu_var(memory_failure_cpu);
}
EXPORT_SYMBOL_GPL(memory_failure_queue);

static void memory_failure_work_func(struct work_struct *work)
{
        struct memory_failure_cpu *mf_cpu;
        struct memory_failure_entry entry = { 0, };
        unsigned long proc_flags;
        int gotten;

        mf_cpu = container_of(work, struct memory_failure_cpu, work);
        for (;;) {
                spin_lock_irqsave(&mf_cpu->lock, proc_flags);
                gotten = kfifo_get(&mf_cpu->fifo, &entry);
                spin_unlock_irqrestore(&mf_cpu->lock, proc_flags);
                if (!gotten)
                        break;
                if (entry.flags & MF_SOFT_OFFLINE)
                        soft_offline_page(entry.pfn, entry.flags);
                else
                        memory_failure(entry.pfn, entry.flags);
        }
}

/*
 * Process memory_failure work queued on the specified CPU.
 * Used to avoid return-to-userspace racing with the memory_failure workqueue.
 */
void memory_failure_queue_kick(int cpu)
{
        struct memory_failure_cpu *mf_cpu;

        mf_cpu = &per_cpu(memory_failure_cpu, cpu);
        cancel_work_sync(&mf_cpu->work);
        memory_failure_work_func(&mf_cpu->work);
}

static int __init memory_failure_init(void)
{
        struct memory_failure_cpu *mf_cpu;
        int cpu;

        for_each_possible_cpu(cpu) {
                mf_cpu = &per_cpu(memory_failure_cpu, cpu);
                spin_lock_init(&mf_cpu->lock);
                INIT_KFIFO(mf_cpu->fifo);
                INIT_WORK(&mf_cpu->work, memory_failure_work_func);
        }

        return 0;
}
core_initcall(memory_failure_init);

#define unpoison_pr_info(fmt, pfn, rs)                  \
({                                                      \
        if (__ratelimit(rs))                            \
                pr_info(fmt, pfn);                      \
})

static inline int clear_page_hwpoison(struct ratelimit_state *rs, struct page *p)
{
        if (TestClearPageHWPoison(p)) {
                unpoison_pr_info("Unpoison: Software-unpoisoned page %#lx\n",
                                 page_to_pfn(p), rs);
                num_poisoned_pages_dec();
                return 1;
        }
        return 0;
}

static inline int unpoison_taken_off_page(struct ratelimit_state *rs,
                                          struct page *p)
{
        if (put_page_back_buddy(p)) {
                unpoison_pr_info("Unpoison: Software-unpoisoned page %#lx\n",
                                 page_to_pfn(p), rs);
                return 0;
        }
        return -EBUSY;
}

/**
 * unpoison_memory - Unpoison a previously poisoned page
 * @pfn: Page number of the to be unpoisoned page
 *
 * Software-unpoison a page that has been poisoned by
 * memory_failure() earlier.
 *
 * This is only done on the software-level, so it only works
 * for linux injected failures, not real hardware failures
 *
 * Returns 0 for success, otherwise -errno.
 */
int unpoison_memory(unsigned long pfn)
{
        struct page *page;
        struct page *p;
        int ret = -EBUSY;
        static DEFINE_RATELIMIT_STATE(unpoison_rs, DEFAULT_RATELIMIT_INTERVAL,
                                        DEFAULT_RATELIMIT_BURST);

        if (!pfn_valid(pfn))
                return -ENXIO;

        p = pfn_to_page(pfn);
        page = compound_head(p);

        mutex_lock(&mf_mutex);

        if (!PageHWPoison(p)) {
                unpoison_pr_info("Unpoison: Page was already unpoisoned %#lx\n",
                                 pfn, &unpoison_rs);
                goto unlock_mutex;
        }

        if (page_count(page) > 1) {
                unpoison_pr_info("Unpoison: Someone grabs the hwpoison page %#lx\n",
                                 pfn, &unpoison_rs);
                goto unlock_mutex;
        }

        if (page_mapped(page)) {
                unpoison_pr_info("Unpoison: Someone maps the hwpoison page %#lx\n",
                                 pfn, &unpoison_rs);
                goto unlock_mutex;
        }

        if (page_mapping(page)) {
                unpoison_pr_info("Unpoison: the hwpoison page has non-NULL mapping %#lx\n",
                                 pfn, &unpoison_rs);
                goto unlock_mutex;
        }

        if (PageSlab(page) || PageTable(page))
                goto unlock_mutex;

        ret = get_hwpoison_page(p, MF_UNPOISON);
        if (!ret) {
                if (clear_page_hwpoison(&unpoison_rs, page))
                        ret = 0;
                else
                        ret = -EBUSY;
        } else if (ret < 0) {
                if (ret == -EHWPOISON) {
                        ret = unpoison_taken_off_page(&unpoison_rs, p);
                } else
                        unpoison_pr_info("Unpoison: failed to grab page %#lx\n",
                                         pfn, &unpoison_rs);
        } else {
                int freeit = clear_page_hwpoison(&unpoison_rs, p);

                put_page(page);
                if (freeit && !(pfn == my_zero_pfn(0) && page_count(p) == 1)) {
                        put_page(page);
                        ret = 0;
                }
        }

unlock_mutex:
        mutex_unlock(&mf_mutex);
        return ret;
}
EXPORT_SYMBOL(unpoison_memory);

static bool isolate_page(struct page *page, struct list_head *pagelist)
{
        bool isolated = false;
        bool lru = PageLRU(page);

        if (PageHuge(page)) {
                isolated = isolate_huge_page(page, pagelist);
        } else {
                if (lru)
                        isolated = !isolate_lru_page(page);
                else
                        isolated = !isolate_movable_page(page, ISOLATE_UNEVICTABLE);

                if (isolated)
                        list_add(&page->lru, pagelist);
        }

        if (isolated && lru)
                inc_node_page_state(page, NR_ISOLATED_ANON +
                                    page_is_file_lru(page));

        /*
         * If we succeed to isolate the page, we grabbed another refcount on
         * the page, so we can safely drop the one we got from get_any_pages().
         * If we failed to isolate the page, it means that we cannot go further
         * and we will return an error, so drop the reference we got from
         * get_any_pages() as well.
         */
        put_page(page);
        return isolated;
}

/*
 * __soft_offline_page handles hugetlb-pages and non-hugetlb pages.
 * If the page is a non-dirty unmapped page-cache page, it simply invalidates.
 * If the page is mapped, it migrates the contents over.
 */
static int __soft_offline_page(struct page *page)
{
        long ret = 0;
        unsigned long pfn = page_to_pfn(page);
        struct page *hpage = compound_head(page);
        char const *msg_page[] = {"page", "hugepage"};
        bool huge = PageHuge(page);
        LIST_HEAD(pagelist);
        struct migration_target_control mtc = {
                .nid = NUMA_NO_NODE,
                .gfp_mask = GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL,
        };

        lock_page(page);
        if (!PageHuge(page))
                wait_on_page_writeback(page);
        if (PageHWPoison(page)) {
                unlock_page(page);
                put_page(page);
                pr_info("soft offline: %#lx page already poisoned\n", pfn);
                return 0;
        }

        if (!PageHuge(page) && PageLRU(page) && !PageSwapCache(page))
                /*
                 * Try to invalidate first. This should work for
                 * non dirty unmapped page cache pages.
                 */
                ret = invalidate_inode_page(page);
        unlock_page(page);

        if (ret) {
                pr_info("soft_offline: %#lx: invalidated\n", pfn);
                page_handle_poison(page, false, true);
                return 0;
        }

        if (isolate_page(hpage, &pagelist)) {
                ret = migrate_pages(&pagelist, alloc_migration_target, NULL,
                        (unsigned long)&mtc, MIGRATE_SYNC, MR_MEMORY_FAILURE, NULL);
                if (!ret) {
                        bool release = !huge;

                        if (!page_handle_poison(page, huge, release))
                                ret = -EBUSY;
                } else {
                        if (!list_empty(&pagelist))
                                putback_movable_pages(&pagelist);

                        pr_info("soft offline: %#lx: %s migration failed %ld, type %pGp\n",
                                pfn, msg_page[huge], ret, &page->flags);
                        if (ret > 0)
                                ret = -EBUSY;
                }
        } else {
                pr_info("soft offline: %#lx: %s isolation failed, page count %d, type %pGp\n",
                        pfn, msg_page[huge], page_count(page), &page->flags);
                ret = -EBUSY;
        }
        return ret;
}

static int soft_offline_in_use_page(struct page *page)
{
        struct page *hpage = compound_head(page);

        if (!PageHuge(page) && PageTransHuge(hpage))
                if (try_to_split_thp_page(page, "soft offline") < 0)
                        return -EBUSY;
        return __soft_offline_page(page);
}

static int soft_offline_free_page(struct page *page)
{
        int rc = 0;

        if (!page_handle_poison(page, true, false))
                rc = -EBUSY;

        return rc;
}

static void put_ref_page(struct page *page)
{
        if (page)
                put_page(page);
}

/**
 * soft_offline_page - Soft offline a page.
 * @pfn: pfn to soft-offline
 * @flags: flags. Same as memory_failure().
 *
 * Returns 0 on success, otherwise negated errno.
 *
 * Soft offline a page, by migration or invalidation,
 * without killing anything. This is for the case when
 * a page is not corrupted yet (so it's still valid to access),
 * but has had a number of corrected errors and is better taken
 * out.
 *
 * The actual policy on when to do that is maintained by
 * user space.
 *
 * This should never impact any application or cause data loss,
 * however it might take some time.
 *
 * This is not a 100% solution for all memory, but tries to be
 * ``good enough'' for the majority of memory.
 */
int soft_offline_page(unsigned long pfn, int flags)
{
        int ret;
        bool try_again = true;
        struct page *page, *ref_page = NULL;

        WARN_ON_ONCE(!pfn_valid(pfn) && (flags & MF_COUNT_INCREASED));

        if (!pfn_valid(pfn))
                return -ENXIO;
        if (flags & MF_COUNT_INCREASED)
                ref_page = pfn_to_page(pfn);

        /* Only online pages can be soft-offlined (esp., not ZONE_DEVICE). */
        page = pfn_to_online_page(pfn);
        if (!page) {
                put_ref_page(ref_page);
                return -EIO;
        }

        mutex_lock(&mf_mutex);

        if (PageHWPoison(page)) {
                pr_info("%s: %#lx page already poisoned\n", __func__, pfn);
                put_ref_page(ref_page);
                mutex_unlock(&mf_mutex);
                return 0;
        }

retry:
        get_online_mems();
        ret = get_hwpoison_page(page, flags | MF_SOFT_OFFLINE);
        put_online_mems();

        if (ret > 0) {
                ret = soft_offline_in_use_page(page);
        } else if (ret == 0) {
                if (soft_offline_free_page(page) && try_again) {
                        try_again = false;
                        flags &= ~MF_COUNT_INCREASED;
                        goto retry;
                }
        }

        mutex_unlock(&mf_mutex);

        return ret;
}