HWPOISON: Add unpoisoning support

[linux-flexiantxendom0-3.2.10.git] / mm / memory-failure.c

/*
 * Copyright (C) 2008, 2009 Intel Corporation
 * Authors: Andi Kleen, Fengguang Wu
 *
 * This software may be redistributed and/or modified under the terms of
 * the GNU General Public License ("GPL") version 2 only as published by the
 * Free Software Foundation.
 *
 * High level machine check handler. Handles pages reported by the
 * hardware as being corrupted usually due to a 2bit ECC memory or cache
 * failure.
 *
 * Handles page cache pages in various states.  The tricky part
 * here is that we can access any page asynchronous to other VM
 * users, because memory failures could happen anytime and anywhere,
 * possibly violating 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.
 *
 * 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
 * mappings. In short it can be quite slow. But since memory corruptions
 * are rare we hope to get away with this.
 */

/*
 * Notebook:
 * - hugetlb needs more code
 * - kcore/oldmem/vmcore/mem/kmem check for hwpoison pages
 * - pass bad pages to kdump next kernel
 */
#define DEBUG 1         /* remove me in 2.6.34 */
#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/page-flags.h>
#include <linux/sched.h>
#include <linux/ksm.h>
#include <linux/rmap.h>
#include <linux/pagemap.h>
#include <linux/swap.h>
#include <linux/backing-dev.h>
#include "internal.h"

int sysctl_memory_failure_early_kill __read_mostly = 0;

int sysctl_memory_failure_recovery __read_mostly = 1;

atomic_long_t mce_bad_pages __read_mostly = ATOMIC_LONG_INIT(0);

/*
 * Send all the processes who have the page mapped an ``action optional''
 * signal.
 */
static int kill_proc_ao(struct task_struct *t, unsigned long addr, int trapno,
                        unsigned long pfn)
{
        struct siginfo si;
        int ret;

        printk(KERN_ERR
                "MCE %#lx: Killing %s:%d early due to hardware memory corruption\n",
                pfn, t->comm, t->pid);
        si.si_signo = SIGBUS;
        si.si_errno = 0;
        si.si_code = BUS_MCEERR_AO;
        si.si_addr = (void *)addr;
#ifdef __ARCH_SI_TRAPNO
        si.si_trapno = trapno;
#endif
        si.si_addr_lsb = PAGE_SHIFT;
        /*
         * 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 noone will do that?
         */
        ret = send_sig_info(SIGBUS, &si, t);  /* synchronous? */
        if (ret < 0)
                printk(KERN_INFO "MCE: Error sending signal to %s:%d: %d\n",
                       t->comm, t->pid, ret);
        return ret;
}

/*
 * When a unknown page type is encountered drain as many buffers as possible
 * in the hope to turn the page into a LRU or free page, which we can handle.
 */
void shake_page(struct page *p)
{
        if (!PageSlab(p)) {
                lru_add_drain_all();
                if (PageLRU(p))
                        return;
                drain_all_pages();
                if (PageLRU(p) || is_free_buddy_page(p))
                        return;
        }
        /*
         * Could call shrink_slab here (which would also
         * shrink other caches). Unfortunately that might
         * also access the corrupted page, which could be fatal.
         */
}
EXPORT_SYMBOL_GPL(shake_page);

/*
 * 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;
        unsigned addr_valid:1;
};

/*
 * 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.
 * TBD would GFP_NOIO be enough?
 */
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 **tkc)
{
        struct to_kill *tk;

        if (*tkc) {
                tk = *tkc;
                *tkc = NULL;
        } else {
                tk = kmalloc(sizeof(struct to_kill), GFP_ATOMIC);
                if (!tk) {
                        printk(KERN_ERR
                "MCE: Out of memory while machine check handling\n");
                        return;
                }
        }
        tk->addr = page_address_in_vma(p, vma);
        tk->addr_valid = 1;

        /*
         * In theory we don't have to kill when the page was
         * munmaped. But it could be also a mremap. Since that's
         * likely very rare kill anyways just out of paranoia, but use
         * a SIGKILL because the error is not contained anymore.
         */
        if (tk->addr == -EFAULT) {
                pr_debug("MCE: Unable to find user space address %lx in %s\n",
                        page_to_pfn(p), tsk->comm);
                tk->addr_valid = 0;
        }
        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 DOIT 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_ao(struct list_head *to_kill, int doit, int trapno,
                          int fail, unsigned long pfn)
{
        struct to_kill *tk, *next;

        list_for_each_entry_safe (tk, next, to_kill, nd) {
                if (doit) {
                        /*
                         * In case something went wrong with munmapping
                         * make sure the process doesn't catch the
                         * signal and then access the memory. Just kill it.
                         * the signal handlers
                         */
                        if (fail || tk->addr_valid == 0) {
                                printk(KERN_ERR
                "MCE %#lx: forcibly killing %s:%d because of failure to unmap corrupted page\n",
                                        pfn, tk->tsk->comm, tk->tsk->pid);
                                force_sig(SIGKILL, tk->tsk);
                        }

                        /*
                         * 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_ao(tk->tsk, tk->addr, trapno,
                                              pfn) < 0)
                                printk(KERN_ERR
                "MCE %#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);
        }
}

static int task_early_kill(struct task_struct *tsk)
{
        if (!tsk->mm)
                return 0;
        if (tsk->flags & PF_MCE_PROCESS)
                return !!(tsk->flags & PF_MCE_EARLY);
        return sysctl_memory_failure_early_kill;
}

/*
 * Collect processes when the error hit an anonymous page.
 */
static void collect_procs_anon(struct page *page, struct list_head *to_kill,
                              struct to_kill **tkc)
{
        struct vm_area_struct *vma;
        struct task_struct *tsk;
        struct anon_vma *av;

        read_lock(&tasklist_lock);
        av = page_lock_anon_vma(page);
        if (av == NULL) /* Not actually mapped anymore */
                goto out;
        for_each_process (tsk) {
                if (!task_early_kill(tsk))
                        continue;
                list_for_each_entry (vma, &av->head, anon_vma_node) {
                        if (!page_mapped_in_vma(page, vma))
                                continue;
                        if (vma->vm_mm == tsk->mm)
                                add_to_kill(tsk, page, vma, to_kill, tkc);
                }
        }
        page_unlock_anon_vma(av);
out:
        read_unlock(&tasklist_lock);
}

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

        /*
         * A note on the locking order between the two locks.
         * We don't rely on this particular order.
         * If you have some other code that needs a different order
         * feel free to switch them around. Or add a reverse link
         * from mm_struct to task_struct, then this could be all
         * done without taking tasklist_lock and looping over all tasks.
         */

        read_lock(&tasklist_lock);
        spin_lock(&mapping->i_mmap_lock);
        for_each_process(tsk) {
                pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);

                if (!task_early_kill(tsk))
                        continue;

                vma_prio_tree_foreach(vma, &iter, &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 == tsk->mm)
                                add_to_kill(tsk, page, vma, to_kill, tkc);
                }
        }
        spin_unlock(&mapping->i_mmap_lock);
        read_unlock(&tasklist_lock);
}

/*
 * Collect the processes who have the corrupted page mapped to kill.
 * This is done in two steps for locking reasons.
 * First preallocate one tokill structure outside the spin locks,
 * so that we can kill at least one process reasonably reliable.
 */
static void collect_procs(struct page *page, struct list_head *tokill)
{
        struct to_kill *tk;

        if (!page->mapping)
                return;

        tk = kmalloc(sizeof(struct to_kill), GFP_NOIO);
        if (!tk)
                return;
        if (PageAnon(page))
                collect_procs_anon(page, tokill, &tk);
        else
                collect_procs_file(page, tokill, &tk);
        kfree(tk);
}

/*
 * Error handlers for various types of pages.
 */

enum outcome {
        FAILED,         /* Error handling failed */
        DELAYED,        /* Will be handled later */
        IGNORED,        /* Error safely ignored */
        RECOVERED,      /* Successfully recovered */
};

static const char *action_name[] = {
        [FAILED] = "Failed",
        [DELAYED] = "Delayed",
        [IGNORED] = "Ignored",
        [RECOVERED] = "Recovered",
};

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

/*
 * 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 *p, unsigned long pfn)
{
        return DELAYED;
}

/*
 * Already poisoned page.
 */
static int me_ignore(struct page *p, unsigned long pfn)
{
        return IGNORED;
}

/*
 * Page in unknown state. Do nothing.
 */
static int me_unknown(struct page *p, unsigned long pfn)
{
        printk(KERN_ERR "MCE %#lx: Unknown page state\n", pfn);
        return FAILED;
}

/*
 * Clean (or cleaned) page cache page.
 */
static int me_pagecache_clean(struct page *p, unsigned long pfn)
{
        int err;
        int ret = FAILED;
        struct address_space *mapping;

        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))
                return RECOVERED;

        /*
         * 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
                 */
                return FAILED;
        }

        /*
         * Truncation is a bit tricky. Enable it per file system for now.
         *
         * Open: to take i_mutex or not for this? Right now we don't.
         */
        if (mapping->a_ops->error_remove_page) {
                err = mapping->a_ops->error_remove_page(mapping, p);
                if (err != 0) {
                        printk(KERN_INFO "MCE %#lx: Failed to punch page: %d\n",
                                        pfn, err);
                } else if (page_has_private(p) &&
                                !try_to_release_page(p, GFP_NOIO)) {
                        pr_debug("MCE %#lx: failed to release buffers\n", pfn);
                } else {
                        ret = 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 = RECOVERED;
                else
                        printk(KERN_INFO "MCE %#lx: Failed to invalidate\n",
                                pfn);
        }
        return ret;
}

/*
 * Dirty cache page page
 * Issues: when the error hit a hole page the error is not properly
 * propagated.
 */
static int me_pagecache_dirty(struct page *p, unsigned long pfn)
{
        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 inbetween 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(p, pfn);
}

/*
 * 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 *p, unsigned long pfn)
{
        ClearPageDirty(p);
        /* Trigger EIO in shmem: */
        ClearPageUptodate(p);

        if (!delete_from_lru_cache(p))
                return DELAYED;
        else
                return FAILED;
}

static int me_swapcache_clean(struct page *p, unsigned long pfn)
{
        delete_from_swap_cache(p);

        if (!delete_from_lru_cache(p))
                return RECOVERED;
        else
                return FAILED;
}

/*
 * Huge pages. Needs work.
 * Issues:
 * No rmap support so we cannot find the original mapper. In theory could walk
 * all MMs and look for the mappings, but that would be non atomic and racy.
 * Need rmap for hugepages for this. Alternatively we could employ a heuristic,
 * like just walking the current process and hoping it has it mapped (that
 * should be usually true for the common "shared database cache" case)
 * Should handle free huge pages and dequeue them too, but this needs to
 * handle huge page accounting correctly.
 */
static int me_huge_page(struct page *p, unsigned long pfn)
{
        return FAILED;
}

/*
 * 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 extremly 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)
#define unevict         (1UL << PG_unevictable)
#define mlock           (1UL << PG_mlocked)
#define writeback       (1UL << PG_writeback)
#define lru             (1UL << PG_lru)
#define swapbacked      (1UL << PG_swapbacked)
#define head            (1UL << PG_head)
#define tail            (1UL << PG_tail)
#define compound        (1UL << PG_compound)
#define slab            (1UL << PG_slab)
#define reserved        (1UL << PG_reserved)

static struct page_state {
        unsigned long mask;
        unsigned long res;
        char *msg;
        int (*action)(struct page *p, unsigned long pfn);
} error_states[] = {
        { reserved,     reserved,       "reserved kernel",      me_ignore },
        /*
         * 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,           "kernel slab",  me_kernel },

#ifdef CONFIG_PAGEFLAGS_EXTENDED
        { head,         head,           "huge",         me_huge_page },
        { tail,         tail,           "huge",         me_huge_page },
#else
        { compound,     compound,       "huge",         me_huge_page },
#endif

        { sc|dirty,     sc|dirty,       "swapcache",    me_swapcache_dirty },
        { sc|dirty,     sc,             "swapcache",    me_swapcache_clean },

        { unevict|dirty, unevict|dirty, "unevictable LRU", me_pagecache_dirty},
        { unevict,      unevict,        "unevictable LRU", me_pagecache_clean},

        { mlock|dirty,  mlock|dirty,    "mlocked LRU",  me_pagecache_dirty },
        { mlock,        mlock,          "mlocked LRU",  me_pagecache_clean },

        { lru|dirty,    lru|dirty,      "LRU",          me_pagecache_dirty },
        { lru|dirty,    lru,            "clean LRU",    me_pagecache_clean },

        /*
         * Catchall entry: must be at end.
         */
        { 0,            0,              "unknown page state",   me_unknown },
};

static void action_result(unsigned long pfn, char *msg, int result)
{
        struct page *page = pfn_to_page(pfn);

        printk(KERN_ERR "MCE %#lx: %s%s page recovery: %s\n",
                pfn,
                PageDirty(page) ? "dirty " : "",
                msg, action_name[result]);
}

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

        result = ps->action(p, pfn);
        action_result(pfn, ps->msg, result);

        count = page_count(p) - 1;
        if (count != 0)
                printk(KERN_ERR
                       "MCE %#lx: %s page still referenced by %d users\n",
                       pfn, ps->msg, count);

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

        return result == RECOVERED ? 0 : -EBUSY;
}

#define N_UNMAP_TRIES 5

/*
 * 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 int hwpoison_user_mappings(struct page *p, unsigned long pfn,
                                  int trapno)
{
        enum ttu_flags ttu = TTU_UNMAP | TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS;
        struct address_space *mapping;
        LIST_HEAD(tokill);
        int ret;
        int i;
        int kill = 1;

        if (PageReserved(p) || PageSlab(p))
                return SWAP_SUCCESS;

        /*
         * 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(p))
                return SWAP_SUCCESS;

        if (PageCompound(p) || PageKsm(p))
                return SWAP_FAIL;

        if (PageSwapCache(p)) {
                printk(KERN_ERR
                       "MCE %#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(p);
        if (!PageDirty(p) && mapping && mapping_cap_writeback_dirty(mapping)) {
                if (page_mkclean(p)) {
                        SetPageDirty(p);
                } else {
                        kill = 0;
                        ttu |= TTU_IGNORE_HWPOISON;
                        printk(KERN_INFO
        "MCE %#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(p, &tokill);

        /*
         * try_to_unmap can fail temporarily due to races.
         * Try a few times (RED-PEN better strategy?)
         */
        for (i = 0; i < N_UNMAP_TRIES; i++) {
                ret = try_to_unmap(p, ttu);
                if (ret == SWAP_SUCCESS)
                        break;
                pr_debug("MCE %#lx: try_to_unmap retry needed %d\n", pfn,  ret);
        }

        if (ret != SWAP_SUCCESS)
                printk(KERN_ERR "MCE %#lx: failed to unmap page (mapcount=%d)\n",
                                pfn, page_mapcount(p));

        /*
         * 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, 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.
         */
        kill_procs_ao(&tokill, !!PageDirty(p), trapno,
                      ret != SWAP_SUCCESS, pfn);

        return ret;
}

int __memory_failure(unsigned long pfn, int trapno, int flags)
{
        struct page_state *ps;
        struct page *p;
        int res;

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

        if (!pfn_valid(pfn)) {
                printk(KERN_ERR
                       "MCE %#lx: memory outside kernel control\n",
                       pfn);
                return -ENXIO;
        }

        p = pfn_to_page(pfn);
        if (TestSetPageHWPoison(p)) {
                action_result(pfn, "already hardware poisoned", IGNORED);
                return 0;
        }

        atomic_long_add(1, &mce_bad_pages);

        /*
         * 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_freeze_refs()/page_unfreeze_refs() mismatch.
         */
        if (!(flags & MF_COUNT_INCREASED) &&
                !get_page_unless_zero(compound_head(p))) {
                if (is_free_buddy_page(p)) {
                        action_result(pfn, "free buddy", DELAYED);
                        return 0;
                } else {
                        action_result(pfn, "high order kernel", IGNORED);
                        return -EBUSY;
                }
        }

        /*
         * 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 __set_page_locked()
         * - 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.
         */
        if (!PageLRU(p))
                lru_add_drain_all();
        if (!PageLRU(p)) {
                action_result(pfn, "non LRU", IGNORED);
                put_page(p);
                return -EBUSY;
        }

        /*
         * Lock the page and wait for writeback to finish.
         * It's very difficult to mess with pages currently under IO
         * and in many cases impossible, so we just avoid it here.
         */
        lock_page_nosync(p);

        /*
         * unpoison always clear PG_hwpoison inside page lock
         */
        if (!PageHWPoison(p)) {
                action_result(pfn, "unpoisoned", IGNORED);
                res = 0;
                goto out;
        }

        wait_on_page_writeback(p);

        /*
         * Now take care of user space mappings.
         * Abort on fail: __remove_from_page_cache() assumes unmapped page.
         */
        if (hwpoison_user_mappings(p, pfn, trapno) != SWAP_SUCCESS) {
                printk(KERN_ERR "MCE %#lx: cannot unmap page, give up\n", pfn);
                res = -EBUSY;
                goto out;
        }

        /*
         * Torn down by someone else?
         */
        if (PageLRU(p) && !PageSwapCache(p) && p->mapping == NULL) {
                action_result(pfn, "already truncated LRU", IGNORED);
                res = 0;
                goto out;
        }

        res = -EBUSY;
        for (ps = error_states;; ps++) {
                if ((p->flags & ps->mask) == ps->res) {
                        res = page_action(ps, p, pfn);
                        break;
                }
        }
out:
        unlock_page(p);
        return res;
}
EXPORT_SYMBOL_GPL(__memory_failure);

/**
 * memory_failure - Handle memory failure of a page.
 * @pfn: Page Number of the corrupted page
 * @trapno: Trap number reported in the signal to user space.
 *
 * 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.
 */
void memory_failure(unsigned long pfn, int trapno)
{
        __memory_failure(pfn, trapno, 0);
}

/**
 * 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 freeit = 0;

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

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

        if (!PageHWPoison(p)) {
                pr_debug("MCE: Page was already unpoisoned %#lx\n", pfn);
                return 0;
        }

        if (!get_page_unless_zero(page)) {
                if (TestClearPageHWPoison(p))
                        atomic_long_dec(&mce_bad_pages);
                pr_debug("MCE: Software-unpoisoned free page %#lx\n", pfn);
                return 0;
        }

        lock_page_nosync(page);
        /*
         * This test is racy because PG_hwpoison is set outside of page lock.
         * That's acceptable because that won't trigger kernel panic. Instead,
         * the PG_hwpoison page will be caught and isolated on the entrance to
         * the free buddy page pool.
         */
        if (TestClearPageHWPoison(p)) {
                pr_debug("MCE: Software-unpoisoned page %#lx\n", pfn);
                atomic_long_dec(&mce_bad_pages);
                freeit = 1;
        }
        unlock_page(page);

        put_page(page);
        if (freeit)
                put_page(page);

        return 0;
}
EXPORT_SYMBOL(unpoison_memory);