swapfile.c

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/*
 *  linux/mm/swapfile.c
 *
 *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
 *  Swap reorganised 29.12.95, Stephen Tweedie
 */

#include <linux/mm.h>
#include <linux/hugetlb.h>
#include <linux/mman.h>
#include <linux/slab.h>
#include <linux/kernel_stat.h>
#include <linux/swap.h>
#include <linux/vmalloc.h>
#include <linux/pagemap.h>
#include <linux/namei.h>
#include <linux/shmem_fs.h>
#include <linux/blkdev.h>
#include <linux/random.h>
#include <linux/writeback.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/init.h>
#include <linux/ksm.h>
#include <linux/rmap.h>
#include <linux/security.h>
#include <linux/backing-dev.h>
#include <linux/mutex.h>
#include <linux/capability.h>
#include <linux/syscalls.h>
#include <linux/memcontrol.h>
#include <linux/poll.h>
#include <linux/oom.h>
#include <linux/frontswap.h>
#include <linux/swapfile.h>
#include <linux/export.h>

#include <asm/pgtable.h>
#include <asm/tlbflush.h>
#include <linux/swapops.h>
#include <linux/page_cgroup.h>

static bool swap_count_continued(struct swap_info_struct *, pgoff_t,
                 unsigned char);
static void free_swap_count_continuations(struct swap_info_struct *);
static sector_t map_swap_entry(swp_entry_t, struct block_device**);

DEFINE_SPINLOCK(swap_lock);
static unsigned int nr_swapfiles;
long nr_swap_pages;
long total_swap_pages;
static int least_priority;

static const char Bad_file[] = "Bad swap file entry ";
static const char Unused_file[] = "Unused swap file entry ";
static const char Bad_offset[] = "Bad swap offset entry ";
static const char Unused_offset[] = "Unused swap offset entry ";

struct swap_list_t swap_list = {-1, -1};

struct swap_info_struct *swap_info[MAX_SWAPFILES];

static DEFINE_MUTEX(swapon_mutex);

static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
/* Activity counter to indicate that a swapon or swapoff has occurred */
static atomic_t proc_poll_event = ATOMIC_INIT(0);

static inline unsigned char swap_count(unsigned char ent)
{
    return ent & ~SWAP_HAS_CACHE;   /* may include SWAP_HAS_CONT flag */
}

/* returns 1 if swap entry is freed */
static int
__try_to_reclaim_swap(struct swap_info_struct *si, unsigned long offset)
{
    swp_entry_t entry = swp_entry(si->type, offset);
    struct page *page;
    int ret = 0;

    page = find_get_page(&swapper_space, entry.val);
    if (!page)
        return 0;
    /*
     * This function is called from scan_swap_map() and it's called
     * by vmscan.c at reclaiming pages. So, we hold a lock on a page, here.
     * We have to use trylock for avoiding deadlock. This is a special
     * case and you should use try_to_free_swap() with explicit lock_page()
     * in usual operations.
     */
    if (trylock_page(page)) {
        ret = try_to_free_swap(page);
        unlock_page(page);
    }
    page_cache_release(page);
    return ret;
}

/*
 * swapon tell device that all the old swap contents can be discarded,
 * to allow the swap device to optimize its wear-levelling.
 */
static int discard_swap(struct swap_info_struct *si)
{
    struct swap_extent *se;
    sector_t start_block;
    sector_t nr_blocks;
    int err = 0;

    /* Do not discard the swap header page! */
    se = &si->first_swap_extent;
    start_block = (se->start_block + 1) << (PAGE_SHIFT - 9);
    nr_blocks = ((sector_t)se->nr_pages - 1) << (PAGE_SHIFT - 9);
    if (nr_blocks) {
        err = blkdev_issue_discard(si->bdev, start_block,
                nr_blocks, GFP_KERNEL, 0);
        if (err)
            return err;
        cond_resched();
    }

    list_for_each_entry(se, &si->first_swap_extent.list, list) {
        start_block = se->start_block << (PAGE_SHIFT - 9);
        nr_blocks = (sector_t)se->nr_pages << (PAGE_SHIFT - 9);

        err = blkdev_issue_discard(si->bdev, start_block,
                nr_blocks, GFP_KERNEL, 0);
        if (err)
            break;

        cond_resched();
    }
    return err;     /* That will often be -EOPNOTSUPP */
}

/*
 * swap allocation tell device that a cluster of swap can now be discarded,
 * to allow the swap device to optimize its wear-levelling.
 */
static void discard_swap_cluster(struct swap_info_struct *si,
                 pgoff_t start_page, pgoff_t nr_pages)
{
    struct swap_extent *se = si->curr_swap_extent;
    int found_extent = 0;

    while (nr_pages) {
        struct list_head *lh;

        if (se->start_page <= start_page &&
            start_page < se->start_page + se->nr_pages) {
            pgoff_t offset = start_page - se->start_page;
            sector_t start_block = se->start_block + offset;
            sector_t nr_blocks = se->nr_pages - offset;

            if (nr_blocks > nr_pages)
                nr_blocks = nr_pages;
            start_page += nr_blocks;
            nr_pages -= nr_blocks;

            if (!found_extent++)
                si->curr_swap_extent = se;

            start_block <<= PAGE_SHIFT - 9;
            nr_blocks <<= PAGE_SHIFT - 9;
            if (blkdev_issue_discard(si->bdev, start_block,
                    nr_blocks, GFP_NOIO, 0))
                break;
        }

        lh = se->list.next;
        se = list_entry(lh, struct swap_extent, list);
    }
}

static int wait_for_discard(void *word)
{
    schedule();
    return 0;
}

#define SWAPFILE_CLUSTER    256
#define LATENCY_LIMIT       256

static unsigned long scan_swap_map(struct swap_info_struct *si,
                   unsigned char usage)
{
    unsigned long offset;
    unsigned long scan_base;
    unsigned long last_in_cluster = 0;
    int latency_ration = LATENCY_LIMIT;
    int found_free_cluster = 0;

    /*
     * We try to cluster swap pages by allocating them sequentially
     * in swap.  Once we've allocated SWAPFILE_CLUSTER pages this
     * way, however, we resort to first-free allocation, starting
     * a new cluster.  This prevents us from scattering swap pages
     * all over the entire swap partition, so that we reduce
     * overall disk seek times between swap pages.  -- sct
     * But we do now try to find an empty cluster.  -Andrea
     * And we let swap pages go all over an SSD partition.  Hugh
     */

    si->flags += SWP_SCANNING;
    scan_base = offset = si->cluster_next;

    if (unlikely(!si->cluster_nr--)) {
        if (si->pages - si->inuse_pages < SWAPFILE_CLUSTER) {
            si->cluster_nr = SWAPFILE_CLUSTER - 1;
            goto checks;
        }
        if (si->flags & SWP_DISCARDABLE) {
            /*
             * Start range check on racing allocations, in case
             * they overlap the cluster we eventually decide on
             * (we scan without swap_lock to allow preemption).
             * It's hardly conceivable that cluster_nr could be
             * wrapped during our scan, but don't depend on it.
             */
            if (si->lowest_alloc)
                goto checks;
            si->lowest_alloc = si->max;
            si->highest_alloc = 0;
        }
        spin_unlock(&swap_lock);

        /*
         * If seek is expensive, start searching for new cluster from
         * start of partition, to minimize the span of allocated swap.
         * But if seek is cheap, search from our current position, so
         * that swap is allocated from all over the partition: if the
         * Flash Translation Layer only remaps within limited zones,
         * we don't want to wear out the first zone too quickly.
         */
        if (!(si->flags & SWP_SOLIDSTATE))
            scan_base = offset = si->lowest_bit;
        last_in_cluster = offset + SWAPFILE_CLUSTER - 1;

        /* Locate the first empty (unaligned) cluster */
        for (; last_in_cluster <= si->highest_bit; offset++) {
            if (si->swap_map[offset])
                last_in_cluster = offset + SWAPFILE_CLUSTER;
            else if (offset == last_in_cluster) {
                spin_lock(&swap_lock);
                offset -= SWAPFILE_CLUSTER - 1;
                si->cluster_next = offset;
                si->cluster_nr = SWAPFILE_CLUSTER - 1;
                found_free_cluster = 1;
                goto checks;
            }
            if (unlikely(--latency_ration < 0)) {
                cond_resched();
                latency_ration = LATENCY_LIMIT;
            }
        }

        offset = si->lowest_bit;
        last_in_cluster = offset + SWAPFILE_CLUSTER - 1;

        /* Locate the first empty (unaligned) cluster */
        for (; last_in_cluster < scan_base; offset++) {
            if (si->swap_map[offset])
                last_in_cluster = offset + SWAPFILE_CLUSTER;
            else if (offset == last_in_cluster) {
                spin_lock(&swap_lock);
                offset -= SWAPFILE_CLUSTER - 1;
                si->cluster_next = offset;
                si->cluster_nr = SWAPFILE_CLUSTER - 1;
                found_free_cluster = 1;
                goto checks;
            }
            if (unlikely(--latency_ration < 0)) {
                cond_resched();
                latency_ration = LATENCY_LIMIT;
            }
        }

        offset = scan_base;
        spin_lock(&swap_lock);
        si->cluster_nr = SWAPFILE_CLUSTER - 1;
        si->lowest_alloc = 0;
    }

checks:
    if (!(si->flags & SWP_WRITEOK))
        goto no_page;
    if (!si->highest_bit)
        goto no_page;
    if (offset > si->highest_bit)
        scan_base = offset = si->lowest_bit;

    /* reuse swap entry of cache-only swap if not busy. */
    if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
        int swap_was_freed;
        spin_unlock(&swap_lock);
        swap_was_freed = __try_to_reclaim_swap(si, offset);
        spin_lock(&swap_lock);
        /* entry was freed successfully, try to use this again */
        if (swap_was_freed)
            goto checks;
        goto scan; /* check next one */
    }

    if (si->swap_map[offset])
        goto scan;

    if (offset == si->lowest_bit)
        si->lowest_bit++;
    if (offset == si->highest_bit)
        si->highest_bit--;
    si->inuse_pages++;
    if (si->inuse_pages == si->pages) {
        si->lowest_bit = si->max;
        si->highest_bit = 0;
    }
    si->swap_map[offset] = usage;
    si->cluster_next = offset + 1;
    si->flags -= SWP_SCANNING;

    if (si->lowest_alloc) {
        /*
         * Only set when SWP_DISCARDABLE, and there's a scan
         * for a free cluster in progress or just completed.
         */
        if (found_free_cluster) {
            /*
             * To optimize wear-levelling, discard the
             * old data of the cluster, taking care not to
             * discard any of its pages that have already
             * been allocated by racing tasks (offset has
             * already stepped over any at the beginning).
             */
            if (offset < si->highest_alloc &&
                si->lowest_alloc <= last_in_cluster)
                last_in_cluster = si->lowest_alloc - 1;
            si->flags |= SWP_DISCARDING;
            spin_unlock(&swap_lock);

            if (offset < last_in_cluster)
                discard_swap_cluster(si, offset,
                    last_in_cluster - offset + 1);

            spin_lock(&swap_lock);
            si->lowest_alloc = 0;
            si->flags &= ~SWP_DISCARDING;

            smp_mb();   /* wake_up_bit advises this */
            wake_up_bit(&si->flags, ilog2(SWP_DISCARDING));

        } else if (si->flags & SWP_DISCARDING) {
            /*
             * Delay using pages allocated by racing tasks
             * until the whole discard has been issued. We
             * could defer that delay until swap_writepage,
             * but it's easier to keep this self-contained.
             */
            spin_unlock(&swap_lock);
            wait_on_bit(&si->flags, ilog2(SWP_DISCARDING),
                wait_for_discard, TASK_UNINTERRUPTIBLE);
            spin_lock(&swap_lock);
        } else {
            /*
             * Note pages allocated by racing tasks while
             * scan for a free cluster is in progress, so
             * that its final discard can exclude them.
             */
            if (offset < si->lowest_alloc)
                si->lowest_alloc = offset;
            if (offset > si->highest_alloc)
                si->highest_alloc = offset;
        }
    }
    return offset;

scan:
    spin_unlock(&swap_lock);
    while (++offset <= si->highest_bit) {
        if (!si->swap_map[offset]) {
            spin_lock(&swap_lock);
            goto checks;
        }
        if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
            spin_lock(&swap_lock);
            goto checks;
        }
        if (unlikely(--latency_ration < 0)) {
            cond_resched();
            latency_ration = LATENCY_LIMIT;
        }
    }
    offset = si->lowest_bit;
    while (++offset < scan_base) {
        if (!si->swap_map[offset]) {
            spin_lock(&swap_lock);
            goto checks;
        }
        if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
            spin_lock(&swap_lock);
            goto checks;
        }
        if (unlikely(--latency_ration < 0)) {
            cond_resched();
            latency_ration = LATENCY_LIMIT;
        }
    }
    spin_lock(&swap_lock);

no_page:
    si->flags -= SWP_SCANNING;
    return 0;
}

swp_entry_t get_swap_page(void)
{
    struct swap_info_struct *si;
    pgoff_t offset;
    int type, next;
    int wrapped = 0;

    spin_lock(&swap_lock);
    if (nr_swap_pages <= 0)
        goto noswap;
    nr_swap_pages--;

    for (type = swap_list.next; type >= 0 && wrapped < 2; type = next) {
        si = swap_info[type];
        next = si->next;
        if (next < 0 ||
            (!wrapped && si->prio != swap_info[next]->prio)) {
            next = swap_list.head;
            wrapped++;
        }

        if (!si->highest_bit)
            continue;
        if (!(si->flags & SWP_WRITEOK))
            continue;

        swap_list.next = next;
        /* This is called for allocating swap entry for cache */
        offset = scan_swap_map(si, SWAP_HAS_CACHE);
        if (offset) {
            spin_unlock(&swap_lock);
            return swp_entry(type, offset);
        }
        next = swap_list.next;
    }

    nr_swap_pages++;
noswap:
    spin_unlock(&swap_lock);
    return (swp_entry_t) {0};
}

/* The only caller of this function is now susupend routine */
swp_entry_t get_swap_page_of_type(int type)
{
    struct swap_info_struct *si;
    pgoff_t offset;

    spin_lock(&swap_lock);
    si = swap_info[type];
    if (si && (si->flags & SWP_WRITEOK)) {
        nr_swap_pages--;
        /* This is called for allocating swap entry, not cache */
        offset = scan_swap_map(si, 1);
        if (offset) {
            spin_unlock(&swap_lock);
            return swp_entry(type, offset);
        }
        nr_swap_pages++;
    }
    spin_unlock(&swap_lock);
    return (swp_entry_t) {0};
}

static struct swap_info_struct *swap_info_get(swp_entry_t entry)
{
    struct swap_info_struct *p;
    unsigned long offset, type;

    if (!entry.val)
        goto out;
    type = swp_type(entry);
    if (type >= nr_swapfiles)
        goto bad_nofile;
    p = swap_info[type];
    if (!(p->flags & SWP_USED))
        goto bad_device;
    offset = swp_offset(entry);
    if (offset >= p->max)
        goto bad_offset;
    if (!p->swap_map[offset])
        goto bad_free;
    spin_lock(&swap_lock);
    return p;

bad_free:
    printk(KERN_ERR "swap_free: %s%08lx\n", Unused_offset, entry.val);
    goto out;
bad_offset:
    printk(KERN_ERR "swap_free: %s%08lx\n", Bad_offset, entry.val);
    goto out;
bad_device:
    printk(KERN_ERR "swap_free: %s%08lx\n", Unused_file, entry.val);
    goto out;
bad_nofile:
    printk(KERN_ERR "swap_free: %s%08lx\n", Bad_file, entry.val);
out:
    return NULL;
}

static unsigned char swap_entry_free(struct swap_info_struct *p,
                     swp_entry_t entry, unsigned char usage)
{
    unsigned long offset = swp_offset(entry);
    unsigned char count;
    unsigned char has_cache;

    count = p->swap_map[offset];
    has_cache = count & SWAP_HAS_CACHE;
    count &= ~SWAP_HAS_CACHE;

    if (usage == SWAP_HAS_CACHE) {
        VM_BUG_ON(!has_cache);
        has_cache = 0;
    } else if (count == SWAP_MAP_SHMEM) {
        /*
         * Or we could insist on shmem.c using a special
         * swap_shmem_free() and free_shmem_swap_and_cache()...
         */
        count = 0;
    } else if ((count & ~COUNT_CONTINUED) <= SWAP_MAP_MAX) {
        if (count == COUNT_CONTINUED) {
            if (swap_count_continued(p, offset, count))
                count = SWAP_MAP_MAX | COUNT_CONTINUED;
            else
                count = SWAP_MAP_MAX;
        } else
            count--;
    }

    if (!count)
        mem_cgroup_uncharge_swap(entry);

    usage = count | has_cache;
    p->swap_map[offset] = usage;

    /* free if no reference */
    if (!usage) {
        if (offset < p->lowest_bit)
            p->lowest_bit = offset;
        if (offset > p->highest_bit)
            p->highest_bit = offset;
        if (swap_list.next >= 0 &&
            p->prio > swap_info[swap_list.next]->prio)
            swap_list.next = p->type;
        nr_swap_pages++;
        p->inuse_pages--;
        frontswap_invalidate_page(p->type, offset);
        if (p->flags & SWP_BLKDEV) {
            struct gendisk *disk = p->bdev->bd_disk;
            if (disk->fops->swap_slot_free_notify)
                disk->fops->swap_slot_free_notify(p->bdev,
                                  offset);
        }
    }

    return usage;
}

/*
 * Caller has made sure that the swapdevice corresponding to entry
 * is still around or has not been recycled.
 */
void swap_free(swp_entry_t entry)
{
    struct swap_info_struct *p;

    p = swap_info_get(entry);
    if (p) {
        swap_entry_free(p, entry, 1);
        spin_unlock(&swap_lock);
    }
}

/*
 * Called after dropping swapcache to decrease refcnt to swap entries.
 */
void swapcache_free(swp_entry_t entry, struct page *page)
{
    struct swap_info_struct *p;
    unsigned char count;

    p = swap_info_get(entry);
    if (p) {
        count = swap_entry_free(p, entry, SWAP_HAS_CACHE);
        if (page)
            mem_cgroup_uncharge_swapcache(page, entry, count != 0);
        spin_unlock(&swap_lock);
    }
}

/*
 * How many references to page are currently swapped out?
 * This does not give an exact answer when swap count is continued,
 * but does include the high COUNT_CONTINUED flag to allow for that.
 */
int page_swapcount(struct page *page)
{
    int count = 0;
    struct swap_info_struct *p;
    swp_entry_t entry;

    entry.val = page_private(page);
    p = swap_info_get(entry);
    if (p) {
        count = swap_count(p->swap_map[swp_offset(entry)]);
        spin_unlock(&swap_lock);
    }
    return count;
}

/*
 * We can write to an anon page without COW if there are no other references
 * to it.  And as a side-effect, free up its swap: because the old content
 * on disk will never be read, and seeking back there to write new content
 * later would only waste time away from clustering.
 */
int reuse_swap_page(struct page *page)
{
    int count;

    VM_BUG_ON(!PageLocked(page));
    if (unlikely(PageKsm(page)))
        return 0;
    count = page_mapcount(page);
    if (count <= 1 && PageSwapCache(page)) {
        count += page_swapcount(page);
        if (count == 1 && !PageWriteback(page)) {
            delete_from_swap_cache(page);
            SetPageDirty(page);
        }
    }
    return count <= 1;
}

/*
 * If swap is getting full, or if there are no more mappings of this page,
 * then try_to_free_swap is called to free its swap space.
 */
int try_to_free_swap(struct page *page)
{
    VM_BUG_ON(!PageLocked(page));

    if (!PageSwapCache(page))
        return 0;
    if (PageWriteback(page))
        return 0;
    if (page_swapcount(page))
        return 0;

    /*
     * Once hibernation has begun to create its image of memory,
     * there's a danger that one of the calls to try_to_free_swap()
     * - most probably a call from __try_to_reclaim_swap() while
     * hibernation is allocating its own swap pages for the image,
     * but conceivably even a call from memory reclaim - will free
     * the swap from a page which has already been recorded in the
     * image as a clean swapcache page, and then reuse its swap for
     * another page of the image.  On waking from hibernation, the
     * original page might be freed under memory pressure, then
     * later read back in from swap, now with the wrong data.
     *
     * Hibration suspends storage while it is writing the image
     * to disk so check that here.
     */
    if (pm_suspended_storage())
        return 0;

    delete_from_swap_cache(page);
    SetPageDirty(page);
    return 1;
}

/*
 * Free the swap entry like above, but also try to
 * free the page cache entry if it is the last user.
 */
int free_swap_and_cache(swp_entry_t entry)
{
    struct swap_info_struct *p;
    struct page *page = NULL;

    if (non_swap_entry(entry))
        return 1;

    p = swap_info_get(entry);
    if (p) {
        if (swap_entry_free(p, entry, 1) == SWAP_HAS_CACHE) {
            page = find_get_page(&swapper_space, entry.val);
            if (page && !trylock_page(page)) {
                page_cache_release(page);
                page = NULL;
            }
        }
        spin_unlock(&swap_lock);
    }
    if (page) {
        /*
         * Not mapped elsewhere, or swap space full? Free it!
         * Also recheck PageSwapCache now page is locked (above).
         */
        if (PageSwapCache(page) && !PageWriteback(page) &&
                (!page_mapped(page) || vm_swap_full())) {
            delete_from_swap_cache(page);
            SetPageDirty(page);
        }
        unlock_page(page);
        page_cache_release(page);
    }
    return p != NULL;
}

#ifdef CONFIG_HIBERNATION
/*
 * Find the swap type that corresponds to given device (if any).
 *
 * @offset - number of the PAGE_SIZE-sized block of the device, starting
 * from 0, in which the swap header is expected to be located.
 *
 * This is needed for the suspend to disk (aka swsusp).
 */
int swap_type_of(dev_t device, sector_t offset, struct block_device **bdev_p)
{
    struct block_device *bdev = NULL;
    int type;

    if (device)
        bdev = bdget(device);

    spin_lock(&swap_lock);
    for (type = 0; type < nr_swapfiles; type++) {
        struct swap_info_struct *sis = swap_info[type];

        if (!(sis->flags & SWP_WRITEOK))
            continue;

        if (!bdev) {
            if (bdev_p)
                *bdev_p = bdgrab(sis->bdev);

            spin_unlock(&swap_lock);
            return type;
        }
        if (bdev == sis->bdev) {
            struct swap_extent *se = &sis->first_swap_extent;

            if (se->start_block == offset) {
                if (bdev_p)
                    *bdev_p = bdgrab(sis->bdev);

                spin_unlock(&swap_lock);
                bdput(bdev);
                return type;
            }
        }
    }
    spin_unlock(&swap_lock);
    if (bdev)
        bdput(bdev);

    return -ENODEV;
}

/*
 * Get the (PAGE_SIZE) block corresponding to given offset on the swapdev
 * corresponding to given index in swap_info (swap type).
 */
sector_t swapdev_block(int type, pgoff_t offset)
{
    struct block_device *bdev;

    if ((unsigned int)type >= nr_swapfiles)
        return 0;
    if (!(swap_info[type]->flags & SWP_WRITEOK))
        return 0;
    return map_swap_entry(swp_entry(type, offset), &bdev);
}

/*
 * Return either the total number of swap pages of given type, or the number
 * of free pages of that type (depending on @free)
 *
 * This is needed for software suspend
 */
unsigned int count_swap_pages(int type, int free)
{
    unsigned int n = 0;

    spin_lock(&swap_lock);
    if ((unsigned int)type < nr_swapfiles) {
        struct swap_info_struct *sis = swap_info[type];

        if (sis->flags & SWP_WRITEOK) {
            n = sis->pages;
            if (free)
                n -= sis->inuse_pages;
        }
    }
    spin_unlock(&swap_lock);
    return n;
}
#endif /* CONFIG_HIBERNATION */

/*
 * No need to decide whether this PTE shares the swap entry with others,
 * just let do_wp_page work it out if a write is requested later - to
 * force COW, vm_page_prot omits write permission from any private vma.
 */
static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
        unsigned long addr, swp_entry_t entry, struct page *page)
{
    struct mem_cgroup *memcg;
    spinlock_t *ptl;
    pte_t *pte;
    int ret = 1;

    if (mem_cgroup_try_charge_swapin(vma->vm_mm, page,
                     GFP_KERNEL, &memcg)) {
        ret = -ENOMEM;
        goto out_nolock;
    }

    pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
    if (unlikely(!pte_same(*pte, swp_entry_to_pte(entry)))) {
        mem_cgroup_cancel_charge_swapin(memcg);
        ret = 0;
        goto out;
    }

    dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
    inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
    get_page(page);
    set_pte_at(vma->vm_mm, addr, pte,
           pte_mkold(mk_pte(page, vma->vm_page_prot)));
    page_add_anon_rmap(page, vma, addr);
    mem_cgroup_commit_charge_swapin(page, memcg);
    swap_free(entry);
    /*
     * Move the page to the active list so it is not
     * immediately swapped out again after swapon.
     */
    activate_page(page);
out:
    pte_unmap_unlock(pte, ptl);
out_nolock:
    return ret;
}

static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
                unsigned long addr, unsigned long end,
                swp_entry_t entry, struct page *page)
{
    pte_t swp_pte = swp_entry_to_pte(entry);
    pte_t *pte;
    int ret = 0;

    /*
     * We don't actually need pte lock while scanning for swp_pte: since
     * we hold page lock and mmap_sem, swp_pte cannot be inserted into the
     * page table while we're scanning; though it could get zapped, and on
     * some architectures (e.g. x86_32 with PAE) we might catch a glimpse
     * of unmatched parts which look like swp_pte, so unuse_pte must
     * recheck under pte lock.  Scanning without pte lock lets it be
     * preemptible whenever CONFIG_PREEMPT but not CONFIG_HIGHPTE.
     */
    pte = pte_offset_map(pmd, addr);
    do {
        /*
         * swapoff spends a _lot_ of time in this loop!
         * Test inline before going to call unuse_pte.
         */
        if (unlikely(pte_same(*pte, swp_pte))) {
            pte_unmap(pte);
            ret = unuse_pte(vma, pmd, addr, entry, page);
            if (ret)
                goto out;
            pte = pte_offset_map(pmd, addr);
        }
    } while (pte++, addr += PAGE_SIZE, addr != end);
    pte_unmap(pte - 1);
out:
    return ret;
}

static inline int unuse_pmd_range(struct vm_area_struct *vma, pud_t *pud,
                unsigned long addr, unsigned long end,
                swp_entry_t entry, struct page *page)
{
    pmd_t *pmd;
    unsigned long next;
    int ret;

    pmd = pmd_offset(pud, addr);
    do {
        next = pmd_addr_end(addr, end);
        if (pmd_none_or_trans_huge_or_clear_bad(pmd))
            continue;
        ret = unuse_pte_range(vma, pmd, addr, next, entry, page);
        if (ret)
            return ret;
    } while (pmd++, addr = next, addr != end);
    return 0;
}

static inline int unuse_pud_range(struct vm_area_struct *vma, pgd_t *pgd,
                unsigned long addr, unsigned long end,
                swp_entry_t entry, struct page *page)
{
    pud_t *pud;
    unsigned long next;
    int ret;

    pud = pud_offset(pgd, addr);
    do {
        next = pud_addr_end(addr, end);
        if (pud_none_or_clear_bad(pud))
            continue;
        ret = unuse_pmd_range(vma, pud, addr, next, entry, page);
        if (ret)
            return ret;
    } while (pud++, addr = next, addr != end);
    return 0;
}

static int unuse_vma(struct vm_area_struct *vma,
                swp_entry_t entry, struct page *page)
{
    pgd_t *pgd;
    unsigned long addr, end, next;
    int ret;

    if (page_anon_vma(page)) {
        addr = page_address_in_vma(page, vma);
        if (addr == -EFAULT)
            return 0;
        else
            end = addr + PAGE_SIZE;
    } else {
        addr = vma->vm_start;
        end = vma->vm_end;
    }

    pgd = pgd_offset(vma->vm_mm, addr);
    do {
        next = pgd_addr_end(addr, end);
        if (pgd_none_or_clear_bad(pgd))
            continue;
        ret = unuse_pud_range(vma, pgd, addr, next, entry, page);
        if (ret)
            return ret;
    } while (pgd++, addr = next, addr != end);
    return 0;
}

static int unuse_mm(struct mm_struct *mm,
                swp_entry_t entry, struct page *page)
{
    struct vm_area_struct *vma;
    int ret = 0;

    if (!down_read_trylock(&mm->mmap_sem)) {
        /*
         * Activate page so shrink_inactive_list is unlikely to unmap
         * its ptes while lock is dropped, so swapoff can make progress.
         */
        activate_page(page);
        unlock_page(page);
        down_read(&mm->mmap_sem);
        lock_page(page);
    }
    for (vma = mm->mmap; vma; vma = vma->vm_next) {
        if (vma->anon_vma && (ret = unuse_vma(vma, entry, page)))
            break;
    }
    up_read(&mm->mmap_sem);
    return (ret < 0)? ret: 0;
}

/*
 * Scan swap_map (or frontswap_map if frontswap parameter is true)
 * from current position to next entry still in use.
 * Recycle to start on reaching the end, returning 0 when empty.
 */
static unsigned int find_next_to_unuse(struct swap_info_struct *si,
                    unsigned int prev, bool frontswap)
{
    unsigned int max = si->max;
    unsigned int i = prev;
    unsigned char count;

    /*
     * No need for swap_lock here: we're just looking
     * for whether an entry is in use, not modifying it; false
     * hits are okay, and sys_swapoff() has already prevented new
     * allocations from this area (while holding swap_lock).
     */
    for (;;) {
        if (++i >= max) {
            if (!prev) {
                i = 0;
                break;
            }
            /*
             * No entries in use at top of swap_map,
             * loop back to start and recheck there.
             */
            max = prev + 1;
            prev = 0;
            i = 1;
        }
        if (frontswap) {
            if (frontswap_test(si, i))
                break;
            else
                continue;
        }
        count = si->swap_map[i];
        if (count && swap_count(count) != SWAP_MAP_BAD)
            break;
    }
    return i;
}

/*
 * We completely avoid races by reading each swap page in advance,
 * and then search for the process using it.  All the necessary
 * page table adjustments can then be made atomically.
 *
 * if the boolean frontswap is true, only unuse pages_to_unuse pages;
 * pages_to_unuse==0 means all pages; ignored if frontswap is false
 */
int try_to_unuse(unsigned int type, bool frontswap,
         unsigned long pages_to_unuse)
{
    struct swap_info_struct *si = swap_info[type];
    struct mm_struct *start_mm;
    unsigned char *swap_map;
    unsigned char swcount;
    struct page *page;
    swp_entry_t entry;
    unsigned int i = 0;
    int retval = 0;

    /*
     * When searching mms for an entry, a good strategy is to
     * start at the first mm we freed the previous entry from
     * (though actually we don't notice whether we or coincidence
     * freed the entry).  Initialize this start_mm with a hold.
     *
     * A simpler strategy would be to start at the last mm we
     * freed the previous entry from; but that would take less
     * advantage of mmlist ordering, which clusters forked mms
     * together, child after parent.  If we race with dup_mmap(), we
     * prefer to resolve parent before child, lest we miss entries
     * duplicated after we scanned child: using last mm would invert
     * that.
     */
    start_mm = &init_mm;
    atomic_inc(&init_mm.mm_users);

    /*
     * Keep on scanning until all entries have gone.  Usually,
     * one pass through swap_map is enough, but not necessarily:
     * there are races when an instance of an entry might be missed.
     */
    while ((i = find_next_to_unuse(si, i, frontswap)) != 0) {
        if (signal_pending(current)) {
            retval = -EINTR;
            break;
        }

        /*
         * Get a page for the entry, using the existing swap
         * cache page if there is one.  Otherwise, get a clean
         * page and read the swap into it.
         */
        swap_map = &si->swap_map[i];
        entry = swp_entry(type, i);
        page = read_swap_cache_async(entry,
                    GFP_HIGHUSER_MOVABLE, NULL, 0);
        if (!page) {
            /*
             * Either swap_duplicate() failed because entry
             * has been freed independently, and will not be
             * reused since sys_swapoff() already disabled
             * allocation from here, or alloc_page() failed.
             */
            if (!*swap_map)
                continue;
            retval = -ENOMEM;
            break;
        }

        /*
         * Don't hold on to start_mm if it looks like exiting.
         */
        if (atomic_read(&start_mm->mm_users) == 1) {
            mmput(start_mm);
            start_mm = &init_mm;
            atomic_inc(&init_mm.mm_users);
        }

        /*
         * Wait for and lock page.  When do_swap_page races with
         * try_to_unuse, do_swap_page can handle the fault much
         * faster than try_to_unuse can locate the entry.  This
         * apparently redundant "wait_on_page_locked" lets try_to_unuse
         * defer to do_swap_page in such a case - in some tests,
         * do_swap_page and try_to_unuse repeatedly compete.
         */
        wait_on_page_locked(page);
        wait_on_page_writeback(page);
        lock_page(page);
        wait_on_page_writeback(page);

        /*
         * Remove all references to entry.
         */
        swcount = *swap_map;
        if (swap_count(swcount) == SWAP_MAP_SHMEM) {
            retval = shmem_unuse(entry, page);
            /* page has already been unlocked and released */
            if (retval < 0)
                break;
            continue;
        }
        if (swap_count(swcount) && start_mm != &init_mm)
            retval = unuse_mm(start_mm, entry, page);

        if (swap_count(*swap_map)) {
            int set_start_mm = (*swap_map >= swcount);
            struct list_head *p = &start_mm->mmlist;
            struct mm_struct *new_start_mm = start_mm;
            struct mm_struct *prev_mm = start_mm;
            struct mm_struct *mm;

            atomic_inc(&new_start_mm->mm_users);
            atomic_inc(&prev_mm->mm_users);
            spin_lock(&mmlist_lock);
            while (swap_count(*swap_map) && !retval &&
                    (p = p->next) != &start_mm->mmlist) {
                mm = list_entry(p, struct mm_struct, mmlist);
                if (!atomic_inc_not_zero(&mm->mm_users))
                    continue;
                spin_unlock(&mmlist_lock);
                mmput(prev_mm);
                prev_mm = mm;

                cond_resched();

                swcount = *swap_map;
                if (!swap_count(swcount)) /* any usage ? */
                    ;
                else if (mm == &init_mm)
                    set_start_mm = 1;
                else
                    retval = unuse_mm(mm, entry, page);

                if (set_start_mm && *swap_map < swcount) {
                    mmput(new_start_mm);
                    atomic_inc(&mm->mm_users);
                    new_start_mm = mm;
                    set_start_mm = 0;
                }
                spin_lock(&mmlist_lock);
            }
            spin_unlock(&mmlist_lock);
            mmput(prev_mm);
            mmput(start_mm);
            start_mm = new_start_mm;
        }
        if (retval) {
            unlock_page(page);
            page_cache_release(page);
            break;
        }

        /*
         * If a reference remains (rare), we would like to leave
         * the page in the swap cache; but try_to_unmap could
         * then re-duplicate the entry once we drop page lock,
         * so we might loop indefinitely; also, that page could
         * not be swapped out to other storage meanwhile.  So:
         * delete from cache even if there's another reference,
         * after ensuring that the data has been saved to disk -
         * since if the reference remains (rarer), it will be
         * read from disk into another page.  Splitting into two
         * pages would be incorrect if swap supported "shared
         * private" pages, but they are handled by tmpfs files.
         *
         * Given how unuse_vma() targets one particular offset
         * in an anon_vma, once the anon_vma has been determined,
         * this splitting happens to be just what is needed to
         * handle where KSM pages have been swapped out: re-reading
         * is unnecessarily slow, but we can fix that later on.
         */
        if (swap_count(*swap_map) &&
             PageDirty(page) && PageSwapCache(page)) {
            struct writeback_control wbc = {
                .sync_mode = WB_SYNC_NONE,
            };

            swap_writepage(page, &wbc);
            lock_page(page);
            wait_on_page_writeback(page);
        }

        /*
         * It is conceivable that a racing task removed this page from
         * swap cache just before we acquired the page lock at the top,
         * or while we dropped it in unuse_mm().  The page might even
         * be back in swap cache on another swap area: that we must not
         * delete, since it may not have been written out to swap yet.
         */
        if (PageSwapCache(page) &&
            likely(page_private(page) == entry.val))
            delete_from_swap_cache(page);

        /*
         * So we could skip searching mms once swap count went
         * to 1, we did not mark any present ptes as dirty: must
         * mark page dirty so shrink_page_list will preserve it.
         */
        SetPageDirty(page);
        unlock_page(page);
        page_cache_release(page);

        /*
         * Make sure that we aren't completely killing
         * interactive performance.
         */
        cond_resched();
        if (frontswap && pages_to_unuse > 0) {
            if (!--pages_to_unuse)
                break;
        }
    }

    mmput(start_mm);
    return retval;
}

/*
 * After a successful try_to_unuse, if no swap is now in use, we know
 * we can empty the mmlist.  swap_lock must be held on entry and exit.
 * Note that mmlist_lock nests inside swap_lock, and an mm must be
 * added to the mmlist just after page_duplicate - before would be racy.
 */
static void drain_mmlist(void)
{
    struct list_head *p, *next;
    unsigned int type;

    for (type = 0; type < nr_swapfiles; type++)
        if (swap_info[type]->inuse_pages)
            return;
    spin_lock(&mmlist_lock);
    list_for_each_safe(p, next, &init_mm.mmlist)
        list_del_init(p);
    spin_unlock(&mmlist_lock);
}

/*
 * Use this swapdev's extent info to locate the (PAGE_SIZE) block which
 * corresponds to page offset for the specified swap entry.
 * Note that the type of this function is sector_t, but it returns page offset
 * into the bdev, not sector offset.
 */
static sector_t map_swap_entry(swp_entry_t entry, struct block_device **bdev)
{
    struct swap_info_struct *sis;
    struct swap_extent *start_se;
    struct swap_extent *se;
    pgoff_t offset;

    sis = swap_info[swp_type(entry)];
    *bdev = sis->bdev;

    offset = swp_offset(entry);
    start_se = sis->curr_swap_extent;
    se = start_se;

    for ( ; ; ) {
        struct list_head *lh;

        if (se->start_page <= offset &&
                offset < (se->start_page + se->nr_pages)) {
            return se->start_block + (offset - se->start_page);
        }
        lh = se->list.next;
        se = list_entry(lh, struct swap_extent, list);
        sis->curr_swap_extent = se;
        BUG_ON(se == start_se);     /* It *must* be present */
    }
}

/*
 * Returns the page offset into bdev for the specified page's swap entry.
 */
sector_t map_swap_page(struct page *page, struct block_device **bdev)
{
    swp_entry_t entry;
    entry.val = page_private(page);
    return map_swap_entry(entry, bdev);
}

/*
 * Free all of a swapdev's extent information
 */
static void destroy_swap_extents(struct swap_info_struct *sis)
{
    while (!list_empty(&sis->first_swap_extent.list)) {
        struct swap_extent *se;

        se = list_entry(sis->first_swap_extent.list.next,
                struct swap_extent, list);
        list_del(&se->list);
        kfree(se);
    }

    if (sis->flags & SWP_FILE) {
        struct file *swap_file = sis->swap_file;
        struct address_space *mapping = swap_file->f_mapping;

        sis->flags &= ~SWP_FILE;
        mapping->a_ops->swap_deactivate(swap_file);
    }
}

/*
 * Add a block range (and the corresponding page range) into this swapdev's
 * extent list.  The extent list is kept sorted in page order.
 *
 * This function rather assumes that it is called in ascending page order.
 */
int
add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
        unsigned long nr_pages, sector_t start_block)
{
    struct swap_extent *se;
    struct swap_extent *new_se;
    struct list_head *lh;

    if (start_page == 0) {
        se = &sis->first_swap_extent;
        sis->curr_swap_extent = se;
        se->start_page = 0;
        se->nr_pages = nr_pages;
        se->start_block = start_block;
        return 1;
    } else {
        lh = sis->first_swap_extent.list.prev;  /* Highest extent */
        se = list_entry(lh, struct swap_extent, list);
        BUG_ON(se->start_page + se->nr_pages != start_page);
        if (se->start_block + se->nr_pages == start_block) {
            /* Merge it */
            se->nr_pages += nr_pages;
            return 0;
        }
    }

    /*
     * No merge.  Insert a new extent, preserving ordering.
     */
    new_se = kmalloc(sizeof(*se), GFP_KERNEL);
    if (new_se == NULL)
        return -ENOMEM;
    new_se->start_page = start_page;
    new_se->nr_pages = nr_pages;
    new_se->start_block = start_block;

    list_add_tail(&new_se->list, &sis->first_swap_extent.list);
    return 1;
}

/*
 * A `swap extent' is a simple thing which maps a contiguous range of pages
 * onto a contiguous range of disk blocks.  An ordered list of swap extents
 * is built at swapon time and is then used at swap_writepage/swap_readpage
 * time for locating where on disk a page belongs.
 *
 * If the swapfile is an S_ISBLK block device, a single extent is installed.
 * This is done so that the main operating code can treat S_ISBLK and S_ISREG
 * swap files identically.
 *
 * Whether the swapdev is an S_ISREG file or an S_ISBLK blockdev, the swap
 * extent list operates in PAGE_SIZE disk blocks.  Both S_ISREG and S_ISBLK
 * swapfiles are handled *identically* after swapon time.
 *
 * For S_ISREG swapfiles, setup_swap_extents() will walk all the file's blocks
 * and will parse them into an ordered extent list, in PAGE_SIZE chunks.  If
 * some stray blocks are found which do not fall within the PAGE_SIZE alignment
 * requirements, they are simply tossed out - we will never use those blocks
 * for swapping.
 *
 * For S_ISREG swapfiles we set S_SWAPFILE across the life of the swapon.  This
 * prevents root from shooting her foot off by ftruncating an in-use swapfile,
 * which will scribble on the fs.
 *
 * The amount of disk space which a single swap extent represents varies.
 * Typically it is in the 1-4 megabyte range.  So we can have hundreds of
 * extents in the list.  To avoid much list walking, we cache the previous
 * search location in `curr_swap_extent', and start new searches from there.
 * This is extremely effective.  The average number of iterations in
 * map_swap_page() has been measured at about 0.3 per page.  - akpm.
 */
static int setup_swap_extents(struct swap_info_struct *sis, sector_t *span)
{
    struct file *swap_file = sis->swap_file;
    struct address_space *mapping = swap_file->f_mapping;
    struct inode *inode = mapping->host;
    int ret;

    if (S_ISBLK(inode->i_mode)) {
        ret = add_swap_extent(sis, 0, sis->max, 0);
        *span = sis->pages;
        return ret;
    }

    if (mapping->a_ops->swap_activate) {
        ret = mapping->a_ops->swap_activate(sis, swap_file, span);
        if (!ret) {
            sis->flags |= SWP_FILE;
            ret = add_swap_extent(sis, 0, sis->max, 0);
            *span = sis->pages;
        }
        return ret;
    }

    return generic_swapfile_activate(sis, swap_file, span);
}

static void enable_swap_info(struct swap_info_struct *p, int prio,
                unsigned char *swap_map,
                unsigned long *frontswap_map)
{
    int i, prev;

    spin_lock(&swap_lock);
    if (prio >= 0)
        p->prio = prio;
    else
        p->prio = --least_priority;
    p->swap_map = swap_map;
    frontswap_map_set(p, frontswap_map);
    p->flags |= SWP_WRITEOK;
    nr_swap_pages += p->pages;
    total_swap_pages += p->pages;

    /* insert swap space into swap_list: */
    prev = -1;
    for (i = swap_list.head; i >= 0; i = swap_info[i]->next) {
        if (p->prio >= swap_info[i]->prio)
            break;
        prev = i;
    }
    p->next = i;
    if (prev < 0)
        swap_list.head = swap_list.next = p->type;
    else
        swap_info[prev]->next = p->type;
    frontswap_init(p->type);
    spin_unlock(&swap_lock);
}

SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
{
    struct swap_info_struct *p = NULL;
    unsigned char *swap_map;
    struct file *swap_file, *victim;
    struct address_space *mapping;
    struct inode *inode;
    struct filename *pathname;
    int oom_score_adj;
    int i, type, prev;
    int err;

    if (!capable(CAP_SYS_ADMIN))
        return -EPERM;

    BUG_ON(!current->mm);

    pathname = getname(specialfile);
    if (IS_ERR(pathname))
        return PTR_ERR(pathname);

    victim = file_open_name(pathname, O_RDWR|O_LARGEFILE, 0);
    err = PTR_ERR(victim);
    if (IS_ERR(victim))
        goto out;

    mapping = victim->f_mapping;
    prev = -1;
    spin_lock(&swap_lock);
    for (type = swap_list.head; type >= 0; type = swap_info[type]->next) {
        p = swap_info[type];
        if (p->flags & SWP_WRITEOK) {
            if (p->swap_file->f_mapping == mapping)
                break;
        }
        prev = type;
    }
    if (type < 0) {
        err = -EINVAL;
        spin_unlock(&swap_lock);
        goto out_dput;
    }
    if (!security_vm_enough_memory_mm(current->mm, p->pages))
        vm_unacct_memory(p->pages);
    else {
        err = -ENOMEM;
        spin_unlock(&swap_lock);
        goto out_dput;
    }
    if (prev < 0)
        swap_list.head = p->next;
    else
        swap_info[prev]->next = p->next;
    if (type == swap_list.next) {
        /* just pick something that's safe... */
        swap_list.next = swap_list.head;
    }
    if (p->prio < 0) {
        for (i = p->next; i >= 0; i = swap_info[i]->next)
            swap_info[i]->prio = p->prio--;
        least_priority++;
    }
    nr_swap_pages -= p->pages;
    total_swap_pages -= p->pages;
    p->flags &= ~SWP_WRITEOK;
    spin_unlock(&swap_lock);

    oom_score_adj = test_set_oom_score_adj(OOM_SCORE_ADJ_MAX);
    err = try_to_unuse(type, false, 0); /* force all pages to be unused */
    compare_swap_oom_score_adj(OOM_SCORE_ADJ_MAX, oom_score_adj);

    if (err) {
        /*
         * reading p->prio and p->swap_map outside the lock is
         * safe here because only sys_swapon and sys_swapoff
         * change them, and there can be no other sys_swapon or
         * sys_swapoff for this swap_info_struct at this point.
         */
        /* re-insert swap space back into swap_list */
        enable_swap_info(p, p->prio, p->swap_map, frontswap_map_get(p));
        goto out_dput;
    }

    destroy_swap_extents(p);
    if (p->flags & SWP_CONTINUED)
        free_swap_count_continuations(p);

    mutex_lock(&swapon_mutex);
    spin_lock(&swap_lock);
    drain_mmlist();

    /* wait for anyone still in scan_swap_map */
    p->highest_bit = 0;     /* cuts scans short */
    while (p->flags >= SWP_SCANNING) {
        spin_unlock(&swap_lock);
        schedule_timeout_uninterruptible(1);
        spin_lock(&swap_lock);
    }

    swap_file = p->swap_file;
    p->swap_file = NULL;
    p->max = 0;
    swap_map = p->swap_map;
    p->swap_map = NULL;
    p->flags = 0;
    frontswap_invalidate_area(type);
    spin_unlock(&swap_lock);
    mutex_unlock(&swapon_mutex);
    vfree(swap_map);
    vfree(frontswap_map_get(p));
    /* Destroy swap account informatin */
    swap_cgroup_swapoff(type);

    inode = mapping->host;
    if (S_ISBLK(inode->i_mode)) {
        struct block_device *bdev = I_BDEV(inode);
        set_blocksize(bdev, p->old_block_size);
        blkdev_put(bdev, FMODE_READ | FMODE_WRITE | FMODE_EXCL);
    } else {
        mutex_lock(&inode->i_mutex);
        inode->i_flags &= ~S_SWAPFILE;
        mutex_unlock(&inode->i_mutex);
    }
    filp_close(swap_file, NULL);
    err = 0;
    atomic_inc(&proc_poll_event);
    wake_up_interruptible(&proc_poll_wait);

out_dput:
    filp_close(victim, NULL);
out:
    putname(pathname);
    return err;
}

#ifdef CONFIG_PROC_FS
static unsigned swaps_poll(struct file *file, poll_table *wait)
{
    struct seq_file *seq = file->private_data;

    poll_wait(file, &proc_poll_wait, wait);

    if (seq->poll_event != atomic_read(&proc_poll_event)) {
        seq->poll_event = atomic_read(&proc_poll_event);
        return POLLIN | POLLRDNORM | POLLERR | POLLPRI;
    }

    return POLLIN | POLLRDNORM;
}

/* iterator */
static void *swap_start(struct seq_file *swap, loff_t *pos)
{
    struct swap_info_struct *si;
    int type;
    loff_t l = *pos;

    mutex_lock(&swapon_mutex);

    if (!l)
        return SEQ_START_TOKEN;

    for (type = 0; type < nr_swapfiles; type++) {
        smp_rmb();  /* read nr_swapfiles before swap_info[type] */
        si = swap_info[type];
        if (!(si->flags & SWP_USED) || !si->swap_map)
            continue;
        if (!--l)
            return si;
    }

    return NULL;
}

static void *swap_next(struct seq_file *swap, void *v, loff_t *pos)
{
    struct swap_info_struct *si = v;
    int type;

    if (v == SEQ_START_TOKEN)
        type = 0;
    else
        type = si->type + 1;

    for (; type < nr_swapfiles; type++) {
        smp_rmb();  /* read nr_swapfiles before swap_info[type] */
        si = swap_info[type];
        if (!(si->flags & SWP_USED) || !si->swap_map)
            continue;
        ++*pos;
        return si;
    }

    return NULL;
}

static void swap_stop(struct seq_file *swap, void *v)
{
    mutex_unlock(&swapon_mutex);
}

static int swap_show(struct seq_file *swap, void *v)
{
    struct swap_info_struct *si = v;
    struct file *file;
    int len;

    if (si == SEQ_START_TOKEN) {
        seq_puts(swap,"Filename\t\t\t\tType\t\tSize\tUsed\tPriority\n");
        return 0;
    }

    file = si->swap_file;
    len = seq_path(swap, &file->f_path, " \t\n\\");
    seq_printf(swap, "%*s%s\t%u\t%u\t%d\n",
            len < 40 ? 40 - len : 1, " ",
            S_ISBLK(file->f_path.dentry->d_inode->i_mode) ?
                "partition" : "file\t",
            si->pages << (PAGE_SHIFT - 10),
            si->inuse_pages << (PAGE_SHIFT - 10),
            si->prio);
    return 0;
}

static const struct seq_operations swaps_op = {
    .start =    swap_start,
    .next =     swap_next,
    .stop =     swap_stop,
    .show =     swap_show
};

static int swaps_open(struct inode *inode, struct file *file)
{
    struct seq_file *seq;
    int ret;

    ret = seq_open(file, &swaps_op);
    if (ret)
        return ret;

    seq = file->private_data;
    seq->poll_event = atomic_read(&proc_poll_event);
    return 0;
}

static const struct file_operations proc_swaps_operations = {
    .open       = swaps_open,
    .read       = seq_read,
    .llseek     = seq_lseek,
    .release    = seq_release,
    .poll       = swaps_poll,
};

static int __init procswaps_init(void)
{
    proc_create("swaps", 0, NULL, &proc_swaps_operations);
    return 0;
}
__initcall(procswaps_init);
#endif /* CONFIG_PROC_FS */

#ifdef MAX_SWAPFILES_CHECK
static int __init max_swapfiles_check(void)
{
    MAX_SWAPFILES_CHECK();
    return 0;
}
late_initcall(max_swapfiles_check);
#endif

static struct swap_info_struct *alloc_swap_info(void)
{
    struct swap_info_struct *p;
    unsigned int type;

    p = kzalloc(sizeof(*p), GFP_KERNEL);
    if (!p)
        return ERR_PTR(-ENOMEM);

    spin_lock(&swap_lock);
    for (type = 0; type < nr_swapfiles; type++) {
        if (!(swap_info[type]->flags & SWP_USED))
            break;
    }
    if (type >= MAX_SWAPFILES) {
        spin_unlock(&swap_lock);
        kfree(p);
        return ERR_PTR(-EPERM);
    }
    if (type >= nr_swapfiles) {
        p->type = type;
        swap_info[type] = p;
        /*
         * Write swap_info[type] before nr_swapfiles, in case a
         * racing procfs swap_start() or swap_next() is reading them.
         * (We never shrink nr_swapfiles, we never free this entry.)
         */
        smp_wmb();
        nr_swapfiles++;
    } else {
        kfree(p);
        p = swap_info[type];
        /*
         * Do not memset this entry: a racing procfs swap_next()
         * would be relying on p->type to remain valid.
         */
    }
    INIT_LIST_HEAD(&p->first_swap_extent.list);
    p->flags = SWP_USED;
    p->next = -1;
    spin_unlock(&swap_lock);

    return p;
}

static int claim_swapfile(struct swap_info_struct *p, struct inode *inode)
{
    int error;

    if (S_ISBLK(inode->i_mode)) {
        p->bdev = bdgrab(I_BDEV(inode));
        error = blkdev_get(p->bdev,
                   FMODE_READ | FMODE_WRITE | FMODE_EXCL,
                   sys_swapon);
        if (error < 0) {
            p->bdev = NULL;
            return -EINVAL;
        }
        p->old_block_size = block_size(p->bdev);
        error = set_blocksize(p->bdev, PAGE_SIZE);
        if (error < 0)
            return error;
        p->flags |= SWP_BLKDEV;
    } else if (S_ISREG(inode->i_mode)) {
        p->bdev = inode->i_sb->s_bdev;
        mutex_lock(&inode->i_mutex);
        if (IS_SWAPFILE(inode))
            return -EBUSY;
    } else
        return -EINVAL;

    return 0;
}

static unsigned long read_swap_header(struct swap_info_struct *p,
                    union swap_header *swap_header,
                    struct inode *inode)
{
    int i;
    unsigned long maxpages;
    unsigned long swapfilepages;

    if (memcmp("SWAPSPACE2", swap_header->magic.magic, 10)) {
        printk(KERN_ERR "Unable to find swap-space signature\n");
        return 0;
    }

    /* swap partition endianess hack... */
    if (swab32(swap_header->info.version) == 1) {
        swab32s(&swap_header->info.version);
        swab32s(&swap_header->info.last_page);
        swab32s(&swap_header->info.nr_badpages);
        for (i = 0; i < swap_header->info.nr_badpages; i++)
            swab32s(&swap_header->info.badpages[i]);
    }
    /* Check the swap header's sub-version */
    if (swap_header->info.version != 1) {
        printk(KERN_WARNING
               "Unable to handle swap header version %d\n",
               swap_header->info.version);
        return 0;
    }

    p->lowest_bit  = 1;
    p->cluster_next = 1;
    p->cluster_nr = 0;

    /*
     * Find out how many pages are allowed for a single swap
     * device. There are two limiting factors: 1) the number
     * of bits for the swap offset in the swp_entry_t type, and
     * 2) the number of bits in the swap pte as defined by the
     * different architectures. In order to find the
     * largest possible bit mask, a swap entry with swap type 0
     * and swap offset ~0UL is created, encoded to a swap pte,
     * decoded to a swp_entry_t again, and finally the swap
     * offset is extracted. This will mask all the bits from
     * the initial ~0UL mask that can't be encoded in either
     * the swp_entry_t or the architecture definition of a
     * swap pte.
     */
    maxpages = swp_offset(pte_to_swp_entry(
            swp_entry_to_pte(swp_entry(0, ~0UL)))) + 1;
    if (maxpages > swap_header->info.last_page) {
        maxpages = swap_header->info.last_page + 1;
        /* p->max is an unsigned int: don't overflow it */
        if ((unsigned int)maxpages == 0)
            maxpages = UINT_MAX;
    }
    p->highest_bit = maxpages - 1;

    if (!maxpages)
        return 0;
    swapfilepages = i_size_read(inode) >> PAGE_SHIFT;
    if (swapfilepages && maxpages > swapfilepages) {
        printk(KERN_WARNING
               "Swap area shorter than signature indicates\n");
        return 0;
    }
    if (swap_header->info.nr_badpages && S_ISREG(inode->i_mode))
        return 0;
    if (swap_header->info.nr_badpages > MAX_SWAP_BADPAGES)
        return 0;

    return maxpages;
}

static int setup_swap_map_and_extents(struct swap_info_struct *p,
                    union swap_header *swap_header,
                    unsigned char *swap_map,
                    unsigned long maxpages,
                    sector_t *span)
{
    int i;
    unsigned int nr_good_pages;
    int nr_extents;

    nr_good_pages = maxpages - 1;   /* omit header page */

    for (i = 0; i < swap_header->info.nr_badpages; i++) {
        unsigned int page_nr = swap_header->info.badpages[i];
        if (page_nr == 0 || page_nr > swap_header->info.last_page)
            return -EINVAL;
        if (page_nr < maxpages) {
            swap_map[page_nr] = SWAP_MAP_BAD;
            nr_good_pages--;
        }
    }

    if (nr_good_pages) {
        swap_map[0] = SWAP_MAP_BAD;
        p->max = maxpages;
        p->pages = nr_good_pages;
        nr_extents = setup_swap_extents(p, span);
        if (nr_extents < 0)
            return nr_extents;
        nr_good_pages = p->pages;
    }
    if (!nr_good_pages) {
        printk(KERN_WARNING "Empty swap-file\n");
        return -EINVAL;
    }

    return nr_extents;
}

SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
{
    struct swap_info_struct *p;
    struct filename *name;
    struct file *swap_file = NULL;
    struct address_space *mapping;
    int i;
    int prio;
    int error;
    union swap_header *swap_header;
    int nr_extents;
    sector_t span;
    unsigned long maxpages;
    unsigned char *swap_map = NULL;
    unsigned long *frontswap_map = NULL;
    struct page *page = NULL;
    struct inode *inode = NULL;

    if (swap_flags & ~SWAP_FLAGS_VALID)
        return -EINVAL;

    if (!capable(CAP_SYS_ADMIN))
        return -EPERM;

    p = alloc_swap_info();
    if (IS_ERR(p))
        return PTR_ERR(p);

    name = getname(specialfile);
    if (IS_ERR(name)) {
        error = PTR_ERR(name);
        name = NULL;
        goto bad_swap;
    }
    swap_file = file_open_name(name, O_RDWR|O_LARGEFILE, 0);
    if (IS_ERR(swap_file)) {
        error = PTR_ERR(swap_file);
        swap_file = NULL;
        goto bad_swap;
    }

    p->swap_file = swap_file;
    mapping = swap_file->f_mapping;

    for (i = 0; i < nr_swapfiles; i++) {
        struct swap_info_struct *q = swap_info[i];

        if (q == p || !q->swap_file)
            continue;
        if (mapping == q->swap_file->f_mapping) {
            error = -EBUSY;
            goto bad_swap;
        }
    }

    inode = mapping->host;
    /* If S_ISREG(inode->i_mode) will do mutex_lock(&inode->i_mutex); */
    error = claim_swapfile(p, inode);
    if (unlikely(error))
        goto bad_swap;

    /*
     * Read the swap header.
     */
    if (!mapping->a_ops->readpage) {
        error = -EINVAL;
        goto bad_swap;
    }
    page = read_mapping_page(mapping, 0, swap_file);
    if (IS_ERR(page)) {
        error = PTR_ERR(page);
        goto bad_swap;
    }
    swap_header = kmap(page);

    maxpages = read_swap_header(p, swap_header, inode);
    if (unlikely(!maxpages)) {
        error = -EINVAL;
        goto bad_swap;
    }

    /* OK, set up the swap map and apply the bad block list */
    swap_map = vzalloc(maxpages);
    if (!swap_map) {
        error = -ENOMEM;
        goto bad_swap;
    }

    error = swap_cgroup_swapon(p->type, maxpages);
    if (error)
        goto bad_swap;

    nr_extents = setup_swap_map_and_extents(p, swap_header, swap_map,
        maxpages, &span);
    if (unlikely(nr_extents < 0)) {
        error = nr_extents;
        goto bad_swap;
    }
    /* frontswap enabled? set up bit-per-page map for frontswap */
    if (frontswap_enabled)
        frontswap_map = vzalloc(maxpages / sizeof(long));

    if (p->bdev) {
        if (blk_queue_nonrot(bdev_get_queue(p->bdev))) {
            p->flags |= SWP_SOLIDSTATE;
            p->cluster_next = 1 + (random32() % p->highest_bit);
        }
        if ((swap_flags & SWAP_FLAG_DISCARD) && discard_swap(p) == 0)
            p->flags |= SWP_DISCARDABLE;
    }

    mutex_lock(&swapon_mutex);
    prio = -1;
    if (swap_flags & SWAP_FLAG_PREFER)
        prio =
          (swap_flags & SWAP_FLAG_PRIO_MASK) >> SWAP_FLAG_PRIO_SHIFT;
    enable_swap_info(p, prio, swap_map, frontswap_map);

    printk(KERN_INFO "Adding %uk swap on %s.  "
            "Priority:%d extents:%d across:%lluk %s%s%s\n",
        p->pages<<(PAGE_SHIFT-10), name->name, p->prio,
        nr_extents, (unsigned long long)span<<(PAGE_SHIFT-10),
        (p->flags & SWP_SOLIDSTATE) ? "SS" : "",
        (p->flags & SWP_DISCARDABLE) ? "D" : "",
        (frontswap_map) ? "FS" : "");

    mutex_unlock(&swapon_mutex);
    atomic_inc(&proc_poll_event);
    wake_up_interruptible(&proc_poll_wait);

    if (S_ISREG(inode->i_mode))
        inode->i_flags |= S_SWAPFILE;
    error = 0;
    goto out;
bad_swap:
    if (inode && S_ISBLK(inode->i_mode) && p->bdev) {
        set_blocksize(p->bdev, p->old_block_size);
        blkdev_put(p->bdev, FMODE_READ | FMODE_WRITE | FMODE_EXCL);
    }
    destroy_swap_extents(p);
    swap_cgroup_swapoff(p->type);
    spin_lock(&swap_lock);
    p->swap_file = NULL;
    p->flags = 0;
    spin_unlock(&swap_lock);
    vfree(swap_map);
    if (swap_file) {
        if (inode && S_ISREG(inode->i_mode)) {
            mutex_unlock(&inode->i_mutex);
            inode = NULL;
        }
        filp_close(swap_file, NULL);
    }
out:
    if (page && !IS_ERR(page)) {
        kunmap(page);
        page_cache_release(page);
    }
    if (name)
        putname(name);
    if (inode && S_ISREG(inode->i_mode))
        mutex_unlock(&inode->i_mutex);
    return error;
}

void si_swapinfo(struct sysinfo *val)
{
    unsigned int type;
    unsigned long nr_to_be_unused = 0;

    spin_lock(&swap_lock);
    for (type = 0; type < nr_swapfiles; type++) {
        struct swap_info_struct *si = swap_info[type];

        if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK))
            nr_to_be_unused += si->inuse_pages;
    }
    val->freeswap = nr_swap_pages + nr_to_be_unused;
    val->totalswap = total_swap_pages + nr_to_be_unused;
    spin_unlock(&swap_lock);
}

/*
 * Verify that a swap entry is valid and increment its swap map count.
 *
 * Returns error code in following case.
 * - success -> 0
 * - swp_entry is invalid -> EINVAL
 * - swp_entry is migration entry -> EINVAL
 * - swap-cache reference is requested but there is already one. -> EEXIST
 * - swap-cache reference is requested but the entry is not used. -> ENOENT
 * - swap-mapped reference requested but needs continued swap count. -> ENOMEM
 */
static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
{
    struct swap_info_struct *p;
    unsigned long offset, type;
    unsigned char count;
    unsigned char has_cache;
    int err = -EINVAL;

    if (non_swap_entry(entry))
        goto out;

    type = swp_type(entry);
    if (type >= nr_swapfiles)
        goto bad_file;
    p = swap_info[type];
    offset = swp_offset(entry);

    spin_lock(&swap_lock);
    if (unlikely(offset >= p->max))
        goto unlock_out;

    count = p->swap_map[offset];
    has_cache = count & SWAP_HAS_CACHE;
    count &= ~SWAP_HAS_CACHE;
    err = 0;

    if (usage == SWAP_HAS_CACHE) {

        /* set SWAP_HAS_CACHE if there is no cache and entry is used */
        if (!has_cache && count)
            has_cache = SWAP_HAS_CACHE;
        else if (has_cache)     /* someone else added cache */
            err = -EEXIST;
        else                /* no users remaining */
            err = -ENOENT;

    } else if (count || has_cache) {

        if ((count & ~COUNT_CONTINUED) < SWAP_MAP_MAX)
            count += usage;
        else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX)
            err = -EINVAL;
        else if (swap_count_continued(p, offset, count))
            count = COUNT_CONTINUED;
        else
            err = -ENOMEM;
    } else
        err = -ENOENT;          /* unused swap entry */

    p->swap_map[offset] = count | has_cache;

unlock_out:
    spin_unlock(&swap_lock);
out:
    return err;

bad_file:
    printk(KERN_ERR "swap_dup: %s%08lx\n", Bad_file, entry.val);
    goto out;
}

/*
 * Help swapoff by noting that swap entry belongs to shmem/tmpfs
 * (in which case its reference count is never incremented).
 */
void swap_shmem_alloc(swp_entry_t entry)
{
    __swap_duplicate(entry, SWAP_MAP_SHMEM);
}

/*
 * Increase reference count of swap entry by 1.
 * Returns 0 for success, or -ENOMEM if a swap_count_continuation is required
 * but could not be atomically allocated.  Returns 0, just as if it succeeded,
 * if __swap_duplicate() fails for another reason (-EINVAL or -ENOENT), which
 * might occur if a page table entry has got corrupted.
 */
int swap_duplicate(swp_entry_t entry)
{
    int err = 0;

    while (!err && __swap_duplicate(entry, 1) == -ENOMEM)
        err = add_swap_count_continuation(entry, GFP_ATOMIC);
    return err;
}

/*
 * @entry: swap entry for which we allocate swap cache.
 *
 * Called when allocating swap cache for existing swap entry,
 * This can return error codes. Returns 0 at success.
 * -EBUSY means there is a swap cache.
 * Note: return code is different from swap_duplicate().
 */
int swapcache_prepare(swp_entry_t entry)
{
    return __swap_duplicate(entry, SWAP_HAS_CACHE);
}

struct swap_info_struct *page_swap_info(struct page *page)
{
    swp_entry_t swap = { .val = page_private(page) };
    BUG_ON(!PageSwapCache(page));
    return swap_info[swp_type(swap)];
}

/*
 * out-of-line __page_file_ methods to avoid include hell.
 */
struct address_space *__page_file_mapping(struct page *page)
{
    VM_BUG_ON(!PageSwapCache(page));
    return page_swap_info(page)->swap_file->f_mapping;
}
EXPORT_SYMBOL_GPL(__page_file_mapping);

pgoff_t __page_file_index(struct page *page)
{
    swp_entry_t swap = { .val = page_private(page) };
    VM_BUG_ON(!PageSwapCache(page));
    return swp_offset(swap);
}
EXPORT_SYMBOL_GPL(__page_file_index);

/*
 * add_swap_count_continuation - called when a swap count is duplicated
 * beyond SWAP_MAP_MAX, it allocates a new page and links that to the entry's
 * page of the original vmalloc'ed swap_map, to hold the continuation count
 * (for that entry and for its neighbouring PAGE_SIZE swap entries).  Called
 * again when count is duplicated beyond SWAP_MAP_MAX * SWAP_CONT_MAX, etc.
 *
 * These continuation pages are seldom referenced: the common paths all work
 * on the original swap_map, only referring to a continuation page when the
 * low "digit" of a count is incremented or decremented through SWAP_MAP_MAX.
 *
 * add_swap_count_continuation(, GFP_ATOMIC) can be called while holding
 * page table locks; if it fails, add_swap_count_continuation(, GFP_KERNEL)
 * can be called after dropping locks.
 */
int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
{
    struct swap_info_struct *si;
    struct page *head;
    struct page *page;
    struct page *list_page;
    pgoff_t offset;
    unsigned char count;

    /*
     * When debugging, it's easier to use __GFP_ZERO here; but it's better
     * for latency not to zero a page while GFP_ATOMIC and holding locks.
     */
    page = alloc_page(gfp_mask | __GFP_HIGHMEM);

    si = swap_info_get(entry);
    if (!si) {
        /*
         * An acceptable race has occurred since the failing
         * __swap_duplicate(): the swap entry has been freed,
         * perhaps even the whole swap_map cleared for swapoff.
         */
        goto outer;
    }

    offset = swp_offset(entry);
    count = si->swap_map[offset] & ~SWAP_HAS_CACHE;

    if ((count & ~COUNT_CONTINUED) != SWAP_MAP_MAX) {
        /*
         * The higher the swap count, the more likely it is that tasks
         * will race to add swap count continuation: we need to avoid
         * over-provisioning.
         */
        goto out;
    }

    if (!page) {
        spin_unlock(&swap_lock);
        return -ENOMEM;
    }

    /*
     * We are fortunate that although vmalloc_to_page uses pte_offset_map,
     * no architecture is using highmem pages for kernel pagetables: so it
     * will not corrupt the GFP_ATOMIC caller's atomic pagetable kmaps.
     */
    head = vmalloc_to_page(si->swap_map + offset);
    offset &= ~PAGE_MASK;

    /*
     * Page allocation does not initialize the page's lru field,
     * but it does always reset its private field.
     */
    if (!page_private(head)) {
        BUG_ON(count & COUNT_CONTINUED);
        INIT_LIST_HEAD(&head->lru);
        set_page_private(head, SWP_CONTINUED);
        si->flags |= SWP_CONTINUED;
    }

    list_for_each_entry(list_page, &head->lru, lru) {
        unsigned char *map;

        /*
         * If the previous map said no continuation, but we've found
         * a continuation page, free our allocation and use this one.
         */
        if (!(count & COUNT_CONTINUED))
            goto out;

        map = kmap_atomic(list_page) + offset;
        count = *map;
        kunmap_atomic(map);

        /*
         * If this continuation count now has some space in it,
         * free our allocation and use this one.
         */
        if ((count & ~COUNT_CONTINUED) != SWAP_CONT_MAX)
            goto out;
    }

    list_add_tail(&page->lru, &head->lru);
    page = NULL;            /* now it's attached, don't free it */
out:
    spin_unlock(&swap_lock);
outer:
    if (page)
        __free_page(page);
    return 0;
}

/*
 * swap_count_continued - when the original swap_map count is incremented
 * from SWAP_MAP_MAX, check if there is already a continuation page to carry
 * into, carry if so, or else fail until a new continuation page is allocated;
 * when the original swap_map count is decremented from 0 with continuation,
 * borrow from the continuation and report whether it still holds more.
 * Called while __swap_duplicate() or swap_entry_free() holds swap_lock.
 */
static bool swap_count_continued(struct swap_info_struct *si,
                 pgoff_t offset, unsigned char count)
{
    struct page *head;
    struct page *page;
    unsigned char *map;

    head = vmalloc_to_page(si->swap_map + offset);
    if (page_private(head) != SWP_CONTINUED) {
        BUG_ON(count & COUNT_CONTINUED);
        return false;       /* need to add count continuation */
    }

    offset &= ~PAGE_MASK;
    page = list_entry(head->lru.next, struct page, lru);
    map = kmap_atomic(page) + offset;

    if (count == SWAP_MAP_MAX)  /* initial increment from swap_map */
        goto init_map;      /* jump over SWAP_CONT_MAX checks */

    if (count == (SWAP_MAP_MAX | COUNT_CONTINUED)) { /* incrementing */
        /*
         * Think of how you add 1 to 999
         */
        while (*map == (SWAP_CONT_MAX | COUNT_CONTINUED)) {
            kunmap_atomic(map);
            page = list_entry(page->lru.next, struct page, lru);
            BUG_ON(page == head);
            map = kmap_atomic(page) + offset;
        }
        if (*map == SWAP_CONT_MAX) {
            kunmap_atomic(map);
            page = list_entry(page->lru.next, struct page, lru);
            if (page == head)
                return false;   /* add count continuation */
            map = kmap_atomic(page) + offset;
init_map:       *map = 0;       /* we didn't zero the page */
        }
        *map += 1;
        kunmap_atomic(map);
        page = list_entry(page->lru.prev, struct page, lru);
        while (page != head) {
            map = kmap_atomic(page) + offset;
            *map = COUNT_CONTINUED;
            kunmap_atomic(map);
            page = list_entry(page->lru.prev, struct page, lru);
        }
        return true;            /* incremented */

    } else {                /* decrementing */
        /*
         * Think of how you subtract 1 from 1000
         */
        BUG_ON(count != COUNT_CONTINUED);
        while (*map == COUNT_CONTINUED) {
            kunmap_atomic(map);
            page = list_entry(page->lru.next, struct page, lru);
            BUG_ON(page == head);
            map = kmap_atomic(page) + offset;
        }
        BUG_ON(*map == 0);
        *map -= 1;
        if (*map == 0)
            count = 0;
        kunmap_atomic(map);
        page = list_entry(page->lru.prev, struct page, lru);
        while (page != head) {
            map = kmap_atomic(page) + offset;
            *map = SWAP_CONT_MAX | count;
            count = COUNT_CONTINUED;
            kunmap_atomic(map);
            page = list_entry(page->lru.prev, struct page, lru);
        }
        return count == COUNT_CONTINUED;
    }
}

/*
 * free_swap_count_continuations - swapoff free all the continuation pages
 * appended to the swap_map, after swap_map is quiesced, before vfree'ing it.
 */
static void free_swap_count_continuations(struct swap_info_struct *si)
{
    pgoff_t offset;

    for (offset = 0; offset < si->max; offset += PAGE_SIZE) {
        struct page *head;
        head = vmalloc_to_page(si->swap_map + offset);
        if (page_private(head)) {
            struct list_head *this, *next;
            list_for_each_safe(this, next, &head->lru) {
                struct page *page;
                page = list_entry(this, struct page, lru);
                list_del(this);
                __free_page(page);
            }
        }
    }
}