Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel...

[mirror_ubuntu-zesty-kernel.git] / mm / swapfile.c

/*
 *  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 <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**);

static 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 ";

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

static 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) {
                struct gendisk *disk = p->bdev->bd_disk;
                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--;
                if ((p->flags & SWP_BLKDEV) &&
                                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)))) {
                if (ret > 0)
                        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 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)
{
        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;
                }
                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.
 */
static int try_to_unuse(unsigned int type)
{
        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)) != 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();
        }

        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);
        }
}

/*
 * 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.
 */
static 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 inode *inode;
        unsigned blocks_per_page;
        unsigned long page_no;
        unsigned blkbits;
        sector_t probe_block;
        sector_t last_block;
        sector_t lowest_block = -1;
        sector_t highest_block = 0;
        int nr_extents = 0;
        int ret;

        inode = sis->swap_file->f_mapping->host;
        if (S_ISBLK(inode->i_mode)) {
                ret = add_swap_extent(sis, 0, sis->max, 0);
                *span = sis->pages;
                goto out;
        }

        blkbits = inode->i_blkbits;
        blocks_per_page = PAGE_SIZE >> blkbits;

        /*
         * Map all the blocks into the extent list.  This code doesn't try
         * to be very smart.
         */
        probe_block = 0;
        page_no = 0;
        last_block = i_size_read(inode) >> blkbits;
        while ((probe_block + blocks_per_page) <= last_block &&
                        page_no < sis->max) {
                unsigned block_in_page;
                sector_t first_block;

                first_block = bmap(inode, probe_block);
                if (first_block == 0)
                        goto bad_bmap;

                /*
                 * It must be PAGE_SIZE aligned on-disk
                 */
                if (first_block & (blocks_per_page - 1)) {
                        probe_block++;
                        goto reprobe;
                }

                for (block_in_page = 1; block_in_page < blocks_per_page;
                                        block_in_page++) {
                        sector_t block;

                        block = bmap(inode, probe_block + block_in_page);
                        if (block == 0)
                                goto bad_bmap;
                        if (block != first_block + block_in_page) {
                                /* Discontiguity */
                                probe_block++;
                                goto reprobe;
                        }
                }

                first_block >>= (PAGE_SHIFT - blkbits);
                if (page_no) {  /* exclude the header page */
                        if (first_block < lowest_block)
                                lowest_block = first_block;
                        if (first_block > highest_block)
                                highest_block = first_block;
                }

                /*
                 * We found a PAGE_SIZE-length, PAGE_SIZE-aligned run of blocks
                 */
                ret = add_swap_extent(sis, page_no, 1, first_block);
                if (ret < 0)
                        goto out;
                nr_extents += ret;
                page_no++;
                probe_block += blocks_per_page;
reprobe:
                continue;
        }
        ret = nr_extents;
        *span = 1 + highest_block - lowest_block;
        if (page_no == 0)
                page_no = 1;    /* force Empty message */
        sis->max = page_no;
        sis->pages = page_no - 1;
        sis->highest_bit = page_no - 1;
out:
        return ret;
bad_bmap:
        printk(KERN_ERR "swapon: swapfile has holes\n");
        ret = -EINVAL;
        goto out;
}

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

        spin_lock(&swap_lock);
        if (prio >= 0)
                p->prio = prio;
        else
                p->prio = --least_priority;
        p->swap_map = swap_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;
        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;
        char *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);
        err = PTR_ERR(pathname);
        if (IS_ERR(pathname))
                goto out;

        victim = filp_open(pathname, O_RDWR|O_LARGEFILE, 0);
        putname(pathname);
        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);
        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);
                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;
        spin_unlock(&swap_lock);
        mutex_unlock(&swapon_mutex);
        vfree(swap_map);
        /* 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:
        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 three 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
         * the different architectures, and 3) the number of free bits
         * in an exceptional radix_tree entry. 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.  Then the same is done for a radix_tree entry.
         */
        maxpages = swp_offset(pte_to_swp_entry(
                        swp_entry_to_pte(swp_entry(0, ~0UL))));
        maxpages = swp_offset(radix_to_swp_entry(
                        swp_to_radix_entry(swp_entry(0, maxpages)))) + 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;
        char *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;
        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 = filp_open(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;
        }

        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);

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

        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);
}

/*
 * 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);
                        }
                }
        }
}