FreeBSD/Linux Kernel Cross Reference
sys/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 <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;
  
          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;
  }
  
  static void enable_swap_info(struct swap_info_struct *p, int prio,
                                  unsigned char *swap_map,
                                  unsigned long *frontswap_map)
  {
          spin_lock(&swap_lock);
          _enable_swap_info(p, prio, swap_map, frontswap_map);
          frontswap_init(p->type);
          spin_unlock(&swap_lock);
  }
  
  static void reinsert_swap_info(struct swap_info_struct *p)
  {
          spin_lock(&swap_lock);
          _enable_swap_info(p, p->prio, p->swap_map, frontswap_map_get(p));
          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 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);
  
          set_current_oom_origin();
          err = try_to_unuse(type, false, 0); /* force all pages to be unused */
          clear_current_oom_origin();
  
          if (err) {
                  /* re-insert swap space back into swap_list */
                  reinsert_swap_info(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);
                          }
                  }
          }
  }

Cache object: 5889c8e20895c4e9755d92bc5e33f40c