Date:	Thu, 15 Oct 2009 12:30:24 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>, hongshin@...il.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 7/9] swap_info: swap count continuations

On Thu, 15 Oct 2009 01:56:01 +0100 (BST)
Hugh Dickins <hugh.dickins@...cali.co.uk> wrote:

> Swap is duplicated (reference count incremented by one) whenever the same
> swap page is inserted into another mm (when forking finds a swap entry in
> place of a pte, or when reclaim unmaps a pte to insert the swap entry).
> 
> swap_info_struct's vmalloc'ed swap_map is the array of these reference
> counts: but what happens when the unsigned short (or unsigned char since
> the preceding patch) is full? (and its high bit is kept for a cache flag)
> 
> We then lose track of it, never freeing, leaving it in use until swapoff:
> at which point we _hope_ that a single pass will have found all instances,
> assume there are no more, and will lose user data if we're wrong.
> 
> Swapping of KSM pages has not yet been enabled; but it is implemented,
> and makes it very easy for a user to overflow the maximum swap count:
> possible with ordinary process pages, but unlikely, even when pid_max
> has been raised from PID_MAX_DEFAULT.
> 
> This patch implements swap count continuations: when the count overflows,
> a continuation page is allocated and linked to the original vmalloc'ed
> map page, and this used to hold the continuation counts for that entry
> and its neighbours.  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.
> 
> Signed-off-by: Hugh Dickins <hugh.dickins@...cali.co.uk>


Hmm...maybe I don't understand the benefit of this style of data structure.

Do we need fine grain chain ? 
Is  array of "unsigned long" counter is bad ?  (too big?)

==
#define EXTENTION_OFFSET_INDEX(offset)	(((offset) & PAGE_MASK)
#define EXTENTION_OFFSET_MASK		(~(PAGE_SIZE/sizeof(long) - 1))
struct swapcount_extention_array {
	unsigned long *map[EXTEND_MAP_SIZE];
};
	
At adding continuation.

int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
{
	struct page *page;
	unsigned long *newmap, *map;
	struct swapcount_extention_array *array;

	newmap = __get_free_page(mask);
	si = swap_info_get(entry);
	array = kmalloc(sizeof(swapcount_extention_array);

	....
	(If overflow)
	page = vmalloc_to_page(si->swap_map + offset);
	if (!PagePrivate(page)) {
		page->praivate = array;
	} else
		kfree(array);

	index = EXTENTION_OFFSET_INDEX(offset);
	pos = EXTENTION_OFFSET_MASK(offset);

	array = page->private;
	if (!array->map[index]) {
		array->map[index] = newmap;
	} else
		free_page(newmap);
	map = array->map[index];
	map[pos] += 1;
	mappage = vaddr_to_page(map);
	get_page(mappage); # increment page->count of array.
==

Hmm? maybe I just don't like chain...

Regards,
-Kame


> ---
> 
>  include/linux/swap.h |   22 ++
>  mm/memory.c          |   19 ++
>  mm/rmap.c            |    6 
>  mm/swapfile.c        |  304 +++++++++++++++++++++++++++++++++--------
>  4 files changed, 287 insertions(+), 64 deletions(-)
> 
> --- si6/include/linux/swap.h	2009-10-14 21:26:49.000000000 +0100
> +++ si7/include/linux/swap.h	2009-10-14 21:26:57.000000000 +0100
> @@ -145,15 +145,18 @@ enum {
>  	SWP_DISCARDABLE = (1 << 2),	/* blkdev supports discard */
>  	SWP_DISCARDING	= (1 << 3),	/* now discarding a free cluster */
>  	SWP_SOLIDSTATE	= (1 << 4),	/* blkdev seeks are cheap */
> +	SWP_CONTINUED	= (1 << 5),	/* swap_map has count continuation */
>  					/* add others here before... */
>  	SWP_SCANNING	= (1 << 8),	/* refcount in scan_swap_map */
>  };
>  
>  #define SWAP_CLUSTER_MAX 32
>  
> -#define SWAP_MAP_MAX	0x7e
> -#define SWAP_MAP_BAD	0x7f
> -#define SWAP_HAS_CACHE	0x80		/* There is a swap cache of entry. */
> +#define SWAP_MAP_MAX	0x3e	/* Max duplication count, in first swap_map */
> +#define SWAP_MAP_BAD	0x3f	/* Note pageblock is bad, in first swap_map */
> +#define SWAP_HAS_CACHE	0x40	/* Flag page is cached, in first swap_map */
> +#define SWAP_CONT_MAX	0x7f	/* Max count, in each swap_map continuation */
> +#define COUNT_CONTINUED	0x80	/* See swap_map continuation for full count */
>  
>  /*
>   * The in-memory structure used to track swap areas.
> @@ -310,9 +313,10 @@ extern long total_swap_pages;
>  extern void si_swapinfo(struct sysinfo *);
>  extern swp_entry_t get_swap_page(void);
>  extern swp_entry_t get_swap_page_of_type(int);
> -extern void swap_duplicate(swp_entry_t);
> -extern int swapcache_prepare(swp_entry_t);
>  extern int valid_swaphandles(swp_entry_t, unsigned long *);
> +extern int add_swap_count_continuation(swp_entry_t, gfp_t);
> +extern int swap_duplicate(swp_entry_t);
> +extern int swapcache_prepare(swp_entry_t);
>  extern void swap_free(swp_entry_t);
>  extern void swapcache_free(swp_entry_t, struct page *page);
>  extern int free_swap_and_cache(swp_entry_t);
> @@ -384,8 +388,14 @@ static inline void show_swap_cache_info(
>  #define free_swap_and_cache(swp)	is_migration_entry(swp)
>  #define swapcache_prepare(swp)		is_migration_entry(swp)
>  
> -static inline void swap_duplicate(swp_entry_t swp)
> +static inline int add_swap_count_continuation(swp_entry_t swp, gfp_t gfp_mask)
>  {
> +	return 0;
> +}
> +
> +static inline int swap_duplicate(swp_entry_t swp)
> +{
> +	return 0;
>  }
>  
>  static inline void swap_free(swp_entry_t swp)
> --- si6/mm/memory.c	2009-09-28 00:28:41.000000000 +0100
> +++ si7/mm/memory.c	2009-10-14 21:26:57.000000000 +0100
> @@ -572,7 +572,7 @@ out:
>   * covered by this vma.
>   */
>  
> -static inline void
> +static inline unsigned long
>  copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  		pte_t *dst_pte, pte_t *src_pte, struct vm_area_struct *vma,
>  		unsigned long addr, int *rss)
> @@ -586,7 +586,9 @@ copy_one_pte(struct mm_struct *dst_mm, s
>  		if (!pte_file(pte)) {
>  			swp_entry_t entry = pte_to_swp_entry(pte);
>  
> -			swap_duplicate(entry);
> +			if (swap_duplicate(entry) < 0)
> +				return entry.val;
> +
>  			/* make sure dst_mm is on swapoff's mmlist. */
>  			if (unlikely(list_empty(&dst_mm->mmlist))) {
>  				spin_lock(&mmlist_lock);
> @@ -635,6 +637,7 @@ copy_one_pte(struct mm_struct *dst_mm, s
>  
>  out_set_pte:
>  	set_pte_at(dst_mm, addr, dst_pte, pte);
> +	return 0;
>  }
>  
>  static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> @@ -645,6 +648,7 @@ static int copy_pte_range(struct mm_stru
>  	spinlock_t *src_ptl, *dst_ptl;
>  	int progress = 0;
>  	int rss[2];
> +	swp_entry_t entry = (swp_entry_t){0};
>  
>  again:
>  	rss[1] = rss[0] = 0;
> @@ -671,7 +675,10 @@ again:
>  			progress++;
>  			continue;
>  		}
> -		copy_one_pte(dst_mm, src_mm, dst_pte, src_pte, vma, addr, rss);
> +		entry.val = copy_one_pte(dst_mm, src_mm, dst_pte, src_pte,
> +							vma, addr, rss);
> +		if (entry.val)
> +			break;
>  		progress += 8;
>  	} while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);
>  
> @@ -681,6 +688,12 @@ again:
>  	add_mm_rss(dst_mm, rss[0], rss[1]);
>  	pte_unmap_unlock(dst_pte - 1, dst_ptl);
>  	cond_resched();
> +
> +	if (entry.val) {
> +		if (add_swap_count_continuation(entry, GFP_KERNEL) < 0)
> +			return -ENOMEM;
> +		progress = 0;
> +	}
>  	if (addr != end)
>  		goto again;
>  	return 0;
> --- si6/mm/rmap.c	2009-10-05 04:33:14.000000000 +0100
> +++ si7/mm/rmap.c	2009-10-14 21:26:57.000000000 +0100
> @@ -822,7 +822,11 @@ static int try_to_unmap_one(struct page
>  			 * Store the swap location in the pte.
>  			 * See handle_pte_fault() ...
>  			 */
> -			swap_duplicate(entry);
> +			if (swap_duplicate(entry) < 0) {
> +				set_pte_at(mm, address, pte, pteval);
> +				ret = SWAP_FAIL;
> +				goto out_unmap;
> +			}
>  			if (list_empty(&mm->mmlist)) {
>  				spin_lock(&mmlist_lock);
>  				if (list_empty(&mm->mmlist))
> --- si6/mm/swapfile.c	2009-10-14 21:26:49.000000000 +0100
> +++ si7/mm/swapfile.c	2009-10-14 21:26:57.000000000 +0100
> @@ -35,11 +35,14 @@
>  #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 DEFINE_SPINLOCK(swap_lock);
>  static unsigned int nr_swapfiles;
>  long nr_swap_pages;
>  long total_swap_pages;
> -static int swap_overflow;
>  static int least_priority;
>  
>  static const char Bad_file[] = "Bad swap file entry ";
> @@ -55,7 +58,7 @@ static DEFINE_MUTEX(swapon_mutex);
>  
>  static inline unsigned char swap_count(unsigned char ent)
>  {
> -	return ent & ~SWAP_HAS_CACHE;
> +	return ent & ~SWAP_HAS_CACHE;	/* may include SWAP_HAS_CONT flag */
>  }
>  
>  /* returns 1 if swap entry is freed */
> @@ -545,8 +548,15 @@ static unsigned char swap_entry_free(str
>  	if (usage == SWAP_HAS_CACHE) {
>  		VM_BUG_ON(!has_cache);
>  		has_cache = 0;
> -	} else if (count < SWAP_MAP_MAX)
> -		count--;
> +	} 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);
> @@ -604,6 +614,8 @@ void swapcache_free(swp_entry_t entry, s
>  
>  /*
>   * 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.
>   */
>  static inline int page_swapcount(struct page *page)
>  {
> @@ -1019,7 +1031,6 @@ static int try_to_unuse(unsigned int typ
>  	swp_entry_t entry;
>  	unsigned int i = 0;
>  	int retval = 0;
> -	int reset_overflow = 0;
>  	int shmem;
>  
>  	/*
> @@ -1034,8 +1045,7 @@ static int try_to_unuse(unsigned int typ
>  	 * 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.  Though it's only a serious concern when an overflowed
> -	 * swap count is reset from SWAP_MAP_MAX, preventing a rescan.
> +	 * that.
>  	 */
>  	start_mm = &init_mm;
>  	atomic_inc(&init_mm.mm_users);
> @@ -1166,36 +1176,6 @@ static int try_to_unuse(unsigned int typ
>  		}
>  
>  		/*
> -		 * How could swap count reach 0x7ffe ?
> -		 * There's no way to repeat a swap page within an mm
> -		 * (except in shmem, where it's the shared object which takes
> -		 * the reference count)?
> -		 * We believe SWAP_MAP_MAX cannot occur.(if occur, unsigned
> -		 * short is too small....)
> -		 * If that's wrong, then we should worry more about
> -		 * exit_mmap() and do_munmap() cases described above:
> -		 * we might be resetting SWAP_MAP_MAX too early here.
> -		 *
> -		 * Yes, that's wrong: though very unlikely, swap count 0x7ffe
> -		 * could surely occur if pid_max raised from PID_MAX_DEFAULT;
> -		 * and we are now lowering SWAP_MAP_MAX to 0x7e, making it
> -		 * much easier to reach.  But the next patch will fix that.
> -		 *
> -		 * We know "Undead"s can happen, they're okay, so don't
> -		 * report them; but do report if we reset SWAP_MAP_MAX.
> -		 */
> -		/* We might release the lock_page() in unuse_mm(). */
> -		if (!PageSwapCache(page) || page_private(page) != entry.val)
> -			goto retry;
> -
> -		if (swap_count(*swap_map) == SWAP_MAP_MAX) {
> -			spin_lock(&swap_lock);
> -			*swap_map = SWAP_HAS_CACHE;
> -			spin_unlock(&swap_lock);
> -			reset_overflow = 1;
> -		}
> -
> -		/*
>  		 * 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,
> @@ -1236,7 +1216,6 @@ static int try_to_unuse(unsigned int typ
>  		 * mark page dirty so shrink_page_list will preserve it.
>  		 */
>  		SetPageDirty(page);
> -retry:
>  		unlock_page(page);
>  		page_cache_release(page);
>  
> @@ -1248,10 +1227,6 @@ retry:
>  	}
>  
>  	mmput(start_mm);
> -	if (reset_overflow) {
> -		printk(KERN_WARNING "swapoff: cleared swap entry overflow\n");
> -		swap_overflow = 0;
> -	}
>  	return retval;
>  }
>  
> @@ -1595,6 +1570,9 @@ SYSCALL_DEFINE1(swapoff, const char __us
>  	up_write(&swap_unplug_sem);
>  
>  	destroy_swap_extents(p);
> +	if (p->flags & SWP_CONTINUED)
> +		free_swap_count_continuations(p);
> +
>  	mutex_lock(&swapon_mutex);
>  	spin_lock(&swap_lock);
>  	drain_mmlist();
> @@ -2080,14 +2058,13 @@ void si_swapinfo(struct sysinfo *val)
>  /*
>   * Verify that a swap entry is valid and increment its swap map count.
>   *
> - * Note: if swap_map[] reaches SWAP_MAP_MAX the entries are treated as
> - * "permanent", but will be reclaimed by the next swapoff.
>   * 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)
>  {
> @@ -2127,15 +2104,14 @@ static int __swap_duplicate(swp_entry_t
>  
>  	} else if (count || has_cache) {
>  
> -		if (count < SWAP_MAP_MAX - 1)
> -			count++;
> -		else if (count <= SWAP_MAP_MAX) {
> -			if (swap_overflow++ < 5)
> -				printk(KERN_WARNING
> -				       "swap_dup: swap entry overflow\n");
> -			count = SWAP_MAP_MAX;
> -		} else
> +		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 */
>  
> @@ -2154,9 +2130,13 @@ bad_file:
>  /*
>   * increase reference count of swap entry by 1.
>   */
> -void swap_duplicate(swp_entry_t entry)
> +int swap_duplicate(swp_entry_t entry)
>  {
> -	__swap_duplicate(entry, 1);
> +	int err = 0;
> +
> +	while (!err && __swap_duplicate(entry, 1) == -ENOMEM)
> +		err = add_swap_count_continuation(entry, GFP_ATOMIC);
> +	return err;
>  }
>  
>  /*
> @@ -2223,3 +2203,219 @@ int valid_swaphandles(swp_entry_t entry,
>  	*offset = ++toff;
>  	return nr_pages? ++nr_pages: 0;
>  }
> +
> +/*
> + * 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, KM_USER0) + offset;
> +		count = *map;
> +		kunmap_atomic(map, KM_USER0);
> +
> +		/*
> +		 * 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, KM_USER0) + 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, KM_USER0);
> +			page = list_entry(page->lru.next, struct page, lru);
> +			BUG_ON(page == head);
> +			map = kmap_atomic(page, KM_USER0) + offset;
> +		}
> +		if (*map == SWAP_CONT_MAX) {
> +			kunmap_atomic(map, KM_USER0);
> +			page = list_entry(page->lru.next, struct page, lru);
> +			if (page == head)
> +				return false;	/* add count continuation */
> +			map = kmap_atomic(page, KM_USER0) + offset;
> +init_map:		*map = 0;		/* we didn't zero the page */
> +		}
> +		*map += 1;
> +		kunmap_atomic(map, KM_USER0);
> +		page = list_entry(page->lru.prev, struct page, lru);
> +		while (page != head) {
> +			map = kmap_atomic(page, KM_USER0) + offset;
> +			*map = COUNT_CONTINUED;
> +			kunmap_atomic(map, KM_USER0);
> +			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, KM_USER0);
> +			page = list_entry(page->lru.next, struct page, lru);
> +			BUG_ON(page == head);
> +			map = kmap_atomic(page, KM_USER0) + offset;
> +		}
> +		BUG_ON(*map == 0);
> +		*map -= 1;
> +		if (*map == 0)
> +			count = 0;
> +		kunmap_atomic(map, KM_USER0);
> +		page = list_entry(page->lru.prev, struct page, lru);
> +		while (page != head) {
> +			map = kmap_atomic(page, KM_USER0) + offset;
> +			*map = SWAP_CONT_MAX | count;
> +			count = COUNT_CONTINUED;
> +			kunmap_atomic(map, KM_USER0);
> +			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);
> +			}
> +		}
> +	}
> +}
> --
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