[net-next-2.6.git] / mm / swap_state.c

/*
 *  linux/mm/swap_state.c
 *
 *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
 *  Swap reorganised 29.12.95, Stephen Tweedie
 *
 *  Rewritten to use page cache, (C) 1998 Stephen Tweedie
 */
#include <linux/module.h>
#include <linux/mm.h>
#include <linux/kernel_stat.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/init.h>
#include <linux/pagemap.h>
#include <linux/buffer_head.h>
#include <linux/backing-dev.h>
#include <linux/pagevec.h>
#include <linux/migrate.h>

#include <asm/pgtable.h>

/*
 * swapper_space is a fiction, retained to simplify the path through
 * vmscan's shrink_page_list, to make sync_page look nicer, and to allow
 * future use of radix_tree tags in the swap cache.
 */
static const struct address_space_operations swap_aops = {
	.writepage	= swap_writepage,
	.sync_page	= block_sync_page,
	.set_page_dirty	= __set_page_dirty_nobuffers,
	.migratepage	= migrate_page,
};

static struct backing_dev_info swap_backing_dev_info = {
	.capabilities	= BDI_CAP_NO_ACCT_DIRTY | BDI_CAP_NO_WRITEBACK,
	.unplug_io_fn	= swap_unplug_io_fn,
};

struct address_space swapper_space = {
	.page_tree	= RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
	.tree_lock	= __RW_LOCK_UNLOCKED(swapper_space.tree_lock),
	.a_ops		= &swap_aops,
	.i_mmap_nonlinear = LIST_HEAD_INIT(swapper_space.i_mmap_nonlinear),
	.backing_dev_info = &swap_backing_dev_info,
};

#define INC_CACHE_INFO(x)	do { swap_cache_info.x++; } while (0)

static struct {
	unsigned long add_total;
	unsigned long del_total;
	unsigned long find_success;
	unsigned long find_total;
} swap_cache_info;

void show_swap_cache_info(void)
{
	printk("Swap cache: add %lu, delete %lu, find %lu/%lu\n",
		swap_cache_info.add_total, swap_cache_info.del_total,
		swap_cache_info.find_success, swap_cache_info.find_total);
	printk("Free swap  = %lukB\n", nr_swap_pages << (PAGE_SHIFT - 10));
	printk("Total swap = %lukB\n", total_swap_pages << (PAGE_SHIFT - 10));
}

/*
 * add_to_swap_cache resembles add_to_page_cache on swapper_space,
 * but sets SwapCache flag and private instead of mapping and index.
 */
int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
{
	int error;

	BUG_ON(!PageLocked(page));
	BUG_ON(PageSwapCache(page));
	BUG_ON(PagePrivate(page));
	error = radix_tree_preload(gfp_mask);
	if (!error) {
		write_lock_irq(&swapper_space.tree_lock);
		error = radix_tree_insert(&swapper_space.page_tree,
						entry.val, page);
		if (!error) {
			page_cache_get(page);
			SetPageSwapCache(page);
			set_page_private(page, entry.val);
			total_swapcache_pages++;
			__inc_zone_page_state(page, NR_FILE_PAGES);
			INC_CACHE_INFO(add_total);
		}
		write_unlock_irq(&swapper_space.tree_lock);
		radix_tree_preload_end();
	}
	return error;
}

/*
 * This must be called only on pages that have
 * been verified to be in the swap cache.
 */
void __delete_from_swap_cache(struct page *page)
{
	BUG_ON(!PageLocked(page));
	BUG_ON(!PageSwapCache(page));
	BUG_ON(PageWriteback(page));
	BUG_ON(PagePrivate(page));

	radix_tree_delete(&swapper_space.page_tree, page_private(page));
	set_page_private(page, 0);
	ClearPageSwapCache(page);
	total_swapcache_pages--;
	__dec_zone_page_state(page, NR_FILE_PAGES);
	INC_CACHE_INFO(del_total);
}

/**
 * add_to_swap - allocate swap space for a page
 * @page: page we want to move to swap
 *
 * Allocate swap space for the page and add the page to the
 * swap cache.  Caller needs to hold the page lock. 
 */
int add_to_swap(struct page * page, gfp_t gfp_mask)
{
	swp_entry_t entry;
	int err;

	BUG_ON(!PageLocked(page));
	BUG_ON(!PageUptodate(page));

	for (;;) {
		entry = get_swap_page();
		if (!entry.val)
			return 0;

		/*
		 * Radix-tree node allocations from PF_MEMALLOC contexts could
		 * completely exhaust the page allocator. __GFP_NOMEMALLOC
		 * stops emergency reserves from being allocated.
		 *
		 * TODO: this could cause a theoretical memory reclaim
		 * deadlock in the swap out path.
		 */
		/*
		 * Add it to the swap cache and mark it dirty
		 */
		err = add_to_swap_cache(page, entry,
				gfp_mask|__GFP_NOMEMALLOC|__GFP_NOWARN);

		switch (err) {
		case 0:				/* Success */
			SetPageDirty(page);
			return 1;
		case -EEXIST:
			/* Raced with "speculative" read_swap_cache_async */
			swap_free(entry);
			continue;
		default:
			/* -ENOMEM radix-tree allocation failure */
			swap_free(entry);
			return 0;
		}
	}
}

/*
 * This must be called only on pages that have
 * been verified to be in the swap cache and locked.
 * It will never put the page into the free list,
 * the caller has a reference on the page.
 */
void delete_from_swap_cache(struct page *page)
{
	swp_entry_t entry;

	entry.val = page_private(page);

	write_lock_irq(&swapper_space.tree_lock);
	__delete_from_swap_cache(page);
	write_unlock_irq(&swapper_space.tree_lock);

	swap_free(entry);
	page_cache_release(page);
}

/* 
 * If we are the only user, then try to free up the swap cache. 
 * 
 * Its ok to check for PageSwapCache without the page lock
 * here because we are going to recheck again inside 
 * exclusive_swap_page() _with_ the lock. 
 * 					- Marcelo
 */
static inline void free_swap_cache(struct page *page)
{
	if (PageSwapCache(page) && !TestSetPageLocked(page)) {
		remove_exclusive_swap_page(page);
		unlock_page(page);
	}
}

/* 
 * Perform a free_page(), also freeing any swap cache associated with
 * this page if it is the last user of the page.
 */
void free_page_and_swap_cache(struct page *page)
{
	free_swap_cache(page);
	page_cache_release(page);
}

/*
 * Passed an array of pages, drop them all from swapcache and then release
 * them.  They are removed from the LRU and freed if this is their last use.
 */
void free_pages_and_swap_cache(struct page **pages, int nr)
{
	struct page **pagep = pages;

	lru_add_drain();
	while (nr) {
		int todo = min(nr, PAGEVEC_SIZE);
		int i;

		for (i = 0; i < todo; i++)
			free_swap_cache(pagep[i]);
		release_pages(pagep, todo, 0);
		pagep += todo;
		nr -= todo;
	}
}

/*
 * Lookup a swap entry in the swap cache. A found page will be returned
 * unlocked and with its refcount incremented - we rely on the kernel
 * lock getting page table operations atomic even if we drop the page
 * lock before returning.
 */
struct page * lookup_swap_cache(swp_entry_t entry)
{
	struct page *page;

	page = find_get_page(&swapper_space, entry.val);

	if (page)
		INC_CACHE_INFO(find_success);

	INC_CACHE_INFO(find_total);
	return page;
}

/* 
 * Locate a page of swap in physical memory, reserving swap cache space
 * and reading the disk if it is not already cached.
 * A failure return means that either the page allocation failed or that
 * the swap entry is no longer in use.
 */
struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
			struct vm_area_struct *vma, unsigned long addr)
{
	struct page *found_page, *new_page = NULL;
	int err;

	do {
		/*
		 * First check the swap cache.  Since this is normally
		 * called after lookup_swap_cache() failed, re-calling
		 * that would confuse statistics.
		 */
		found_page = find_get_page(&swapper_space, entry.val);
		if (found_page)
			break;

		/*
		 * Get a new page to read into from swap.
		 */
		if (!new_page) {
			new_page = alloc_page_vma(gfp_mask, vma, addr);
			if (!new_page)
				break;		/* Out of memory */
		}

		/*
		 * Swap entry may have been freed since our caller observed it.
		 */
		if (!swap_duplicate(entry))
			break;

		/*
		 * Associate the page with swap entry in the swap cache.
		 * May fail (-EEXIST) if there is already a page associated
		 * with this entry in the swap cache: added by a racing
		 * read_swap_cache_async, or add_to_swap or shmem_writepage
		 * re-using the just freed swap entry for an existing page.
		 * May fail (-ENOMEM) if radix-tree node allocation failed.
		 */
		SetPageLocked(new_page);
		err = add_to_swap_cache(new_page, entry, gfp_mask & GFP_KERNEL);
		if (!err) {
			/*
			 * Initiate read into locked page and return.
			 */
			lru_cache_add_active(new_page);
			swap_readpage(NULL, new_page);
			return new_page;
		}
		ClearPageLocked(new_page);
		swap_free(entry);
	} while (err != -ENOMEM);

	if (new_page)
		page_cache_release(new_page);
	return found_page;
}

/**
 * swapin_readahead - swap in pages in hope we need them soon
 * @entry: swap entry of this memory
 * @vma: user vma this address belongs to
 * @addr: target address for mempolicy
 *
 * Returns the struct page for entry and addr, after queueing swapin.
 *
 * Primitive swap readahead code. We simply read an aligned block of
 * (1 << page_cluster) entries in the swap area. This method is chosen
 * because it doesn't cost us any seek time.  We also make sure to queue
 * the 'original' request together with the readahead ones...
 *
 * This has been extended to use the NUMA policies from the mm triggering
 * the readahead.
 *
 * Caller must hold down_read on the vma->vm_mm if vma is not NULL.
 */
struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
			struct vm_area_struct *vma, unsigned long addr)
{
	int nr_pages;
	struct page *page;
	unsigned long offset;
	unsigned long end_offset;

	/*
	 * Get starting offset for readaround, and number of pages to read.
	 * Adjust starting address by readbehind (for NUMA interleave case)?
	 * No, it's very unlikely that swap layout would follow vma layout,
	 * more likely that neighbouring swap pages came from the same node:
	 * so use the same "addr" to choose the same node for each swap read.
	 */
	nr_pages = valid_swaphandles(entry, &offset);
	for (end_offset = offset + nr_pages; offset < end_offset; offset++) {
		/* Ok, do the async read-ahead now */
		page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
						gfp_mask, vma, addr);
		if (!page)
			break;
		page_cache_release(page);
	}
	lru_add_drain();	/* Push any new pages onto the LRU now */
	return read_swap_cache_async(entry, gfp_mask, vma, addr);
}
Commit	Line	Data
1da177e4 LT	1	/*
	2	* linux/mm/swap_state.c
	3	*
	4	* Copyright (C) 1991, 1992, 1993, 1994 Linus Torvalds
	5	* Swap reorganised 29.12.95, Stephen Tweedie
	6	*
	7	* Rewritten to use page cache, (C) 1998 Stephen Tweedie
	8	*/
	9	#include <linux/module.h>
	10	#include <linux/mm.h>
	11	#include <linux/kernel_stat.h>
	12	#include <linux/swap.h>
46017e95	13	#include <linux/swapops.h>
1da177e4 LT	14	#include <linux/init.h>
	15	#include <linux/pagemap.h>
	16	#include <linux/buffer_head.h>
	17	#include <linux/backing-dev.h>
c484d410	18	#include <linux/pagevec.h>
b20a3503	19	#include <linux/migrate.h>
1da177e4 LT	20
	21	#include <asm/pgtable.h>
	22
	23	/*
	24	* swapper_space is a fiction, retained to simplify the path through
2706a1b8	25	* vmscan's shrink_page_list, to make sync_page look nicer, and to allow
1da177e4 LT	26	* future use of radix_tree tags in the swap cache.
1da177e4 LT	27	*/
f5e54d6e	28	static const struct address_space_operations swap_aops = {
1da177e4 LT	29	.writepage = swap_writepage,
	30	.sync_page = block_sync_page,
	31	.set_page_dirty = __set_page_dirty_nobuffers,
e965f963	32	.migratepage = migrate_page,
1da177e4 LT	33	};
	34
	35	static struct backing_dev_info swap_backing_dev_info = {
	36	.capabilities = BDI_CAP_NO_ACCT_DIRTY \| BDI_CAP_NO_WRITEBACK,
	37	.unplug_io_fn = swap_unplug_io_fn,
	38	};
	39
	40	struct address_space swapper_space = {
	41	.page_tree = RADIX_TREE_INIT(GFP_ATOMIC\|__GFP_NOWARN),
e4d91918	42	.tree_lock = __RW_LOCK_UNLOCKED(swapper_space.tree_lock),
1da177e4 LT	43	.a_ops = &swap_aops,
	44	.i_mmap_nonlinear = LIST_HEAD_INIT(swapper_space.i_mmap_nonlinear),
	45	.backing_dev_info = &swap_backing_dev_info,
	46	};
1da177e4 LT	47
	48	#define INC_CACHE_INFO(x) do { swap_cache_info.x++; } while (0)
	49
	50	static struct {
	51	unsigned long add_total;
	52	unsigned long del_total;
	53	unsigned long find_success;
	54	unsigned long find_total;
1da177e4 LT	55	} swap_cache_info;
	56
	57	void show_swap_cache_info(void)
	58	{
bb63be0a	59	printk("Swap cache: add %lu, delete %lu, find %lu/%lu\n",
1da177e4	60	swap_cache_info.add_total, swap_cache_info.del_total,
bb63be0a	61	swap_cache_info.find_success, swap_cache_info.find_total);
1da177e4 LT	62	printk("Free swap = %lukB\n", nr_swap_pages << (PAGE_SHIFT - 10));
	63	printk("Total swap = %lukB\n", total_swap_pages << (PAGE_SHIFT - 10));
	64	}
	65
	66	/*
f000944d	67	* add_to_swap_cache resembles add_to_page_cache on swapper_space,
1da177e4 LT	68	* but sets SwapCache flag and private instead of mapping and index.
1da177e4 LT	69	*/
73b1262f	70	int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
1da177e4 LT	71	{
	72	int error;
	73
b55ed816	74	BUG_ON(!PageLocked(page));
1da177e4 LT	75	BUG_ON(PageSwapCache(page));
1da177e4 LT	76	BUG_ON(PagePrivate(page));
35c754d7 BS	77	error = radix_tree_preload(gfp_mask);
35c754d7 BS	78	if (!error) {
1da177e4 LT	79	write_lock_irq(&swapper_space.tree_lock);
	80	error = radix_tree_insert(&swapper_space.page_tree,
	81	entry.val, page);
	82	if (!error) {
	83	page_cache_get(page);
1da177e4	84	SetPageSwapCache(page);
4c21e2f2	85	set_page_private(page, entry.val);
1da177e4	86	total_swapcache_pages++;
347ce434	87	__inc_zone_page_state(page, NR_FILE_PAGES);
bb63be0a	88	INC_CACHE_INFO(add_total);
1da177e4 LT	89	}
	90	write_unlock_irq(&swapper_space.tree_lock);
	91	radix_tree_preload_end();
fa1de900	92	}
1da177e4 LT	93	return error;
	94	}
	95
1da177e4 LT	96	/*
	97	* This must be called only on pages that have
	98	* been verified to be in the swap cache.
	99	*/
	100	void __delete_from_swap_cache(struct page *page)
	101	{
	102	BUG_ON(!PageLocked(page));
	103	BUG_ON(!PageSwapCache(page));
	104	BUG_ON(PageWriteback(page));
3279ffd9	105	BUG_ON(PagePrivate(page));
1da177e4	106
4c21e2f2 HD	107	radix_tree_delete(&swapper_space.page_tree, page_private(page));
4c21e2f2 HD	108	set_page_private(page, 0);
1da177e4 LT	109	ClearPageSwapCache(page);
1da177e4 LT	110	total_swapcache_pages--;
347ce434	111	__dec_zone_page_state(page, NR_FILE_PAGES);
1da177e4 LT	112	INC_CACHE_INFO(del_total);
	113	}
	114
	115	/**
	116	* add_to_swap - allocate swap space for a page
	117	* @page: page we want to move to swap
	118	*
	119	* Allocate swap space for the page and add the page to the
	120	* swap cache. Caller needs to hold the page lock.
	121	*/
1480a540	122	int add_to_swap(struct page * page, gfp_t gfp_mask)
1da177e4 LT	123	{
1da177e4 LT	124	swp_entry_t entry;
1da177e4 LT	125	int err;
1da177e4 LT	126
e74ca2b4	127	BUG_ON(!PageLocked(page));
0ed361de	128	BUG_ON(!PageUptodate(page));
1da177e4 LT	129
	130	for (;;) {
	131	entry = get_swap_page();
	132	if (!entry.val)
	133	return 0;
	134
bd53b714 NP	135	/*
	136	* Radix-tree node allocations from PF_MEMALLOC contexts could
	137	* completely exhaust the page allocator. __GFP_NOMEMALLOC
	138	* stops emergency reserves from being allocated.
1da177e4	139	*
bd53b714 NP	140	* TODO: this could cause a theoretical memory reclaim
bd53b714 NP	141	* deadlock in the swap out path.
1da177e4	142	*/
1da177e4 LT	143	/*
	144	* Add it to the swap cache and mark it dirty
	145	*/
f000944d	146	err = add_to_swap_cache(page, entry,
1480a540	147	gfp_mask\|__GFP_NOMEMALLOC\|__GFP_NOWARN);
1da177e4 LT	148
	149	switch (err) {
	150	case 0: /* Success */
1da177e4	151	SetPageDirty(page);
1da177e4 LT	152	return 1;
	153	case -EEXIST:
	154	/* Raced with "speculative" read_swap_cache_async */
1da177e4 LT	155	swap_free(entry);
	156	continue;
	157	default:
	158	/* -ENOMEM radix-tree allocation failure */
	159	swap_free(entry);
	160	return 0;
	161	}
	162	}
	163	}
	164
	165	/*
	166	* This must be called only on pages that have
	167	* been verified to be in the swap cache and locked.
	168	* It will never put the page into the free list,
	169	* the caller has a reference on the page.
	170	*/
	171	void delete_from_swap_cache(struct page *page)
	172	{
	173	swp_entry_t entry;
	174
4c21e2f2	175	entry.val = page_private(page);
1da177e4 LT	176
	177	write_lock_irq(&swapper_space.tree_lock);
	178	__delete_from_swap_cache(page);
	179	write_unlock_irq(&swapper_space.tree_lock);
	180
	181	swap_free(entry);
	182	page_cache_release(page);
	183	}
	184
1da177e4 LT	185	/*
	186	* If we are the only user, then try to free up the swap cache.
	187	*
	188	* Its ok to check for PageSwapCache without the page lock
	189	* here because we are going to recheck again inside
	190	* exclusive_swap_page() _with_ the lock.
	191	* - Marcelo
	192	*/
	193	static inline void free_swap_cache(struct page *page)
	194	{
	195	if (PageSwapCache(page) && !TestSetPageLocked(page)) {
	196	remove_exclusive_swap_page(page);
	197	unlock_page(page);
	198	}
	199	}
	200
	201	/*
	202	* Perform a free_page(), also freeing any swap cache associated with
b8072f09	203	* this page if it is the last user of the page.
1da177e4 LT	204	*/
	205	void free_page_and_swap_cache(struct page *page)
	206	{
	207	free_swap_cache(page);
	208	page_cache_release(page);
	209	}
	210
	211	/*
	212	* Passed an array of pages, drop them all from swapcache and then release
	213	* them. They are removed from the LRU and freed if this is their last use.
	214	*/
	215	void free_pages_and_swap_cache(struct page **pages, int nr)
	216	{
1da177e4 LT	217	struct page **pagep = pages;
	218
	219	lru_add_drain();
	220	while (nr) {
c484d410	221	int todo = min(nr, PAGEVEC_SIZE);
1da177e4 LT	222	int i;
	223
	224	for (i = 0; i < todo; i++)
	225	free_swap_cache(pagep[i]);
	226	release_pages(pagep, todo, 0);
	227	pagep += todo;
	228	nr -= todo;
	229	}
	230	}
	231
	232	/*
	233	* Lookup a swap entry in the swap cache. A found page will be returned
	234	* unlocked and with its refcount incremented - we rely on the kernel
	235	* lock getting page table operations atomic even if we drop the page
	236	* lock before returning.
	237	*/
	238	struct page * lookup_swap_cache(swp_entry_t entry)
	239	{
	240	struct page *page;
	241
	242	page = find_get_page(&swapper_space, entry.val);
	243
	244	if (page)
	245	INC_CACHE_INFO(find_success);
	246
	247	INC_CACHE_INFO(find_total);
	248	return page;
	249	}
	250
	251	/*
	252	* Locate a page of swap in physical memory, reserving swap cache space
	253	* and reading the disk if it is not already cached.
	254	* A failure return means that either the page allocation failed or that
	255	* the swap entry is no longer in use.
	256	*/
02098fea	257	struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
1da177e4 LT	258	struct vm_area_struct *vma, unsigned long addr)
	259	{
	260	struct page found_page, new_page = NULL;
	261	int err;
	262
	263	do {
	264	/*
	265	* First check the swap cache. Since this is normally
	266	* called after lookup_swap_cache() failed, re-calling
	267	* that would confuse statistics.
	268	*/
	269	found_page = find_get_page(&swapper_space, entry.val);
	270	if (found_page)
	271	break;
	272
	273	/*
	274	* Get a new page to read into from swap.
	275	*/
	276	if (!new_page) {
02098fea	277	new_page = alloc_page_vma(gfp_mask, vma, addr);
1da177e4 LT	278	if (!new_page)
	279	break; /* Out of memory */
	280	}
	281
f000944d HD	282	/*
	283	* Swap entry may have been freed since our caller observed it.
	284	*/
	285	if (!swap_duplicate(entry))
	286	break;
	287
1da177e4 LT	288	/*
1da177e4 LT	289	* Associate the page with swap entry in the swap cache.
f000944d HD	290	* May fail (-EEXIST) if there is already a page associated
	291	* with this entry in the swap cache: added by a racing
	292	* read_swap_cache_async, or add_to_swap or shmem_writepage
	293	* re-using the just freed swap entry for an existing page.
1da177e4 LT	294	* May fail (-ENOMEM) if radix-tree node allocation failed.
1da177e4 LT	295	*/
f000944d HD	296	SetPageLocked(new_page);
f000944d HD	297	err = add_to_swap_cache(new_page, entry, gfp_mask & GFP_KERNEL);
1da177e4 LT	298	if (!err) {
	299	/*
	300	* Initiate read into locked page and return.
	301	*/
	302	lru_cache_add_active(new_page);
	303	swap_readpage(NULL, new_page);
	304	return new_page;
	305	}
f000944d HD	306	ClearPageLocked(new_page);
	307	swap_free(entry);
	308	} while (err != -ENOMEM);
1da177e4 LT	309
	310	if (new_page)
	311	page_cache_release(new_page);
	312	return found_page;
	313	}
46017e95 HD	314
	315	/**
	316	* swapin_readahead - swap in pages in hope we need them soon
	317	* @entry: swap entry of this memory
	318	* @vma: user vma this address belongs to
	319	* @addr: target address for mempolicy
	320	*
	321	* Returns the struct page for entry and addr, after queueing swapin.
	322	*
	323	* Primitive swap readahead code. We simply read an aligned block of
	324	* (1 << page_cluster) entries in the swap area. This method is chosen
	325	* because it doesn't cost us any seek time. We also make sure to queue
	326	* the 'original' request together with the readahead ones...
	327	*
	328	* This has been extended to use the NUMA policies from the mm triggering
	329	* the readahead.
	330	*
	331	* Caller must hold down_read on the vma->vm_mm if vma is not NULL.
	332	*/
02098fea	333	struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
46017e95 HD	334	struct vm_area_struct *vma, unsigned long addr)
	335	{
	336	int nr_pages;
	337	struct page *page;
	338	unsigned long offset;
	339	unsigned long end_offset;
	340
	341	/*
	342	* Get starting offset for readaround, and number of pages to read.
	343	* Adjust starting address by readbehind (for NUMA interleave case)?
	344	* No, it's very unlikely that swap layout would follow vma layout,
	345	* more likely that neighbouring swap pages came from the same node:
	346	* so use the same "addr" to choose the same node for each swap read.
	347	*/
	348	nr_pages = valid_swaphandles(entry, &offset);
	349	for (end_offset = offset + nr_pages; offset < end_offset; offset++) {
	350	/* Ok, do the async read-ahead now */
	351	page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
02098fea	352	gfp_mask, vma, addr);
46017e95 HD	353	if (!page)
	354	break;
	355	page_cache_release(page);
	356	}
	357	lru_add_drain(); /* Push any new pages onto the LRU now */
02098fea	358	return read_swap_cache_async(entry, gfp_mask, vma, addr);
46017e95	359	}