mm/swap_state.c - kernel/msm-4.9 - Gitiles

 /*
  *  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/mm.h>
 #include <linux/gfp.h>
 #include <linux/kernel_stat.h>
 #include <linux/swap.h>
 #include <linux/swapops.h>
 #include <linux/init.h>
 #include <linux/pagemap.h>
 #include <linux/backing-dev.h>
 #include <linux/blkdev.h>
 #include <linux/pagevec.h>
 #include <linux/migrate.h>
 #include <linux/page_cgroup.h>

 #include <asm/pgtable.h>

 /*
  * swapper_space is a fiction, retained to simplify the path through
  * vmscan's shrink_page_list.
  */
 static const struct address_space_operations swap_aops = {
 	.writepage	= swap_writepage,
 	.set_page_dirty	= swap_set_page_dirty,
 	.migratepage	= migrate_page,
 };

 static struct backing_dev_info swap_backing_dev_info = {
 	.name		= "swap",
 	.capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
 };

 struct address_space swapper_spaces[MAX_SWAPFILES] = {
 	[0 ... MAX_SWAPFILES - 1] = {
 		.page_tree	= RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
 		.a_ops		= &swap_aops,
 		.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;

 unsigned long total_swapcache_pages(void)
 {
 	int i;
 	unsigned long ret = 0;

 	for (i = 0; i < MAX_SWAPFILES; i++)
 		ret += swapper_spaces[i].nrpages;
 	return ret;
 }

 void show_swap_cache_info(void)
 {
 	printk("%lu pages in swap cache\n", total_swapcache_pages());
 	printk("Swap cache stats: 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  = %ldkB\n",
 		get_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_locked 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)
 {
 	int error;
 	struct address_space *address_space;

 	VM_BUG_ON(!PageLocked(page));
 	VM_BUG_ON(PageSwapCache(page));
 	VM_BUG_ON(!PageSwapBacked(page));

 	page_cache_get(page);
 	SetPageSwapCache(page);
 	set_page_private(page, entry.val);

 	address_space = swap_address_space(entry);
 	spin_lock_irq(&address_space->tree_lock);
 	error = radix_tree_insert(&address_space->page_tree,
 					entry.val, page);
 	if (likely(!error)) {
 		address_space->nrpages++;
 		__inc_zone_page_state(page, NR_FILE_PAGES);
 		INC_CACHE_INFO(add_total);
 	}
 	spin_unlock_irq(&address_space->tree_lock);

 	if (unlikely(error)) {
 		/*
 		 * Only the context which have set SWAP_HAS_CACHE flag
 		 * would call add_to_swap_cache().
 		 * So add_to_swap_cache() doesn't returns -EEXIST.
 		 */
 		VM_BUG_ON(error == -EEXIST);
 		set_page_private(page, 0UL);
 		ClearPageSwapCache(page);
 		page_cache_release(page);
 	}

 	return error;
 }


 int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
 {
 	int error;

 	error = radix_tree_preload(gfp_mask);
 	if (!error) {
 		error = __add_to_swap_cache(page, entry);
 		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)
 {
 	swp_entry_t entry;
 	struct address_space *address_space;

 	VM_BUG_ON(!PageLocked(page));
 	VM_BUG_ON(!PageSwapCache(page));
 	VM_BUG_ON(PageWriteback(page));

 	entry.val = page_private(page);
 	address_space = swap_address_space(entry);
 	radix_tree_delete(&address_space->page_tree, page_private(page));
 	set_page_private(page, 0);
 	ClearPageSwapCache(page);
 	address_space->nrpages--;
 	__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)
 {
 	swp_entry_t entry;
 	int err;

 	VM_BUG_ON(!PageLocked(page));
 	VM_BUG_ON(!PageUptodate(page));

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

 	if (unlikely(PageTransHuge(page)))
 		if (unlikely(split_huge_page(page))) {
 			swapcache_free(entry, NULL);
 			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_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);

 	if (!err) {	/* Success */
 		SetPageDirty(page);
 		return 1;
 	} else {	/* -ENOMEM radix-tree allocation failure */
 		/*
 		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
 		 * clear SWAP_HAS_CACHE flag.
 		 */
 		swapcache_free(entry, NULL);
 		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;
 	struct address_space *address_space;

 	entry.val = page_private(page);

 	address_space = swap_address_space(entry);
 	spin_lock_irq(&address_space->tree_lock);
 	__delete_from_swap_cache(page);
 	spin_unlock_irq(&address_space->tree_lock);

 	swapcache_free(entry, page);
 	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
  * try_to_free_swap() _with_ the lock.
  * 					- Marcelo
  */
 static inline void free_swap_cache(struct page *page)
 {
 	if (PageSwapCache(page) && !page_mapped(page) && trylock_page(page)) {
 		try_to_free_swap(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(swap_address_space(entry), 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(swap_address_space(entry),
 					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 */
 		}

 		/*
 		 * call radix_tree_preload() while we can wait.
 		 */
 		err = radix_tree_preload(gfp_mask & GFP_KERNEL);
 		if (err)
 			break;

 		/*
 		 * Swap entry may have been freed since our caller observed it.
 		 */
 		err = swapcache_prepare(entry);
 		if (err == -EEXIST) {	/* seems racy */
 			radix_tree_preload_end();
 			continue;
 		}
 		if (err) {		/* swp entry is obsolete ? */
 			radix_tree_preload_end();
 			break;
 		}

 		/* May fail (-ENOMEM) if radix-tree node allocation failed. */
 		__set_page_locked(new_page);
 		SetPageSwapBacked(new_page);
 		err = __add_to_swap_cache(new_page, entry);
 		if (likely(!err)) {
 			radix_tree_preload_end();
 			/*
 			 * Initiate read into locked page and return.
 			 */
 			lru_cache_add_anon(new_page);
 			swap_readpage(new_page);
 			return new_page;
 		}
 		radix_tree_preload_end();
 		ClearPageSwapBacked(new_page);
 		__clear_page_locked(new_page);
 		/*
 		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
 		 * clear SWAP_HAS_CACHE flag.
 		 */
 		swapcache_free(entry, NULL);
 	} 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
  * @gfp_mask: memory allocation flags
  * @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)
 {
 	struct page *page;
 	unsigned long offset = swp_offset(entry);
 	unsigned long start_offset, end_offset;
 	unsigned long mask = (1UL << page_cluster) - 1;
 	struct blk_plug plug;

 	/* Read a page_cluster sized and aligned cluster around offset. */
 	start_offset = offset & ~mask;
 	end_offset = offset | mask;
 	if (!start_offset)	/* First page is swap header. */
 		start_offset++;

 	blk_start_plug(&plug);
 	for (offset = start_offset; 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)
 			continue;
 		page_cache_release(page);
 	}
 	blk_finish_plug(&plug);

 	lru_add_drain();	/* Push any new pages onto the LRU now */
 	return read_swap_cache_async(entry, gfp_mask, vma, addr);
 }
	/*
	* 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/mm.h>
	#include <linux/gfp.h>
	#include <linux/kernel_stat.h>
	#include <linux/swap.h>
	#include <linux/swapops.h>
	#include <linux/init.h>
	#include <linux/pagemap.h>
	#include <linux/backing-dev.h>
	#include <linux/blkdev.h>
	#include <linux/pagevec.h>
	#include <linux/migrate.h>
	#include <linux/page_cgroup.h>

	#include <asm/pgtable.h>

	/*
	* swapper_space is a fiction, retained to simplify the path through
	* vmscan's shrink_page_list.
	*/
	static const struct address_space_operations swap_aops = {
	.writepage = swap_writepage,
	.set_page_dirty = swap_set_page_dirty,
	.migratepage = migrate_page,
	};

	static struct backing_dev_info swap_backing_dev_info = {
	.name = "swap",
	.capabilities = BDI_CAP_NO_ACCT_AND_WRITEBACK \| BDI_CAP_SWAP_BACKED,
	};

	struct address_space swapper_spaces[MAX_SWAPFILES] = {
	[0 ... MAX_SWAPFILES - 1] = {
	.page_tree = RADIX_TREE_INIT(GFP_ATOMIC\|__GFP_NOWARN),
	.a_ops = &swap_aops,
	.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;

	unsigned long total_swapcache_pages(void)
	{
	int i;
	unsigned long ret = 0;

	for (i = 0; i < MAX_SWAPFILES; i++)
	ret += swapper_spaces[i].nrpages;
	return ret;
	}

	void show_swap_cache_info(void)
	{
	printk("%lu pages in swap cache\n", total_swapcache_pages());
	printk("Swap cache stats: 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 = %ldkB\n",
	get_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_locked 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)
	{
	int error;
	struct address_space *address_space;

	VM_BUG_ON(!PageLocked(page));
	VM_BUG_ON(PageSwapCache(page));
	VM_BUG_ON(!PageSwapBacked(page));

	page_cache_get(page);
	SetPageSwapCache(page);
	set_page_private(page, entry.val);

	address_space = swap_address_space(entry);
	spin_lock_irq(&address_space->tree_lock);
	error = radix_tree_insert(&address_space->page_tree,
	entry.val, page);
	if (likely(!error)) {
	address_space->nrpages++;
	__inc_zone_page_state(page, NR_FILE_PAGES);
	INC_CACHE_INFO(add_total);
	}
	spin_unlock_irq(&address_space->tree_lock);

	if (unlikely(error)) {
	/*
	* Only the context which have set SWAP_HAS_CACHE flag
	* would call add_to_swap_cache().
	* So add_to_swap_cache() doesn't returns -EEXIST.
	*/
	VM_BUG_ON(error == -EEXIST);
	set_page_private(page, 0UL);
	ClearPageSwapCache(page);
	page_cache_release(page);
	}

	return error;
	}


	int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
	{
	int error;

	error = radix_tree_preload(gfp_mask);
	if (!error) {
	error = __add_to_swap_cache(page, entry);
	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)
	{
	swp_entry_t entry;
	struct address_space *address_space;

	VM_BUG_ON(!PageLocked(page));
	VM_BUG_ON(!PageSwapCache(page));
	VM_BUG_ON(PageWriteback(page));

	entry.val = page_private(page);
	address_space = swap_address_space(entry);
	radix_tree_delete(&address_space->page_tree, page_private(page));
	set_page_private(page, 0);
	ClearPageSwapCache(page);
	address_space->nrpages--;
	__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)
	{
	swp_entry_t entry;
	int err;

	VM_BUG_ON(!PageLocked(page));
	VM_BUG_ON(!PageUptodate(page));

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

	if (unlikely(PageTransHuge(page)))
	if (unlikely(split_huge_page(page))) {
	swapcache_free(entry, NULL);
	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_HIGH\|__GFP_NOMEMALLOC\|__GFP_NOWARN);

	if (!err) { /* Success */
	SetPageDirty(page);
	return 1;
	} else { /* -ENOMEM radix-tree allocation failure */
	/*
	* add_to_swap_cache() doesn't return -EEXIST, so we can safely
	* clear SWAP_HAS_CACHE flag.
	*/
	swapcache_free(entry, NULL);
	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;
	struct address_space *address_space;

	entry.val = page_private(page);

	address_space = swap_address_space(entry);
	spin_lock_irq(&address_space->tree_lock);
	__delete_from_swap_cache(page);
	spin_unlock_irq(&address_space->tree_lock);

	swapcache_free(entry, page);
	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
	* try_to_free_swap() _with_ the lock.
	* - Marcelo
	*/
	static inline void free_swap_cache(struct page *page)
	{
	if (PageSwapCache(page) && !page_mapped(page) && trylock_page(page)) {
	try_to_free_swap(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(swap_address_space(entry), 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(swap_address_space(entry),
	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 */
	}

	/*
	* call radix_tree_preload() while we can wait.
	*/
	err = radix_tree_preload(gfp_mask & GFP_KERNEL);
	if (err)
	break;

	/*
	* Swap entry may have been freed since our caller observed it.
	*/
	err = swapcache_prepare(entry);
	if (err == -EEXIST) { /* seems racy */
	radix_tree_preload_end();
	continue;
	}
	if (err) { /* swp entry is obsolete ? */
	radix_tree_preload_end();
	break;
	}

	/* May fail (-ENOMEM) if radix-tree node allocation failed. */
	__set_page_locked(new_page);
	SetPageSwapBacked(new_page);
	err = __add_to_swap_cache(new_page, entry);
	if (likely(!err)) {
	radix_tree_preload_end();
	/*
	* Initiate read into locked page and return.
	*/
	lru_cache_add_anon(new_page);
	swap_readpage(new_page);
	return new_page;
	}
	radix_tree_preload_end();
	ClearPageSwapBacked(new_page);
	__clear_page_locked(new_page);
	/*
	* add_to_swap_cache() doesn't return -EEXIST, so we can safely
	* clear SWAP_HAS_CACHE flag.
	*/
	swapcache_free(entry, NULL);
	} 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
	* @gfp_mask: memory allocation flags
	* @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)
	{
	struct page *page;
	unsigned long offset = swp_offset(entry);
	unsigned long start_offset, end_offset;
	unsigned long mask = (1UL << page_cluster) - 1;
	struct blk_plug plug;

	/* Read a page_cluster sized and aligned cluster around offset. */
	start_offset = offset & ~mask;
	end_offset = offset \| mask;
	if (!start_offset) /* First page is swap header. */
	start_offset++;

	blk_start_plug(&plug);
	for (offset = start_offset; 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)
	continue;
	page_cache_release(page);
	}
	blk_finish_plug(&plug);

	lru_add_drain(); /* Push any new pages onto the LRU now */
	return read_swap_cache_async(entry, gfp_mask, vma, addr);
	}