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Commit 6922c0c7 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
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tmpfs: convert shmem_writepage and enable swap 



Convert shmem_writepage() to use shmem_delete_from_page_cache() to use
shmem_radix_tree_replace() to substitute swap entry for page pointer
atomically in the radix tree.

As with shmem_add_to_page_cache(), it's not entirely satisfactory to be
copying such code from delete_from_swap_cache, but again judged easier
to sell than making its other callers go through the extras.

Remove the toy implementation's shmem_put_swap() and shmem_get_swap(),
now unreferenced, and the hack to disable swap: it's now good to go.

The way things have worked out, info->lock no longer helps to guard the
shmem_swaplist: we increment swapped under shmem_swaplist_mutex only.
That global mutex exclusion between shmem_writepage() and shmem_unuse()
is not pretty, and we ought to find another way; but it's been forced on
us by recent race discoveries, not a consequence of this patchset.

And what has become of the WARN_ON_ONCE(1) free_swap_and_cache() if a
swap entry was found already present? That's no longer possible, the
(unknown) one inserting this page into filecache would hit the swap
entry occupying that slot.

Signed-off-by: default avatarHugh Dickins <hughd@google.com>
Acked-by: default avatarRik van Riel <riel@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent aa3b1895

mm/shmem.c

+37 −51
Original line number Diff line number Diff line
@@ -6,7 +6,8 @@ 
 *		 2000-2001 Christoph Rohland

 *		 2000-2001 SAP AG

 *		 2002 Red Hat Inc.

 * Copyright (C) 2002-2005 Hugh Dickins.

 * Copyright (C) 2002-2011 Hugh Dickins.

 * Copyright (C) 2011 Google Inc.

 * Copyright (C) 2002-2005 VERITAS Software Corporation.

 * Copyright (C) 2004 Andi Kleen, SuSE Labs

 *
@@ -219,19 +220,6 @@ static void shmem_recalc_inode(struct inode *inode) 
	}

}



static void shmem_put_swap(struct shmem_inode_info *info, pgoff_t index,

			   swp_entry_t swap)

{

	if (index < SHMEM_NR_DIRECT)

		info->i_direct[index] = swap;

}



static swp_entry_t shmem_get_swap(struct shmem_inode_info *info, pgoff_t index)

{

	return (index < SHMEM_NR_DIRECT) ?

		info->i_direct[index] : (swp_entry_t){0};

}



/*

 * Replace item expected in radix tree by a new item, while holding tree lock.

 */
@@ -299,6 +287,25 @@ static int shmem_add_to_page_cache(struct page *page, 
	return error;

}



/*

 * Like delete_from_page_cache, but substitutes swap for page.

 */

static void shmem_delete_from_page_cache(struct page *page, void *radswap)

{

	struct address_space *mapping = page->mapping;

	int error;



	spin_lock_irq(&mapping->tree_lock);

	error = shmem_radix_tree_replace(mapping, page->index, page, radswap);

	page->mapping = NULL;

	mapping->nrpages--;

	__dec_zone_page_state(page, NR_FILE_PAGES);

	__dec_zone_page_state(page, NR_SHMEM);

	spin_unlock_irq(&mapping->tree_lock);

	page_cache_release(page);

	BUG_ON(error);

}



/*

 * Like find_get_pages, but collecting swap entries as well as pages.

 */
@@ -664,14 +671,10 @@ int shmem_unuse(swp_entry_t swap, struct page *page) 
	mutex_lock(&shmem_swaplist_mutex);

	list_for_each_safe(this, next, &shmem_swaplist) {

		info = list_entry(this, struct shmem_inode_info, swaplist);

		if (!info->swapped) {

			spin_lock(&info->lock);

			if (!info->swapped)

				list_del_init(&info->swaplist);

			spin_unlock(&info->lock);

		}

		if (info->swapped)

			found = shmem_unuse_inode(info, swap, page);

		else

			list_del_init(&info->swaplist);

		cond_resched();

		if (found)

			break;
@@ -694,10 +697,10 @@ int shmem_unuse(swp_entry_t swap, struct page *page) 
static int shmem_writepage(struct page *page, struct writeback_control *wbc)

{

	struct shmem_inode_info *info;

	swp_entry_t swap, oswap;

	struct address_space *mapping;

	pgoff_t index;

	struct inode *inode;

	swp_entry_t swap;

	pgoff_t index;



	BUG_ON(!PageLocked(page));

	mapping = page->mapping;
@@ -720,55 +723,38 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) 
		WARN_ON_ONCE(1);	/* Still happens? Tell us about it! */

		goto redirty;

	}



	/*

	 * Disable even the toy swapping implementation, while we convert

	 * functions one by one to having swap entries in the radix tree.

	 */

	if (index < ULONG_MAX)

		goto redirty;



	swap = get_swap_page();

	if (!swap.val)

		goto redirty;



	/*

	 * Add inode to shmem_unuse()'s list of swapped-out inodes,

	 * if it's not already there.  Do it now because we cannot take

	 * mutex while holding spinlock, and must do so before the page

	 * is moved to swap cache, when its pagelock no longer protects

	 * if it's not already there.  Do it now before the page is

	 * moved to swap cache, when its pagelock no longer protects

	 * the inode from eviction.  But don't unlock the mutex until

	 * we've taken the spinlock, because shmem_unuse_inode() will

	 * prune a !swapped inode from the swaplist under both locks.

	 * we've incremented swapped, because shmem_unuse_inode() will

	 * prune a !swapped inode from the swaplist under this mutex.

	 */

	mutex_lock(&shmem_swaplist_mutex);

	if (list_empty(&info->swaplist))

		list_add_tail(&info->swaplist, &shmem_swaplist);



	spin_lock(&info->lock);

	mutex_unlock(&shmem_swaplist_mutex);



	oswap = shmem_get_swap(info, index);

	if (oswap.val) {

		WARN_ON_ONCE(1);	/* Still happens? Tell us about it! */

		free_swap_and_cache(oswap);

		shmem_put_swap(info, index, (swp_entry_t){0});

		info->swapped--;

	}

	shmem_recalc_inode(inode);



	if (add_to_swap_cache(page, swap, GFP_ATOMIC) == 0) {

		delete_from_page_cache(page);

		shmem_put_swap(info, index, swap);

		info->swapped++;

		swap_shmem_alloc(swap);

		shmem_delete_from_page_cache(page, swp_to_radix_entry(swap));



		spin_lock(&info->lock);

		info->swapped++;

		shmem_recalc_inode(inode);

		spin_unlock(&info->lock);



		mutex_unlock(&shmem_swaplist_mutex);

		BUG_ON(page_mapped(page));

		swap_writepage(page, wbc);

		return 0;

	}



	spin_unlock(&info->lock);

	mutex_unlock(&shmem_swaplist_mutex);

	swapcache_free(swap, NULL);

redirty:

	set_page_dirty(page);