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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: Hugh Dickins <hughd@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2011-08-03 16:21:25 -07:00 committed by Linus Torvalds
parent aa3b189551
commit 6922c0c7ab
1 changed files with 37 additions and 51 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

88
mm/shmem.c
View File

@ -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 @@ out:
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);