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@xxxxxxxxxx>
---
mm/shmem.c | 88 +++++++++++++++++++++------------------------------
1 file changed, 37 insertions(+), 51 deletions(-)
--- linux.orig/mm/shmem.c 2011-06-14 00:45:20.685161581 -0700
+++ linux/mm/shmem.c 2011-06-14 00:54:36.499917716 -0700
@@ -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 in
}
}
-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.
*/
@@ -300,6 +288,25 @@ static int shmem_add_to_page_cache(struc
}
/*
+ * 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.
*/
static unsigned shmem_find_get_pages_and_swap(struct address_space *mapping,
@@ -664,14 +671,10 @@ int shmem_unuse(swp_entry_t swap, struct
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 *
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);
--
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