mm: filemap: fix mapping->nrpages double accounting in fuse

commit 3ddf40e8c31964b744ff10abb48c8e36a83ec6e7 upstream.

Commit 22f2ac51b6d6 ("mm: workingset: fix crash in shadow node shrinker
caused by replace_page_cache_page()") switched replace_page_cache() from
raw radix tree operations to page_cache_tree_insert() but didn't take
into account that the latter function, unlike the raw radix tree op,
handles mapping->nrpages.  As a result, that counter is bumped for each
page replacement rather than balanced out even.

The mapping->nrpages counter is used to skip needless radix tree walks
when invalidating, truncating, syncing inodes without pages, as well as
statistics for userspace.  Since the error is positive, we'll do more
page cache tree walks than necessary; we won't miss a necessary one.
And we'll report more buffer pages to userspace than there are.  The
error is limited to fuse inodes.

Fixes: 22f2ac51b6d6 ("mm: workingset: fix crash in shadow node shrinker caused by replace_page_cache_page()")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/mm/filemap.c b/mm/filemap.c
index 0762825618700c1485292339fc0bab7c90cb12e7..7377ff2c1854fa63a89264ff8f5f02f441c7c30f 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -545,7 +545,6 @@ int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
                __delete_from_page_cache(old, NULL);
                error = page_cache_tree_insert(mapping, new, NULL);
                BUG_ON(error);
-               mapping->nrpages++;
                __inc_zone_page_state(new, NR_FILE_PAGES);
                if (PageSwapBacked(new))
                        __inc_zone_page_state(new, NR_SHMEM);