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    Subject[PATCH stable 4.4 3/4] mm: filemap: fix mapping->nrpages double accounting in fuse
    Date
    From: Johannes Weiner <hannes@cmpxchg.org>

    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>
    Cc: stable@vger.kernel.org
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Michal Hocko <mhocko@suse.com>
    ---
    mm/filemap.c | 1 -
    1 file changed, 1 deletion(-)

    diff --git a/mm/filemap.c b/mm/filemap.c
    index 7ad648c9780c..c588d1222b2a 100644
    --- a/mm/filemap.c
    +++ b/mm/filemap.c
    @@ -590,7 +590,6 @@ int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
    __delete_from_page_cache(old, NULL, memcg);
    error = page_cache_tree_insert(mapping, new, NULL);
    BUG_ON(error);
    - mapping->nrpages++;

    /*
    * hugetlb pages do not participate in page cache accounting.
    --
    2.9.3
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