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Subject[PATCH 101/241] tmpfs: fix shmem_getpage_gfp() VM_BUG_ON
Date -stable review patch.  If anyone has any objections, please let me know.


From: Hugh Dickins <>

commit 215c02bc33bbd5ff4d7379a909462d11f0103218 upstream.

Fuzzing with trinity hit the "impossible" VM_BUG_ON(error) (which Fedora
has converted to WARNING) in shmem_getpage_gfp():

WARNING: at mm/shmem.c:1151 shmem_getpage_gfp+0xa5c/0xa70()
Pid: 29795, comm: trinity-child4 Not tainted 3.7.0-rc2+ #49
Call Trace:

Thanks to Johannes for pointing to truncation: free_swap_and_cache()
only does a trylock on the page, so the page lock we've held since
before confirming swap is not enough to protect against truncation.

What cleanup is needed in this case? Just delete_from_swap_cache(),
which takes care of the memcg uncharge.

Signed-off-by: Hugh Dickins <>
Reported-by: Dave Jones <>
Cc: Johannes Weiner <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Herton Ronaldo Krzesinski <>
mm/shmem.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 06d48ca..c7f7a77 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1154,8 +1154,20 @@ repeat:
if (!error) {
error = shmem_add_to_page_cache(page, mapping, index,
gfp, swp_to_radix_entry(swap));
- /* We already confirmed swap, and make no allocation */
- VM_BUG_ON(error);
+ /*
+ * We already confirmed swap under page lock, and make
+ * no memory allocation here, so usually no possibility
+ * of error; but free_swap_and_cache() only trylocks a
+ * page, so it is just possible that the entry has been
+ * truncated or holepunched since swap was confirmed.
+ * shmem_undo_range() will have done some of the
+ * unaccounting, now delete_from_swap_cache() will do
+ * the rest (including mem_cgroup_uncharge_swapcache).
+ * Reset swap.val? No, leave it so "failed" goes back to
+ * "repeat": reading a hole and writing should succeed.
+ */
+ if (error)
+ delete_from_swap_cache(page);
if (error)
goto failed;

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