tmpfs: fix shmem_getpage_gfp() VM_BUG_ON
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:
warn_slowpath_common+0x7f/0xc0
warn_slowpath_null+0x1a/0x20
shmem_getpage_gfp+0xa5c/0xa70
shmem_fault+0x4f/0xa0
__do_fault+0x71/0x5c0
handle_pte_fault+0x97/0xae0
handle_mm_fault+0x289/0x350
__do_page_fault+0x18e/0x530
do_page_fault+0x2b/0x50
page_fault+0x28/0x30
tracesys+0xe1/0xe6
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 <hughd@google.com>
Reported-by: Dave Jones <davej@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/shmem.c b/mm/shmem.c
index 67afba5..dc12264 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1145,8 +1145,20 @@
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;
