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:
[<ffffffff8107100f>] warn_slowpath_common+0x7f/0xc0
[<ffffffff8107106a>] warn_slowpath_null+0x1a/0x20
[<ffffffff811903fc>] shmem_getpage_gfp+0xa5c/0xa70
[<ffffffff81190e4f>] shmem_fault+0x4f/0xa0
[<ffffffff8119f391>] __do_fault+0x71/0x5c0
[<ffffffff811a2767>] handle_pte_fault+0x97/0xae0
[<ffffffff811a4a39>] handle_mm_fault+0x289/0x350
[<ffffffff816d091e>] __do_page_fault+0x18e/0x530
[<ffffffff816d0ceb>] do_page_fault+0x2b/0x50
[<ffffffff816cd3b8>] page_fault+0x28/0x30
[<ffffffff816d5688>] 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.
Reported-by: Dave Jones <davej@xxxxxxxxxx>
Hypothesis-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
mm/shmem.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
--- 3.7-rc4/mm/shmem.c 2012-10-14 16:16:58.361309122 -0700
+++ linux/mm/shmem.c 2012-11-01 14:31:04.288185742 -0700
@@ -1145,8 +1145,22 @@ 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) {
+ VM_BUG_ON(error != -ENOENT);
+ delete_from_swap_cache(page);
+ }
}
if (error)
goto failed;
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>