From eb0bb60c8d97c9173c6a662420d269b6c86cec8c Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Fri, 9 Nov 2012 14:03:40 +1100 Subject: [PATCH] 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. Reported-by: Dave Jones Cc: Johannes Weiner Signed-off-by: Hugh Dickins Cc: Signed-off-by: Andrew Morton --- mm/shmem.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/mm/shmem.c b/mm/shmem.c index 67afba5117f2..0c6c0fd51997 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -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; -- 2.39.5