From: Hugh Dickins <[email protected]> Subject: 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: [<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. Signed-off-by: Hugh Dickins <[email protected]> Reported-by: Dave Jones <[email protected]> Cc: Johannes Weiner <[email protected]> Cc: <[email protected]> Signed-off-by: Andrew Morton <[email protected]> --- mm/shmem.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) diff -puN mm/shmem.c~tmpfs-fix-shmem_getpage_gfp-vm_bug_on mm/shmem.c --- a/mm/shmem.c~tmpfs-fix-shmem_getpage_gfp-vm_bug_on +++ a/mm/shmem.c @@ -1145,8 +1145,20 @@ 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; _ -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
