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 67afba5117f2ebe80ef54e487d068fdf99e986bb..dc12264f44f571ea6c88c8e2d5ad42ce1c7087bd 100644 (file)
--- a/mm/shmem.c
+++ b/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;