Commit 215c02bc authored Nov 16, 2012 by Hugh Dickins Committed by Linus Torvalds Nov 16, 2012

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>

parent 498c2280

mm/shmem.c

+14 −2

Original line number	Diff line number	Diff line
		@@ -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;

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