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mm: try_to_unmap_cluster() should lock_page() before mlocking

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A BUG_ON(!PageLocked) was triggered in mlock_vma_page() by Sasha Levin
fuzzing with trinity.  The call site try_to_unmap_cluster() does not lock
the pages other than its check_page parameter (which is already locked).

The BUG_ON in mlock_vma_page() is not documented and its purpose is
somewhat unclear, but apparently it serializes against page migration,
which could otherwise fail to transfer the PG_mlocked flag.  This would
not be fatal, as the page would be eventually encountered again, but
NR_MLOCK accounting would become distorted nevertheless.  This patch adds
a comment to the BUG_ON in mlock_vma_page() and munlock_vma_page() to that
effect.

The call site try_to_unmap_cluster() is fixed so that for page !=
check_page, trylock_page() is attempted (to avoid possible deadlocks as we
already have check_page locked) and mlock_vma_page() is performed only
upon success.  If the page lock cannot be obtained, the page is left
without PG_mlocked, which is again not a problem in the whole unevictable
memory design.

Signed-off-by: Vlastimil Babka <[email protected]>
Signed-off-by: Bob Liu <[email protected]>
Reported-by: Sasha Levin <[email protected]>
Cc: Wanpeng Li <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Vlastimil Babka 2014-04-07 15:37:50 -07:00 • committed by Linus Torvalds
commit 57e68e9cd6
2 changed files with 14 additions and 2 deletions
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14
mm/rmap.c
View file

@ -1332,9 +1332,19 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
BUG_ON(!page || PageAnon(page));
if (locked_vma) {
mlock_vma_page(page); /* no-op if already mlocked */
if (page == check_page)
if (page == check_page) {
/* we know we have check_page locked */
mlock_vma_page(page);
ret = SWAP_MLOCK;
} else if (trylock_page(page)) {
/*
* If we can lock the page, perform mlock.
* Otherwise leave the page alone, it will be
* eventually encountered again later.
*/
mlock_vma_page(page);
unlock_page(page);
}
continue; /* don't unmap */
}