Commit 57e68e9c authored Apr 07, 2014 by Vlastimil Babka Committed by Linus Torvalds Apr 07, 2014

mm: try_to_unmap_cluster() should lock_page() before mlocking



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 <vbabka@suse.cz>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.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 3a025760

mm/mlock.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -79,6 +79,7 @@ void clear_page_mlock(struct page *page)
		*/
		void mlock_vma_page(struct page *page)
		{
		/* Serialize with page migration */
		BUG_ON(!PageLocked(page));

		if (!TestSetPageMlocked(page)) {
		@@ -174,6 +175,7 @@ unsigned int munlock_vma_page(struct page *page)
		unsigned int nr_pages;
		struct zone *zone = page_zone(page);

		/* For try_to_munlock() and to serialize with page migration */
		BUG_ON(!PageLocked(page));

		/*

mm/rmap.c

+12 −2

Original line number	Diff line number	Diff line
		@@ -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 */
		}

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