Commit 67b9509b authored May 24, 2010 by Mel Gorman Committed by Linus Torvalds May 25, 2010

mm: migration: do not try to migrate unmapped anonymous pages



rmap_walk_anon() was triggering errors in memory compaction that look like
use-after-free errors.  The problem is that between the page being
isolated from the LRU and rcu_read_lock() being taken, the mapcount of the
page dropped to 0 and the anon_vma gets freed.  This can happen during
memory compaction if pages being migrated belong to a process that exits
before migration completes.  Hence, the use-after-free race looks like

 1. Page isolated for migration
 2. Process exits
 3. page_mapcount(page) drops to zero so anon_vma was no longer reliable
 4. unmap_and_move() takes the rcu_lock but the anon_vma is already garbage
 4. call try_to_unmap, looks up tha anon_vma and "locks" it but the lock
    is garbage.

This patch checks the mapcount after the rcu lock is taken.  If the
mapcount is zero, the anon_vma is assumed to be freed and no further
action is taken.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 7f60c214

mm/migrate.c

+11 −0

Original line number	Diff line number	Diff line
		@@ -600,6 +600,17 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
		if (PageAnon(page)) {
		rcu_read_lock();
		rcu_locked = 1;

		/*
		* If the page has no mappings any more, just bail. An
		* unmapped anon page is likely to be freed soon but worse,
		* it's possible its anon_vma disappeared between when
		* the page was isolated and when we reached here while
		* the RCU lock was not held
		*/
		if (!page_mapped(page))
		goto rcu_unlock;

		anon_vma = page_anon_vma(page);
		atomic_inc(&anon_vma->external_refcount);
		}

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