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authorMel Gorman <mel@csn.ul.ie>2010-05-24 14:32:19 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2010-05-25 08:06:58 -0700
commit67b9509b2c68ae38cecb83a239881cb0ddf087dc (patch)
treea8cee4f4bec57ab44a993ce81df2a10a4980f0f8
parent7f60c214fd3a360461f3286c6908084f7f8b1950 (diff)
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>
-rw-r--r--mm/migrate.c11
1 files changed, 11 insertions, 0 deletions
diff --git a/mm/migrate.c b/mm/migrate.c
index 42a3d24d1107..b114635962dc 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -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);
}