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From: Mel Gorman on 18 Feb 2010 13:10
rmap_walk_anon() was triggering errors in memory compaction that looks like
use-after-free errors in anon_vma. The problem appears to be 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 was freed. This patch
skips the migration of anon pages that are not mapped by anyone.

Signed-off-by: Mel Gorman <mel(a)csn.ul.ie>
---
mm/migrate.c | 10 ++++++++++
1 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 63addfa..1ce6a2f 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -606,6 +606,16 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
* just care Anon page here.
*/
if (PageAnon(page)) {
+ /*
+ * 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_mapcount(page))
+ goto uncharge;
+
rcu_read_lock();
rcu_locked = 1;
anon_vma = page_anon_vma(page);
--
1.6.5

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