[PATCH 02/12] mm,migration: Do not try to migrate unmapped anonymous pages

[Mel Gorman <mel@csn.ul.ie>]

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@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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