[PATCH 12/12] mm: Check the anon_vma is still valid in rmap_walk_anon()

[Mel Gorman]

Despite the additional locking added around rmap_walk_anon, bad
references still manage to trigger on ppc64. The most likely cause is a
use-after-free but it's not clear if it's due to a locking problem or
something ppc64 specific. This patch somewhat works around the problem
by checking the contents of the anon_vma make sense before using it but
it needs reviewing by eyes familiar with the page migration code to try
spot where the real problem lies.

Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
---
 mm/rmap.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index fb695d3..462ac86 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1237,6 +1237,8 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
 	anon_vma = page_anon_vma(page);
 	if (!anon_vma)
 		goto out_rcu_unlock;
+	if (!anon_vma->head.next)
+		goto out_rcu_unlock;
 	spin_lock(&anon_vma->lock);
 
 	/*
-- 
1.6.5

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