[PATCH 11/12] mm: Take the RCU read lock in rmap_walk_anon

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

rmap_walk_anon() does not use page_lock_anon_vma() for looking up and
locking an anon_vma. One important difference between page_lock_anon_vma()
and rmap_walk_anon() is that the page_lock_anon_vma() takes the RCU lock
before the lookup so that the anon_vma does not disappear. There does not
appear to be locking in place that prevents the anon_vma disappearing before
the spinlock is taken.

This patch puts a rcu_read_lock() around the anon_vma lookup similar to
what page_lock_anon_vma() does to prevent an accidental use-after-free.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
---
 mm/rmap.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index b468d5f..fb695d3 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1233,9 +1233,10 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
 	 * This needs to be reviewed later: avoiding page_lock_anon_vma()
 	 * is risky, and currently limits the usefulness of rmap_walk().
 	 */
+	rcu_read_lock();
 	anon_vma = page_anon_vma(page);
 	if (!anon_vma)
-		return ret;
+		goto out_rcu_unlock;
 	spin_lock(&anon_vma->lock);
 
 	/*
@@ -1256,6 +1257,10 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
 
 out_anon_unlock:
 	spin_unlock(&anon_vma->lock);
+
+out_rcu_unlock:
+	rcu_read_unlock();
+
 	return ret;
 }
 
-- 
1.6.5

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