[patch 1/2] mm: memcontrol: handle potential crash when rmap races with task exit

[Johannes Weiner <hannes@cmpxchg.org>]

page_referenced() counts only references of mm's that are associated
with the memcg hierarchy that is being reclaimed.  However, if it
races with the owner of the mm exiting, mm->owner may be NULL.  Don't
crash, just ignore the reference.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: stable@kernel.org [3.5]
---
 include/linux/memcontrol.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 8d9489f..8686294 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -91,7 +91,7 @@ int mm_match_cgroup(const struct mm_struct *mm, const struct mem_cgroup *cgroup)
 
 	rcu_read_lock();
 	memcg = mem_cgroup_from_task(rcu_dereference((mm)->owner));
-	match = __mem_cgroup_same_or_subtree(cgroup, memcg);
+	match = memcg && __mem_cgroup_same_or_subtree(cgroup, memcg);
 	rcu_read_unlock();
 	return match;
 }
-- 
1.7.11.4

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