[PATCH] memcg: fix use_hierarchy css_is_ancestor oops regression

[Hugh Dickins]

If use_hierarchy is set, reclaim testing soon oopses in css_is_ancestor()
called from __mem_cgroup_same_or_subtree() called from page_referenced():
when processes are exiting, it's easy for mm_match_cgroup() to pass along
a NULL memcg coming from a NULL mm->owner.

Check for that in __mem_cgroup_same_or_subtree().  Return true or false?
False because we cannot know if it was in the hierarchy, but also false
because it's better not to count a reference from an exiting process.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
This a 3.5-rc issue: not needed for stable.

 mm/memcontrol.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 3.5-rc2/mm/memcontrol.c	2012-05-30 08:17:19.400008280 -0700
+++ linux/mm/memcontrol.c	2012-06-10 08:39:39.618182396 -0700
@@ -1148,7 +1148,7 @@ bool __mem_cgroup_same_or_subtree(const
 {
 	if (root_memcg == memcg)
 		return true;
-	if (!root_memcg->use_hierarchy)
+	if (!root_memcg->use_hierarchy || !memcg)
 		return false;
 	return css_is_ancestor(&memcg->css, &root_memcg->css);
 }
--
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