linux-kernel - [PATCH] memcg: fix use_hierarchy css_is

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Date:	Sun, 10 Jun 2012 11:54:47 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Johannes Weiner <hannes@...xchg.org>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: [PATCH] memcg: fix use_hierarchy css_is_ancestor oops regression

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@...gle.com>
---
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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