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

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Date:	Mon, 11 Jun 2012 11:29:25 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] memcg: fix use_hierarchy css_is_ancestor oops regression

Johannes Weiner wrote:
> On Sun, Jun 10, 2012 at 11:54:47AM -0700, Hugh Dickins wrote:
>> 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>
>
> Looks like an older version of the patch that introduced it slipped
> into the tree, Konstantin noted this problem during review.  The final
> version did
>
> 	match = memcg&&  __mem_cgroup_same_or_subtree(root, memcg);
>
> in the caller because of it.
>
> Do you think it would be cleaner this way, since this is also the
> place where that memcg is looked up, and so the "can return NULL"
> handling after mem_cgroup_from_task() would be in the same place?

I agree, it cleaner, but nevertheless:

Acked-by: Konstantin Khlebnikov <khlebnikov@...nvz.org>

Thanks, Hugh!

>
> But either way,
>
> Acked-by: Johannes Weiner<hannes@...xchg.org>
>
> Thanks, Hugh!
>
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