linux-kernel - Re: [PATCH 1/8] cgroup: implement cgroup_is

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Message-ID: <20130408213646.GB17159@mtj.dyndns.org>
Date:	Mon, 8 Apr 2013 14:36:46 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Li Zefan <lizefan@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Glauber Costa <glommer@...allels.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 1/8] cgroup: implement cgroup_is_ancestor()

On Mon, Apr 08, 2013 at 08:03:44PM +0200, Michal Hocko wrote:
> __mem_cgroup_same_or_subtree relies on css_is_ancestor if hierarchy is
> enabled for ages. This, however, is not correct because use_hierarchy
> doesn't need to be true all the way up the cgroup hierarchy. Consider
> the following example:
> root (use_hierarchy=0)
>  \
>   A (use_hierarchy=0)
>    \
>     B (use_hierarchy=1)
>      \
>       C (use_hierarchy=1)
> 
> __mem_cgroup_same_or_subtree(A, C) would return true even though C is
> not from the same hierarchy subtree. The bug shouldn't be critical but
> at least dump_tasks might print unrelated tasks (via
> task_in_mem_cgroup).

Huh?  Isn't that avoided by the !root_memcg->use_hierarchy test?

> @@ -1470,9 +1470,12 @@ bool __mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
>  {
>  	if (root_memcg == memcg)
>  		return true;
> -	if (!root_memcg->use_hierarchy || !memcg)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^
> +	if (!memcg)
>  		return false;
> -	return css_is_ancestor(&memcg->css, &root_memcg->css);
> +	while ((memcg = parent_mem_cgroup(memcg)))
> +		if (memcg == root_memcg)
> +			return true;
> +	return false;
>  }
>  
>  static bool mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
> -- 
> 1.7.10.4
> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
tejun
--
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