linux-kernel - Re: [PATCH v3 2/6] memcg: root_cgroup cannot reach mem_cgroup_move

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Message-ID: <20121029135203.GA20757@dhcp22.suse.cz>
Date:	Mon, 29 Oct 2012 14:52:03 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <bsingharora@...il.com>
Subject: Re: [PATCH v3 2/6] memcg: root_cgroup cannot reach
 mem_cgroup_move_parent

On Mon 29-10-12 17:48:00, Glauber Costa wrote:
> On 10/26/2012 03:37 PM, Michal Hocko wrote:
> > The root cgroup cannot be destroyed so we never hit it down the
> > mem_cgroup_pre_destroy path and mem_cgroup_force_empty_write shouldn't
> > even try to do anything if called for the root.
> > 
> > This means that mem_cgroup_move_parent doesn't have to bother with the
> > root cgroup and it can assume it can always move charges upwards.
> > 
> > Signed-off-by: Michal Hocko <mhocko@...e.cz>
> > Reviewed-by: Tejun Heo <tj@...nel.org>
> 
> I think it would be safer to have this folded in the last patch, to
> avoid a weird intermediate state (specially for force_empty).

force_empty excludes root cgroup explicitly so there is no way to fail
here. I have kept VM_BUG_ON for future reference but it also can go away
completely.

> Being a single statement, it doesn't confuse review so much.
> 
> However, this is also pretty much just a nitpick, do as you prefer.
> 
> Reviewed-by: Glauber Costa <glommer@...allels.com>
> 

-- 
Michal Hocko
SUSE Labs
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