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Date: Wed, 11 Nov 2009 10:49:34 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Subject: Re: [PATCH -mmotm 3/3] memcg: remove memcg_tasklist
Message-Id: <20091111104934.eee96fc4.kamezawa.hiroyu@jp.fujitsu.com>
In-Reply-To: <20091111103906.5c3563bb.nishimura@mxp.nes.nec.co.jp>
References: <20091106141011.3ded1551.nishimura@mxp.nes.nec.co.jp>
	<20091111103533.c634ff8d.nishimura@mxp.nes.nec.co.jp>
	<20091111103906.5c3563bb.nishimura@mxp.nes.nec.co.jp>
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To: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Andrew Morton <akpm@linux-foundation.org>, Balbir Singh <balbir@linux.vnet.ibm.com>, linux-mm <linux-mm@kvack.org>
List-ID: <linux-mm.kvack.org>

On Wed, 11 Nov 2009 10:39:06 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> memcg_tasklist was introduced at commit 7f4d454d(memcg: avoid deadlock caused
> by race between oom and cpuset_attach) instead of cgroup_mutex to fix a deadlock
> problem.  The cgroup_mutex, which was removed by the commit, in
> mem_cgroup_out_of_memory() was originally introduced at commit c7ba5c9e
> (Memory controller: OOM handling).
> 
> IIUC, the intention of this cgroup_mutex was to prevent task move during
> select_bad_process() so that situations like below can be avoided.
> 
>   Assume cgroup "foo" has exceeded its limit and is about to trigger oom.
>   1. Process A, which has been in cgroup "baa" and uses large memory, is just
>      moved to cgroup "foo". Process A can be the candidates for being killed.
>   2. Process B, which has been in cgroup "foo" and uses large memory, is just
>      moved from cgroup "foo". Process B can be excluded from the candidates for
>      being killed.
> 
> But these race window exists anyway even if we hold a lock, because
> __mem_cgroup_try_charge() decides wether it should trigger oom or not outside
> of the lock. So the original cgroup_mutex in mem_cgroup_out_of_memory and thus
> current memcg_tasklist has no use. And IMHO, those races are not so critical
> for users.
> 
> This patch removes it and make codes simpler.
> 
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

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