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Date: Tue, 24 Nov 2009 23:00:29 +0900
From: Daisuke Nishimura <d-nishimura@mtf.biglobe.ne.jp>
To: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
       stable <stable@kernel.org>, David Rientjes <rientjes@google.com>,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Subject: Re: [BUGFIX][PATCH -mmotm] memcg: avoid oom-killing innocent task
 in  case of use_hierarchy
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On Tue, 24 Nov 2009 19:01:54 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> On Tue, Nov 24, 2009 at 11:27 AM, Daisuke Nishimura
> <nishimura@mxp.nes.nec.co.jp> wrote:
> > task_in_mem_cgroup(), which is called by select_bad_process() to check whether
> > a task can be a candidate for being oom-killed from memcg's limit, checks
> > "curr->use_hierarchy"("curr" is the mem_cgroup the task belongs to).
> >
> > But this check return true(it's false positive) when:
> >
> >        <some path>/00          use_hierarchy == 0      <- hitting limit
> >          <some path>/00/aa     use_hierarchy == 1      <- "curr"
> >
> > This leads to killing an innocent task in 00/aa. This patch is a fix for this
> > bug. And this patch also fixes the arg for mem_cgroup_print_oom_info(). We
> > should print information of mem_cgroup which the task being killed, not current,
> > belongs to.
> >
> 
> Quick Question: What happens if <some path>/00 has no tasks in it
> after your patches?
> 
Nothing would happen because <some path>/00 never hit its limit.

The bug that this patch fixes is:

- create a dir <some path>/00 and set some limits.
- create a sub dir <some path>/00/aa w/o any limits, and enable hierarchy.
- run some programs in both in 00 and 00/aa. programs in 00 should be
  big enough to cause oom by its limit.
- when oom happens by 00's limit, tasks in 00/aa can also be killed.


Regards,
Daisuke Nishimura.
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