linux-kernel - Re: [RFC][PATCH 5/4] memcg: don't call res_counter

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Date:	Thu, 15 Jan 2009 09:47:50 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Li Zefan <lizf@...fujitsu.com>, Paul Menage <menage@...gle.com>
Subject: Re: [RFC][PATCH 5/4] memcg: don't call res_counter_uncharge when
	obsolete

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-01-15 12:24:16]:

> On Wed, 14 Jan 2009 19:25:39 +0530
> Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> 
> > * Daisuke Nishimura <nishimura@....nes.nec.co.jp> [2009-01-14 17:51:21]:
> > 
> > > This is a new one. Please review.
> > > 
> > > ===
> > > From: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> > > 
> > > mem_cgroup_get ensures that the memcg that has been got can be accessed
> > > even after the directory has been removed, but it doesn't ensure that parents
> > > of it can be accessed: parents might have been freed already by rmdir.
> > > 
> > > This causes a bug in case of use_hierarchy==1, because res_counter_uncharge
> > > climb up the tree.
> > > 
> > > Check if the memcg is obsolete, and don't call res_counter_uncharge when obsole.
> > > 
> > > Signed-off-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> > 
> > I liked the earlier, EBUSY approach that ensured that parents could
> > not go away if children exist. IMHO, the code has gotten too complex
> > and has too many corner cases. Time to revisit it.
> > 
> 
> But I don't like -EBUSY ;)
> 
> When rmdir() returns -EBUSY even if there are no (visible) children and tasks,
> our customer will take kdump and send it to me "please explain this kernel bug"
> 
> I'm sure it will happen ;)
>

OK, but memory.stat can show why the group is busy and with
move_to_parent() such issues should not occur right? I'll relook at
the code. Thanks for your input.

-- 
	Balbir
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