linux-kernel - Re: [PATCH mmotm] memcg: fix mem_cgroup_update_mapped_file

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Date:	Thu, 30 Apr 2009 10:22:40 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Hugh Dickins <hugh@...itas.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mmotm] memcg: fix mem_cgroup_update_mapped_file_stat
	oops

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-04-30 09:06:46]:

> On Wed, 29 Apr 2009 22:13:33 +0100 (BST)
> Hugh Dickins <hugh@...itas.com> wrote:
> 
> > CONFIG_SPARSEMEM=y CONFIG_CGROUP_MEM_RES_CTLR=y cgroup_disable=memory
> > bootup is oopsing in mem_cgroup_update_mapped_file_stat().  !SPARSEMEM
> > is fine because its lookup_page_cgroup() contains an explicit check for
> > NULL node_page_cgroup, but the SPARSEMEM version was missing a check for
> > NULL section->page_cgroup.
> > 
> Ouch, it's curious this bug alive now.. thank you.
> 
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> 
> I think this patch itself is sane but.. Balbir, could you see "caller" ?
> It seems strange.

Ideally we need to have a disabled check in
mem_cgroup_update_mapped_file_stat(), but it seems as if this fix is
better and fixes a larger scenario and the root cause of
lookup_page_cgroup() OOPSing. It would not hurt to check for
mem_cgroup_disabled() though, but too many checks might spoil the
party for frequent operations.

Kame, do you mean you wanted me to check if I am using
lookup_page_cgroup() correctly?

Hugh, Thank you very much for finding and fixing the problem!
Acked-by: Balbir Singh <balbir@...ux.vnet.ibm.com>
 

-- 
	Balbir
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