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

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Date:	Fri, 1 May 2009 21:02:06 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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

* Hugh Dickins <hugh@...itas.com> [2009-05-01 14:55:51]:

> On Thu, 30 Apr 2009, KAMEZAWA Hiroyuki wrote:
> > 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.
> 
> I agree with you, it seems strange for it to come alive only now;
> but I've not investigated further, may I leave that to you?
> 
> Could it be that all those checks on NULL lookup_page_cgroup()
> actually date from before you reworked page cgroup assignment,
> and they're now redundant?  If so, you'd do better to remove
> all the checks, and Balbir put an explicit check in his code.
>

I agree, it needs investigation. I would propose converting them to a
VM_BUG_ON() and then consider removing them, just to catch potential
problems, in case we miss anything.
 
> Alternatively, could the SPARSEMEM case have been corrupting or
> otherwise misbehaving in a hidden way until now?  Seems unlikely.

Agreed.

-- 
	Balbir
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