Re: [PATCH mmotm] memcg: fix mem_cgroup_update_mapped_file_stat oops

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* Hugh Dickins <hugh@veritas.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@veritas.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@jp.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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