linux-kernel - Re: [RFC][PATCH 1/4] memcg: fix for mem_cgroup_get_reclaim_stat_from

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Message-Id: <20090109134736.a995fc49.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 9 Jan 2009 13:47:36 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	lizf@...fujitsu.com, menage@...gle.com,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>
Subject: Re: [RFC][PATCH 1/4] memcg: fix for
 mem_cgroup_get_reclaim_stat_from_page

On Fri, 9 Jan 2009 10:02:57 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> >  	pc = lookup_page_cgroup(page);
> > +	smp_rmb();
> 
> Do you really need the read memory barrier?
> 
Necessary.

> > +	if (!PageCgroupUsed(pc))
> > +		return NULL;
> > +
> 
> In this case we've hit a case where the page is valid and the pc is
> not. This does fix the problem, but won't this impact us getting
> correct reclaim stats and thus indirectly impact the working of
> pressure?
> 
 - If retruns NULL, only global LRU's status is updated. 

Because this page is not belongs to any memcg, we cannot update
any counters. But yes, your point is a concern.

Maybe moving acitvate_page() to
==
do_swap_page()
{
    
- activate_page()
   mem_cgroup_try_charge()..
   ....
   mem_cgroup_commit_charge()....
   ....
+  activate_page()   
}
==
is necessary. How do you think, kosaki ?

Thanks,
-Kame

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