linux-kernel - Re: [BUGFIX][PATCH 1/5] memcg use correct scan number at reclaim

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Message-ID: <20090312040054.GE23583@balbir.in.ibm.com>
Date:	Thu, 12 Mar 2009 09:30:54 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [BUGFIX][PATCH 1/5] memcg use correct scan number at reclaim

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-03-12 12:51:24]:

> On Thu, 12 Mar 2009 09:19:18 +0530
> Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> 
> > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-03-12 09:55:16]:
> > 
> > > Andrew, this [1/5] is a bug fix, others are not.
> > > 
> > > ==
> > > From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > > 
> > > Even when page reclaim is under mem_cgroup, # of scan page is determined by
> > > status of global LRU. Fix that.
> > > 
> > > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > > ---
> > >  mm/vmscan.c |    2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > Index: mmotm-2.6.29-Mar10/mm/vmscan.c
> > > ===================================================================
> > > --- mmotm-2.6.29-Mar10.orig/mm/vmscan.c
> > > +++ mmotm-2.6.29-Mar10/mm/vmscan.c
> > > @@ -1470,7 +1470,7 @@ static void shrink_zone(int priority, st
> > >  		int file = is_file_lru(l);
> > >  		int scan;
> > > 
> > > -		scan = zone_page_state(zone, NR_LRU_BASE + l);
> > > +		scan = zone_nr_pages(zone, sc, l);
> > 
> > I have the exact same patch in my patch queue. BTW, mem_cgroup_zone_nr_pages is
> > buggy. We don't hold any sort of lock while extracting
> > MEM_CGROUP_ZSTAT (ideally we need zone->lru_lock). Without that how do
> > we guarantee that MEM_CGRUP_ZSTAT is not changing at the same time as
> > we are reading it?
> > 
> Is it big problem ? We don't need very precise value and ZSTAT just have
> increment/decrement. So, I tend to ignore this small race.
> (and it's unsigned long, not long long.)
>

The assumption is that unsigned long read is atomic even on 32 bit
systems? What if we get pre-empted in the middle of reading the data
and don't return back for long? The data can be highly in-accurate.
No? 

-- 
	Balbir
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