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Date: Wed, 14 Apr 2010 10:40:10 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
       LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
       Mel Gorman <mel@csn.ul.ie>, Rik van Riel <riel@redhat.com>,
       Minchan Kim <minchan.kim@gmail.com>,
       Balbir Singh <balbir@linux.vnet.ibm.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Christoph Lameter <cl@linux-foundation.org>,
       Andrea Arcangeli <aarcange@redhat.com>,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC][BUGFIX][PATCH] memcg: fix underflow of mapped_file stat
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On Wed, 14 Apr 2010 10:03:08 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> > > @@ -2563,6 +2565,15 @@ void mem_cgroup_end_migration(struct mem
> > >  	 */
> > >  	if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
> > >  		mem_cgroup_uncharge_page(target);
> > > +	else {
> > > +		/*
> > > +		 * When a migrated file cache is remapped, it's not charged.
> > > +		 * Verify it. Because we're under lock_page(), there are
> > > +		 * no race with uncharge.
> > > +		 */
> > > +		if (page_mapped(target))
> > > +			mem_cgroup_update_file_mapped(mem, target, 1);
> > > +	}
> > We cannot rely on page lock, because when we succeeded in page migration,
> > "target" = "newpage" has already unlocked in move_to_new_page(). So the "target"
> > can be removed from the radix-tree theoretically(it's not related to this
> > underflow problem, though).
> > Shouldn't we call lock_page(target) and check "if (!target->mapping)" to handle
> > this case(maybe in another patch) ?
> > 
> Sounds reasonable. I think about that.
> 

Ah, PageCgroupUsed() is already checked under lock_page_cgroup(). It's enough.

Thanks,
-Kame