linux-kernel - Re: [PATCH -v2 4/6] memcg: make sure that memcg is not offline when charging

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Date:	Wed, 5 Feb 2014 11:45:43 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org
Subject: Re: [PATCH -v2 4/6] memcg: make sure that memcg is not offline when
 charging

On Wed, Feb 05, 2014 at 05:19:40PM +0100, Michal Hocko wrote:
> On Wed 05-02-14 10:28:21, Johannes Weiner wrote:
> > Here is the only exception to the above: swapout records maintain
> > permanent css references, so they prevent css_free() from running.
> > For that reason alone we should run one optimistic reparenting in
> > css_offline() to make sure one swap record does not pin gigabytes of
> > pages in an offlined cgroup, which is unreachable for reclaim.  But
> 
> How can reparenting help for swapped out pages? Or did you mean to at
> least get rid of swapcache pages?

I was thinking primarily of page cache.  There could be a lot of it
left in the group and once css_tryget() is disabled we can't reclaim
it anymore.  So we'd clean that out at offline time optimistically and
at css_free() we catch any charges raced that showed up afterwards.
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