linux-kernel - Re: [patch v2 3/6] memcg: rework mem_cgroup

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Message-ID: <20121128091745.GC12309@dhcp22.suse.cz>
Date:	Wed, 28 Nov 2012 10:17:45 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Johannes Weiner <hannes@...xchg.org>,
	Ying Han <yinghan@...gle.com>, Tejun Heo <htejun@...il.com>,
	Glauber Costa <glommer@...allels.com>,
	Li Zefan <lizefan@...wei.com>
Subject: Re: [patch v2 3/6] memcg: rework mem_cgroup_iter to use cgroup
 iterators

On Wed 28-11-12 17:47:59, KAMEZAWA Hiroyuki wrote:
> (2012/11/27 3:47), Michal Hocko wrote:
[...]
> > +		/*
> > +		 * Even if we found a group we have to make sure it is alive.
> > +		 * css && !memcg means that the groups should be skipped and
> > +		 * we should continue the tree walk.
> > +		 * last_visited css is safe to use because it is protected by
> > +		 * css_get and the tree walk is rcu safe.
> > +		 */
> > +		if (css == &root->css || (css && css_tryget(css)))
> > +			memcg = mem_cgroup_from_css(css);
> 
> Could you note that this iterator will never visit dangling(removed)
> memcg, somewhere ?

OK, I can add it to the function comment but the behavior hasn't changed
so I wouldn't like to confuse anybody.

> Hmm, I'm not sure but it may be trouble at shrkinking dangling
> kmem_cache(slab).

We do not shrink slab at all. Those objects that are in a dead memcg
wait for their owner tho release them which will make the dangling group
eventually go away

> 
> Costa, how do you think ?
> 
> I guess there is no problem with swap and not against the way you go.

Yes, swap should be OK. Pages charged against removed memcg will
fallback to the the current's mm (try_get_mem_cgroup_from_page and
__mem_cgroup_try_charge_swapin)

[...]
-- 
Michal Hocko
SUSE Labs
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