linux-kernel - Re: [PATCH v3 06/13] memcg: kmem controller infrastructure

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Message-ID: <50658B3B.9020303@parallels.com>
Date:	Fri, 28 Sep 2012 15:34:19 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	<linux-kernel@...r.kernel.org>, <cgroups@...r.kernel.org>,
	<kamezawa.hiroyu@...fujitsu.com>, <devel@...nvz.org>,
	Tejun Heo <tj@...nel.org>, <linux-mm@...ck.org>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v3 06/13] memcg: kmem controller infrastructure

On 09/27/2012 05:44 PM, Michal Hocko wrote:
>> > the reference count aquired by mem_cgroup_get will still prevent the
>> > memcg from going away, no?
> Yes but you are outside of the rcu now and we usually do css_get before
> we rcu_unlock. mem_cgroup_get just makes sure the group doesn't get
> deallocated but it could be gone before you call it. Or I am just
> confused - these 2 levels of ref counting is really not nice.
> 
> Anyway, I have just noticed that __mem_cgroup_try_charge does
> VM_BUG_ON(css_is_removed(&memcg->css)) on a given memcg so you should
> keep css ref count up as well.
> 

IIRC, css_get will prevent the cgroup directory from being removed.
Because some allocations are expected to outlive the cgroup, we
specifically don't want that.
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