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Date: Fri, 28 Sep 2012 15:34:19 +0400
From: Glauber Costa <glommer@parallels.com>
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To: Michal Hocko <mhocko@suse.cz>
CC: <linux-kernel@vger.kernel.org>, <cgroups@vger.kernel.org>,
        <kamezawa.hiroyu@jp.fujitsu.com>, <devel@openvz.org>,
        Tejun Heo <tj@kernel.org>, <linux-mm@kvack.org>,
        Suleiman Souhlal <suleiman@google.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, Mel Gorman <mgorman@suse.de>,
        David Rientjes <rientjes@google.com>, Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@cs.helsinki.fi>,
        Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH v3 06/13] memcg: kmem controller infrastructure
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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.