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

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Message-ID: <20121001094846.GC8622@dhcp22.suse.cz>
Date:	Mon, 1 Oct 2012 11:48:46 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Glauber Costa <glommer@...allels.com>
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 Fri 28-09-12 15:34:19, Glauber Costa wrote:
> 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.

Yes, but how do you guarantee that the above VM_BUG_ON doesn't trigger?
Task could have been moved to another group between mem_cgroup_from_task
and mem_cgroup_get, no?
-- 
Michal Hocko
SUSE Labs
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