Date: Mon, 1 Oct 2012 13:51:57 +0200
From: Michal Hocko <mhocko@suse.cz>
To: Glauber Costa <glommer@parallels.com>
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
Message-ID: <20121001115157.GE8622@dhcp22.suse.cz>
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 <5064392D.5040707@parallels.com>
 <20120927134432.GE29104@dhcp22.suse.cz>
 <50658B3B.9020303@parallels.com>
 <20121001094846.GC8622@dhcp22.suse.cz>
 <50696BC5.8040808@parallels.com>
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On Mon 01-10-12 14:09:09, Glauber Costa wrote:
> On 10/01/2012 01:48 PM, Michal Hocko wrote:
> > 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?
> > 
> 
> Ok, after reading this again (and again), you seem to be right. It
> concerns me, however, that simply getting the css would lead us to a
> double get/put pair, since try_charge will have to do it anyway.

That happens only for !*ptr case and you provide a memcg here, don't
you.

> I considered just letting try_charge selecting the memcg, but that is
> not really what we want, since if that memcg will fail kmem allocations,
> we simply won't issue try charge, but return early.
> 
> Any immediate suggestions on how to handle this ?

I would do the same thing __mem_cgroup_try_charge does.
retry:
	rcu_read_lock();
	p = rcu_dereference(mm->owner);
	if (!css_tryget(&memcg->css)) {
		rcu_read_unlock();
		goto retry;
	}
	rcu_read_unlock();

-- 
Michal Hocko
SUSE Labs
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