linux-kernel - Re: [PATCH v3 16/28] memcg: kmem controller charge/uncharge infrastructure

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Message-ID: <20120530153256.GA27008@somewhere.redhat.com>
Date:	Wed, 30 May 2012 17:33:00 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	linux-mm@...ck.org, kamezawa.hiroyu@...fujitsu.com,
	Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
	Greg Thelen <gthelen@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>, devel@...nvz.org,
	David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH v3 16/28] memcg: kmem controller charge/uncharge
 infrastructure

On Wed, May 30, 2012 at 05:55:38PM +0400, Glauber Costa wrote:
> On 05/30/2012 05:53 PM, Frederic Weisbecker wrote:
> >On Wed, May 30, 2012 at 05:37:57PM +0400, Glauber Costa wrote:
> >>On 05/30/2012 05:37 PM, Frederic Weisbecker wrote:
> >>>Right. __mem_cgroup_get_kmem_cache() fetches the memcg of the owner
> >>>and calls memcg_create_cache_enqueue() which does css_tryget(&memcg->css).
> >>>After this tryget I think you're fine. And in-between you're safe against
> >>>css_set removal due to rcu_read_lock().
> >>>
> >>>I'm less clear with __mem_cgroup_new_kmem_page() though...
> >>
> >>That one does not get memcg->css but it does call mem_cgroup_get(),
> >>that does prevent against the memcg structure being freed, which I
> >>believe to be good enough.
> >
> >What if the owner calls cgroup_exit() between mem_cgroup_from_task()
> >and mem_cgroup_get()? The css_set which contains the memcg gets freed.
> >Also the reference on the memcg doesn't even prevent the css_set to
> >be removed, does it?
> It doesn't, but we don't really care. The css can go away, if the
> memcg structure stays.

Ah right, the memcg itself is only freed at destroy time.

> The caches will outlive the memcg anyway,
> since it is possible that you delete it, with some caches still
> holding objects that
> are not freed (they will be marked as dead).

I guess I need to look at how the destroy path is handled in your patchset
then. Or how you ensure that __mem_cgroup_new_kmem_page() can't race against
destroy.
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