From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752392Ab2JALwE (ORCPT ); Mon, 1 Oct 2012 07:52:04 -0400 Received: from cantor2.suse.de ([195.135.220.15]:59190 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751821Ab2JALwC (ORCPT ); Mon, 1 Oct 2012 07:52:02 -0400 Date: Mon, 1 Oct 2012 13:51:57 +0200 From: Michal Hocko To: Glauber Costa Cc: linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, kamezawa.hiroyu@jp.fujitsu.com, devel@openvz.org, Tejun Heo , linux-mm@kvack.org, Suleiman Souhlal , Frederic Weisbecker , Mel Gorman , David Rientjes , Christoph Lameter , Pekka Enberg , Johannes Weiner Subject: Re: [PATCH v3 06/13] memcg: kmem controller infrastructure Message-ID: <20121001115157.GE8622@dhcp22.suse.cz> References: <1347977050-29476-1-git-send-email-glommer@parallels.com> <1347977050-29476-7-git-send-email-glommer@parallels.com> <20120926155108.GE15801@dhcp22.suse.cz> <5064392D.5040707@parallels.com> <20120927134432.GE29104@dhcp22.suse.cz> <50658B3B.9020303@parallels.com> <20121001094846.GC8622@dhcp22.suse.cz> <50696BC5.8040808@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50696BC5.8040808@parallels.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH v3 06/13] memcg: kmem controller infrastructure Date: Mon, 1 Oct 2012 13:51:57 +0200 Message-ID: <20121001115157.GE8622@dhcp22.suse.cz> References: <1347977050-29476-1-git-send-email-glommer@parallels.com> <1347977050-29476-7-git-send-email-glommer@parallels.com> <20120926155108.GE15801@dhcp22.suse.cz> <5064392D.5040707@parallels.com> <20120927134432.GE29104@dhcp22.suse.cz> <50658B3B.9020303@parallels.com> <20121001094846.GC8622@dhcp22.suse.cz> <50696BC5.8040808@parallels.com> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <50696BC5.8040808@parallels.com> Sender: owner-linux-mm@kvack.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Glauber Costa Cc: linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, kamezawa.hiroyu@jp.fujitsu.com, devel@openvz.org, Tejun Heo , linux-mm@kvack.org, Suleiman Souhlal , Frederic Weisbecker , Mel Gorman , David Rientjes , Christoph Lameter , Pekka Enberg , Johannes Weiner 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org