Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752451Ab2JAMHz (ORCPT ); Mon, 1 Oct 2012 08:07:55 -0400 Received: from mx2.parallels.com ([64.131.90.16]:47619 "EHLO mx2.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750981Ab2JAMHx (ORCPT ); Mon, 1 Oct 2012 08:07:53 -0400 Message-ID: <506986C6.3050908@parallels.com> Date: Mon, 1 Oct 2012 16:04:22 +0400 From: Glauber Costa User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120911 Thunderbird/15.0.1 MIME-Version: 1.0 To: Michal Hocko CC: , , , , Tejun Heo , , Suleiman Souhlal , Frederic Weisbecker , Mel Gorman , David Rientjes , Christoph Lameter , Pekka Enberg , Johannes Weiner Subject: Re: [PATCH v3 06/13] memcg: kmem controller infrastructure 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> <20121001115157.GE8622@dhcp22.suse.cz> <506983B8.8010805@parallels.com> <20121001115847.GF8622@dhcp22.suse.cz> In-Reply-To: <20121001115847.GF8622@dhcp22.suse.cz> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3059 Lines: 70 On 10/01/2012 03:58 PM, Michal Hocko wrote: > On Mon 01-10-12 15:51:20, Glauber Costa wrote: >> On 10/01/2012 03:51 PM, Michal Hocko wrote: >>> 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. >>> >> >> if (*ptr) { /* css should be a valid one */ >> memcg = *ptr; >> VM_BUG_ON(css_is_removed(&memcg->css)); >> if (mem_cgroup_is_root(memcg)) >> goto done; >> if (consume_stock(memcg, nr_pages)) >> goto done; >> css_get(&memcg->css); >> >> >> The way I read this, this will still issue a css_get here, unless >> consume_stock suceeds (assuming non-root) >> >> So we'd still have to have a wrapping get/put pair outside the charge. > > That is correct but it assumes that the css is valid so somebody upwards > made sure css will not go away. This would suggest css_get is not > necessary here but I guess the primary intention here is to make the > code easier so that we do not have to check whether we took css > reference on the return path. > In any case, umem would also suffer from double reference, so I'm fine taking it here as well, since a solution for that is orthogonal. I still need mem_cgroup_get() to make sure the data structure stays around, but we only need to do it once at first charge. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/