Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752579Ab2JALy4 (ORCPT ); Mon, 1 Oct 2012 07:54:56 -0400 Received: from mx2.parallels.com ([64.131.90.16]:45213 "EHLO mx2.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752304Ab2JALyz (ORCPT ); Mon, 1 Oct 2012 07:54:55 -0400 Message-ID: <506983B8.8010805@parallels.com> Date: Mon, 1 Oct 2012 15:51:20 +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> In-Reply-To: <20121001115157.GE8622@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: 2279 Lines: 54 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. -- 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/