From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx153.postini.com [74.125.245.153]) by kanga.kvack.org (Postfix) with SMTP id F1E966B006E for ; Mon, 1 Oct 2012 07:54:57 -0400 (EDT) Message-ID: <506983B8.8010805@parallels.com> Date: Mon, 1 Oct 2012 15:51:20 +0400 From: Glauber Costa MIME-Version: 1.0 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: owner-linux-mm@kvack.org List-ID: To: Michal Hocko 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 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, 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