From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx202.postini.com [74.125.245.202]) by kanga.kvack.org (Postfix) with SMTP id 8F6136B005D for ; Mon, 1 Oct 2012 07:58:49 -0400 (EDT) Date: Mon, 1 Oct 2012 13:58:47 +0200 From: Michal Hocko Subject: Re: [PATCH v3 06/13] memcg: kmem controller infrastructure Message-ID: <20121001115847.GF8622@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> <20121001115157.GE8622@dhcp22.suse.cz> <506983B8.8010805@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <506983B8.8010805@parallels.com> Sender: owner-linux-mm@kvack.org List-ID: 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 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. -- 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