From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx201.postini.com [74.125.245.201]) by kanga.kvack.org (Postfix) with SMTP id 7D64B6B005A for ; Tue, 30 Oct 2012 06:36:03 -0400 (EDT) Date: Tue, 30 Oct 2012 11:35:59 +0100 From: Michal Hocko Subject: Re: [PATCH v3 3/6] memcg: Simplify mem_cgroup_force_empty_list error handling Message-ID: <20121030103559.GA7394@dhcp22.suse.cz> References: <1351251453-6140-1-git-send-email-mhocko@suse.cz> <1351251453-6140-4-git-send-email-mhocko@suse.cz> <508E8B95.406@parallels.com> <20121029150022.a595b866.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121029150022.a595b866.akpm@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Glauber Costa , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Tejun Heo , Li Zefan , Johannes Weiner , KAMEZAWA Hiroyuki , Balbir Singh On Mon 29-10-12 15:00:22, Andrew Morton wrote: > On Mon, 29 Oct 2012 17:58:45 +0400 > Glauber Costa wrote: > > > > + * move charges to its parent or the root cgroup if the group has no > > > + * parent (aka use_hierarchy==0). > > > + * Although this might fail (get_page_unless_zero, isolate_lru_page or > > > + * mem_cgroup_move_account fails) the failure is always temporary and > > > + * it signals a race with a page removal/uncharge or migration. In the > > > + * first case the page is on the way out and it will vanish from the LRU > > > + * on the next attempt and the call should be retried later. > > > + * Isolation from the LRU fails only if page has been isolated from > > > + * the LRU since we looked at it and that usually means either global > > > + * reclaim or migration going on. The page will either get back to the > > > + * LRU or vanish. > > > > I just wonder for how long can it go in the worst case? > > If the kernel is uniprocessor and the caller is SCHED_FIFO: ad infinitum! You are right, if the rmdir (resp. echo > force_empty) at SCHED_FIFO races with put_page (on a shared page) which gets preempted after put_page_testzero and before __page_cache_release then we are screwed: put_page(page) put_page_testzero mem_cgroup_force_empty_list page = list_entry(list->prev, struct page, lru); mem_cgroup_move_parent(page) get_page_unless_zero cond_resched() The race window is really small but it is definitely possible. I am not happy about this state and it should be probably mentioned in the patch description but I do not see any way around (except for hacks like sched_setscheduler for the current which is, ehm...) and still keep do_not_fail contract here. Can we consider this as a corner case (it is much easier to kill a machine with SCHED_FIFO than this anyway) or the concern is really strong and we should come with a solution before this can get merged? -- 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