From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755517AbcEaW3i (ORCPT ); Tue, 31 May 2016 18:29:38 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38411 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750730AbcEaW3h (ORCPT ); Tue, 31 May 2016 18:29:37 -0400 Date: Wed, 1 Jun 2016 00:29:33 +0200 From: Oleg Nesterov To: Michal Hocko Cc: linux-mm@kvack.org, Tetsuo Handa , David Rientjes , Vladimir Davydov , Andrew Morton , LKML Subject: Re: [PATCH 6/6] mm, oom: fortify task_will_free_mem Message-ID: <20160531222933.GD26582@redhat.com> References: <1464613556-16708-1-git-send-email-mhocko@kernel.org> <1464613556-16708-7-git-send-email-mhocko@kernel.org> <20160530173505.GA25287@redhat.com> <20160531074624.GE26128@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160531074624.GE26128@dhcp22.suse.cz> User-Agent: Mutt/1.5.24 (2015-08-30) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Tue, 31 May 2016 22:29:36 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/31, Michal Hocko wrote: > > On Mon 30-05-16 19:35:05, Oleg Nesterov wrote: > > > > Well, let me suggest this again. I think it should do > > > > > > if (SIGNAL_GROUP_COREDUMP) > > return false; > > > > if (SIGNAL_GROUP_EXIT) > > return true; > > > > if (thread_group_empty() && PF_EXITING) > > return true; > > > > return false; > > > > we do not need fatal_signal_pending(), in this case SIGNAL_GROUP_EXIT should > > be set (ignoring some bugs with sub-namespaces which we need to fix anyway). > > OK, so we shouldn't care about race when the fatal_signal is set on the > task until it reaches do_group_exit? if fatal_signal() is true then (ignoring exec and coredump) SIGNAL_GROUP_EXIT is already set (again, ignoring the bugs with sub-namespace inits). At the same time, SIGKILL can be already dequeued when the task exits, so fatal_signal_pending() can be "false negative". > > And. I think this needs smp_rmb() at the end of the loop (assuming we have the > > process_shares_mm() check here). We need it to ensure that we read p->mm before > > we read next_task(), to avoid the race with exit() + clone(CLONE_VM). > > Why don't we need the same barrier in oom_kill_process? Because it calls do_send_sig_info() which takes ->siglock and copy_process() takes the same lock. Not a barrier, but acts the same way. > Which barrier it > would pair with? With the barrier implied by list_add_tail_rcu(&p->tasks, &init_task.tasks). > Anyway I think this would deserve it's own patch. > Barriers are always tricky and it is better to have them in a small > patch with a full explanation. OK, agreed. I am not sure I can read the new patch correctly, it depends on the previous changes... but afaics it looks good. Cosmetic/subjective nit, feel free to ignore, > +bool task_will_free_mem(struct task_struct *task) > +{ > + struct mm_struct *mm = NULL; unnecessary initialization ;) > + struct task_struct *p; > + bool ret; > + > + /* > + * If the process has passed exit_mm we have to skip it because > + * we have lost a link to other tasks sharing this mm, we do not > + * have anything to reap and the task might then get stuck waiting > + * for parent as zombie and we do not want it to hold TIF_MEMDIE > + */ > + p = find_lock_task_mm(task); > + if (!p) > + return false; > + > + if (!__task_will_free_mem(p)) { > + task_unlock(p); > + return false; > + } We can call the 1st __task_will_free_mem(p) before find_lock_task_mm(). In the likely case (I think) it should return false. And since __task_will_free_mem() has no other callers perhaps it should go into oom_kill.c too. Oleg.