From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964829AbcFAHDp (ORCPT ); Wed, 1 Jun 2016 03:03:45 -0400 Received: from mail-wm0-f65.google.com ([74.125.82.65]:35034 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932236AbcFAHDn (ORCPT ); Wed, 1 Jun 2016 03:03:43 -0400 Date: Wed, 1 Jun 2016 09:03:40 +0200 From: Michal Hocko To: Oleg Nesterov 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: <20160601070340.GB26601@dhcp22.suse.cz> 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> <20160531222933.GD26582@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160531222933.GD26582@redhat.com> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 01-06-16 00:29:33, Oleg Nesterov wrote: > 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". Thanks for the clarification. I guess I got the point but this is a land of surprises so one can never be sure... > > > 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. Ahh ok, so an implicit barrier. > > Which barrier it > > would pair with? > > With the barrier implied by list_add_tail_rcu(&p->tasks, &init_task.tasks). Ahh I see. rcu_assign_pointer that is, right? > > 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. cool > 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 ;) fixed > > + 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. OK > > And since __task_will_free_mem() has no other callers perhaps it should go into > oom_kill.c too. ok I will resend the whole series with the fixups later during this week. Thanks again for your review. -- Michal Hocko SUSE Labs