From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lb0-f198.google.com (mail-lb0-f198.google.com [209.85.217.198]) by kanga.kvack.org (Postfix) with ESMTP id 765C66B0261 for ; Tue, 31 May 2016 03:46:27 -0400 (EDT) Received: by mail-lb0-f198.google.com with SMTP id rs7so94793125lbb.2 for ; Tue, 31 May 2016 00:46:27 -0700 (PDT) Received: from mail-wm0-f66.google.com (mail-wm0-f66.google.com. [74.125.82.66]) by mx.google.com with ESMTPS id y19si49034975wjw.14.2016.05.31.00.46.26 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 31 May 2016 00:46:26 -0700 (PDT) Received: by mail-wm0-f66.google.com with SMTP id a136so29927780wme.0 for ; Tue, 31 May 2016 00:46:26 -0700 (PDT) Date: Tue, 31 May 2016 09:46:24 +0200 From: Michal Hocko Subject: Re: [PATCH 6/6] mm, oom: fortify task_will_free_mem Message-ID: <20160531074624.GE26128@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160530173505.GA25287@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Oleg Nesterov Cc: linux-mm@kvack.org, Tetsuo Handa , David Rientjes , Vladimir Davydov , Andrew Morton , LKML On Mon 30-05-16 19:35:05, Oleg Nesterov wrote: > On 05/30, Michal Hocko wrote: > > > > task_will_free_mem is rather weak. > > I was thinking about the similar change because I noticed that try_oom_reaper() > is very, very wrong. > > To the point I think that we need another change for stable which simply removes > spin_lock_irq(sighand->siglock) from try_oom_reaper(). It buys nothing, we can > check signal_group_exit() (which is wrong too ;) lockless, and at the same time > the kernel can crash because we can hit ->siglock == NULL. OK, I have sent a separate patch http://lkml.kernel.org/r/1464679423-30218-1-git-send-email-mhocko@kernel.org and rebase the series on top. This would be 4.7 material. Thanks for catching that! > So I do think this change is good in general. > > I think that task_will_free_mem() should be un-inlined, and __task_will_free_mem() > should go into mm/oom-kill.c... but this is minor. I was thinking about it as well but then thought that this would be harder to review. But OK, I will do that. > > -static inline bool task_will_free_mem(struct task_struct *task) > > +static inline bool __task_will_free_mem(struct task_struct *task) > > { > > struct signal_struct *sig = task->signal; > > > > @@ -119,16 +119,69 @@ static inline bool task_will_free_mem(struct task_struct *task) > > if (sig->flags & SIGNAL_GROUP_COREDUMP) > > return false; > > > > - if (!(task->flags & PF_EXITING)) > > + if (!(task->flags & PF_EXITING || fatal_signal_pending(task))) > > return false; > > > > /* Make sure that the whole thread group is going down */ > > - if (!thread_group_empty(task) && !(sig->flags & SIGNAL_GROUP_EXIT)) > > + if (!thread_group_empty(task) && > > + !(sig->flags & SIGNAL_GROUP_EXIT || fatal_signal_pending(task))) > > return false; > > > > return true; > > } > > 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? > At the same time, we do not want to return false if PF_EXITING is not set > if SIGNAL_GROUP_EXIT is set. makes sense. > > +static inline bool task_will_free_mem(struct task_struct *task) > > +{ > > + struct mm_struct *mm = NULL; > > + 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; > > + } > > + > > + mm = p->mm; > > + if (atomic_read(&mm->mm_users) <= 1) { > > this is sub-optimal, we should probably take signal->live or ->nr_threads > into account... but OK, we can do this later. Yes I would prefer to add a more complex checks later. We want mm_has_external_refs for other purposes as well. > > + rcu_read_lock(); > > + for_each_process(p) { > > + ret = __task_will_free_mem(p); > > + if (!ret) > > + break; > > + } > > + rcu_read_unlock(); > > Yes, I agree very much. > > But it seems you forgot to add the process_shares_mm() check into this loop? Yes. Dunno where it got lost but it surely wasn't in the previous version either. I definitely screwed somewhere... > and perhaps it also makes sense to add > > if (same_thread_group(tsk, p)) > continue; > > This should not really matter, we know that __task_will_free_mem(p) should return > true. Just to make it more clear. ok > 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? Which barrier it would pair with? 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. Thanks for your review. It was really helpful! The whole pile is currently in my k.org git tree in attempts/process-share-mm-oom-sanitization branch if somebody wants to see the full series. My current diff on top of the patch ---