From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f199.google.com (mail-ig0-f199.google.com [209.85.213.199]) by kanga.kvack.org (Postfix) with ESMTP id 89FC86B007E for ; Thu, 26 May 2016 11:25:34 -0400 (EDT) Received: by mail-ig0-f199.google.com with SMTP id sq19so144598953igc.0 for ; Thu, 26 May 2016 08:25:34 -0700 (PDT) Received: from www262.sakura.ne.jp (www262.sakura.ne.jp. [2001:e42:101:1:202:181:97:72]) by mx.google.com with ESMTPS id s197si3077127ois.185.2016.05.26.08.25.33 for (version=TLS1 cipher=AES128-SHA bits=128/128); Thu, 26 May 2016 08:25:33 -0700 (PDT) Subject: Re: [PATCH 1/6] mm, oom: do not loop over all tasks if there are noexternal tasks sharing mm From: Tetsuo Handa References: <1464266415-15558-1-git-send-email-mhocko@kernel.org> <1464266415-15558-2-git-send-email-mhocko@kernel.org> <201605262330.EEB52182.OtMFOJHFLOSFVQ@I-love.SAKURA.ne.jp> <20160526145930.GF23675@dhcp22.suse.cz> In-Reply-To: <20160526145930.GF23675@dhcp22.suse.cz> Message-Id: <201605270025.IAC48454.QSHOOMFOLtFJFV@I-love.SAKURA.ne.jp> Date: Fri, 27 May 2016 00:25:23 +0900 Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: owner-linux-mm@kvack.org List-ID: To: mhocko@kernel.org Cc: linux-mm@kvack.org, rientjes@google.com, oleg@redhat.com, vdavydov@parallels.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org Michal Hocko wrote: > On Thu 26-05-16 23:30:06, Tetsuo Handa wrote: > > Michal Hocko wrote: > > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c > > > index 5bb2f7698ad7..0e33e912f7e4 100644 > > > --- a/mm/oom_kill.c > > > +++ b/mm/oom_kill.c > > > @@ -820,6 +820,13 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p, > > > task_unlock(victim); > > > > > > /* > > > + * skip expensive iterations over all tasks if we know that there > > > + * are no users outside of threads in the same thread group > > > + */ > > > + if (atomic_read(&mm->mm_users) <= get_nr_threads(victim)) > > > + goto oom_reap; > > > > Is this really safe? Isn't it possible that victim thread's thread group has > > more than atomic_read(&mm->mm_users) threads which are past exit_mm() and blocked > > at exit_task_work() which are before __exit_signal() from release_task() from > > exit_notify()? > > You are right. The race window between exit_mm and __exit_signal is > really large. I thought about == check instead but that wouldn't work > for the same reason, dang, it looked so promissing. > > Scratch this patch then. > I think that remembering whether this mm might be shared between multiple thread groups at clone() time (i.e. whether clone(CLONE_VM without CLONE_SIGHAND) was ever requested on this mm) is safe (given that that thread already got SIGKILL or is exiting). By the way, in oom_kill_process(), how (p->flags & PF_KTHREAD) case can become true when process_shares_mm() is true? Even if it can become true, why can't we reap that mm? Is (p->flags & PF_KTHREAD) case only for not to send SIGKILL rather than not to reap that mm? -- 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