From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f71.google.com (mail-pl0-f71.google.com [209.85.160.71]) by kanga.kvack.org (Postfix) with ESMTP id B8AAD6B0003 for ; Mon, 16 Jul 2018 02:13:22 -0400 (EDT) Received: by mail-pl0-f71.google.com with SMTP id w1-v6so3848477ply.12 for ; Sun, 15 Jul 2018 23:13:22 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id c4-v6si4890755pfa.285.2018.07.15.23.13.20 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 15 Jul 2018 23:13:21 -0700 (PDT) Date: Mon, 16 Jul 2018 08:13:17 +0200 From: Michal Hocko Subject: Re: [patch -mm] mm, oom: remove oom_lock from exit_mmap Message-ID: <20180716061317.GA17280@dhcp22.suse.cz> References: <20180713142612.GD19960@dhcp22.suse.cz> <44d26c25-6e09-49de-5e90-3c16115eb337@i-love.sakura.ne.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <44d26c25-6e09-49de-5e90-3c16115eb337@i-love.sakura.ne.jp> Sender: owner-linux-mm@kvack.org List-ID: To: Tetsuo Handa Cc: David Rientjes , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Sat 14-07-18 06:18:58, Tetsuo Handa wrote: > On 2018/07/13 23:26, Michal Hocko wrote: > > On Thu 12-07-18 14:34:00, David Rientjes wrote: > > [...] > >> diff --git a/mm/oom_kill.c b/mm/oom_kill.c > >> index 0fe4087d5151..e6328cef090f 100644 > >> --- a/mm/oom_kill.c > >> +++ b/mm/oom_kill.c > >> @@ -488,9 +488,11 @@ void __oom_reap_task_mm(struct mm_struct *mm) > >> * Tell all users of get_user/copy_from_user etc... that the content > >> * is no longer stable. No barriers really needed because unmapping > >> * should imply barriers already and the reader would hit a page fault > >> - * if it stumbled over a reaped memory. > >> + * if it stumbled over a reaped memory. If MMF_UNSTABLE is already set, > >> + * reaping as already occurred so nothing left to do. > >> */ > >> - set_bit(MMF_UNSTABLE, &mm->flags); > >> + if (test_and_set_bit(MMF_UNSTABLE, &mm->flags)) > >> + return; > > > > This could lead to pre mature oom victim selection > > oom_reaper exiting victim > > oom_reap_task exit_mmap > > __oom_reap_task_mm __oom_reap_task_mm > > test_and_set_bit(MMF_UNSTABLE) # wins the race > > test_and_set_bit(MMF_UNSTABLE) > > set_bit(MMF_OOM_SKIP) # new victim can be selected now. > > > > Besides that, why should we back off in the first place. We can > > race the two without any problems AFAICS. We already do have proper > > synchronization between the two due to mmap_sem and MMF_OOM_SKIP. > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > index fc41c0543d7f..4642964f7741 100644 > > --- a/mm/mmap.c > > +++ b/mm/mmap.c > > @@ -3073,9 +3073,7 @@ void exit_mmap(struct mm_struct *mm) > > * which clears VM_LOCKED, otherwise the oom reaper cannot > > * reliably test it. > > */ > > - mutex_lock(&oom_lock); > > __oom_reap_task_mm(mm); > > - mutex_unlock(&oom_lock); > > > > set_bit(MMF_OOM_SKIP, &mm->flags); > > David and Michal are using different version as a baseline here. > David is making changes using timeout based back off (in linux-next.git) > which is inappropriately trying to use MMF_UNSTABLE for two purposes. > > Michal is making changes using current code (in linux.git) which does not > address David's concern. Yes I have based it on top of Linus tree because the point of this patch is to get rid of the locking which is no longer needed. I do not see what concern are you talking about. > > My version ( https://marc.info/?l=linux-mm&m=153119509215026 ) is > making changes using current code which also provides oom-badness > based back off in order to address David's concern. > > > down_write(&mm->mmap_sem); > > Anyway, I suggest doing > > mutex_lock(&oom_lock); > set_bit(MMF_OOM_SKIP, &mm->flags); > mutex_unlock(&oom_lock); Why do we need it? > like I mentioned at > http://lkml.kernel.org/r/201807130620.w6D6KiAJ093010@www262.sakura.ne.jp > even if we make changes on top of linux-next's timeout based back off. says : (3) Prevent from selecting new OOM victim when there is an !MMF_OOM_SKIP mm : which current thread should wait for. [...] : Regarding (A), we can reduce the range oom_lock serializes from : "__oom_reap_task_mm()" to "setting MMF_OOM_SKIP", for oom_lock is useful for (3). But why there is a lock needed for this? This doesn't make much sense to me. If we do not have MMF_OOM_SKIP set we still should have mm_is_oom_victim so no new task should be selected. If we race with the oom reaper than ok, we would just not select a new victim and retry later. -- Michal Hocko SUSE Labs