From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f69.google.com (mail-ed1-f69.google.com [209.85.208.69]) by kanga.kvack.org (Postfix) with ESMTP id 690616B0003 for ; Mon, 16 Jul 2018 07:15:11 -0400 (EDT) Received: by mail-ed1-f69.google.com with SMTP id g11-v6so539369edi.8 for ; Mon, 16 Jul 2018 04:15:11 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id i20-v6si197465edb.403.2018.07.16.04.15.09 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 16 Jul 2018 04:15:09 -0700 (PDT) Date: Mon, 16 Jul 2018 13:15:08 +0200 From: Michal Hocko Subject: Re: [patch -mm] mm, oom: remove oom_lock from exit_mmap Message-ID: <20180716111508.GL17280@dhcp22.suse.cz> References: <20180713142612.GD19960@dhcp22.suse.cz> <44d26c25-6e09-49de-5e90-3c16115eb337@i-love.sakura.ne.jp> <20180716061317.GA17280@dhcp22.suse.cz> <916d7e1d-66ea-00d9-c943-ef3d2e082584@i-love.sakura.ne.jp> <20180716074410.GB17280@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Mon 16-07-18 19:38:21, Tetsuo Handa wrote: > On 2018/07/16 16:44, Michal Hocko wrote: > >> If setting MMF_OOM_SKIP is guarded by oom_lock, we can enforce > >> last second allocation attempt like below. > >> > >> CPU 0 CPU 1 > >> > >> mutex_trylock(&oom_lock) in __alloc_pages_may_oom() succeeds. > >> get_page_from_freelist() fails. > >> Enters out_of_memory(). > >> > >> __oom_reap_task_mm() reclaims some memory. > >> mutex_lock(&oom_lock); > >> > >> select_bad_process() does not select new victim because MMF_OOM_SKIP is not yet set. > >> Leaves out_of_memory(). > >> mutex_unlock(&oom_lock) in __alloc_pages_may_oom() is called. > >> > >> Sets MMF_OOM_SKIP. > >> mutex_unlock(&oom_lock); > >> > >> get_page_from_freelist() likely succeeds before reaching __alloc_pages_may_oom() again. > >> Saved one OOM victim from being needlessly killed. > >> > >> That is, guarding setting MMF_OOM_SKIP works as if synchronize_rcu(); it waits for anybody > >> who already acquired (or started waiting for) oom_lock to release oom_lock, in order to > >> prevent select_bad_process() from needlessly selecting new OOM victim. > > > > Hmm, is this a practical problem though? Do we really need to have a > > broader locking context just to defeat this race? > > Yes, for you think that select_bad_process() might take long time. It is possible > that MMF_OOM_SKIP is set while the owner of oom_lock is preempted. It is not such > a small window that select_bad_process() finds an mm which got MMF_OOM_SKIP > immediately before examining that mm. I only do care if the race is practical to hit. And that is why I would like a simplification first (so drop the oom_lock in the oom_reaper path) and then follow up with some decent justification on top. -- Michal Hocko SUSE Labs