Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965516AbdGTOLm (ORCPT ); Thu, 20 Jul 2017 10:11:42 -0400 Received: from mx2.suse.de ([195.135.220.15]:44466 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S934974AbdGTOLl (ORCPT ); Thu, 20 Jul 2017 10:11:41 -0400 Date: Thu, 20 Jul 2017 16:11:38 +0200 From: Michal Hocko To: Tetsuo Handa Cc: linux-mm@kvack.org, hannes@cmpxchg.org, rientjes@google.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] oom_reaper: close race without using oom_lock Message-ID: <20170720141138.GJ9058@dhcp22.suse.cz> References: <1500386810-4881-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> <20170718141602.GB19133@dhcp22.suse.cz> <201707190551.GJE30718.OFHOQMFJtVSFOL@I-love.SAKURA.ne.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201707190551.GJE30718.OFHOQMFJtVSFOL@I-love.SAKURA.ne.jp> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1894 Lines: 40 On Wed 19-07-17 05:51:03, Tetsuo Handa wrote: > Michal Hocko wrote: > > On Tue 18-07-17 23:06:50, Tetsuo Handa wrote: > > > Commit e2fe14564d3316d1 ("oom_reaper: close race with exiting task") > > > guarded whole OOM reaping operations using oom_lock. But there was no > > > need to guard whole operations. We needed to guard only setting of > > > MMF_OOM_REAPED flag because get_page_from_freelist() in > > > __alloc_pages_may_oom() is called with oom_lock held. > > > > > > If we change to guard only setting of MMF_OOM_SKIP flag, the OOM reaper > > > can start reaping operations as soon as wake_oom_reaper() is called. > > > But since setting of MMF_OOM_SKIP flag at __mmput() is not guarded with > > > oom_lock, guarding only the OOM reaper side is not sufficient. > > > > > > If we change the OOM killer side to ignore MMF_OOM_SKIP flag once, > > > there is no need to guard setting of MMF_OOM_SKIP flag, and we can > > > guarantee a chance to call get_page_from_freelist() in > > > __alloc_pages_may_oom() without depending on oom_lock serialization. > > > > > > This patch makes MMF_OOM_SKIP act as if MMF_OOM_REAPED, and adds a new > > > flag which acts as if MMF_OOM_SKIP, in order to close both race window > > > (the OOM reaper side and __mmput() side) without using oom_lock. > > > > Why do we need this patch when > > http://lkml.kernel.org/r/20170626130346.26314-1-mhocko@kernel.org > > already removes the lock and solves another problem at once? > > We haven't got an answer from Hugh and/or Andrea whether that patch is safe. So what? I haven't see anybody disputing the correctness. And to be honest I really dislike your patch. Yet another round kind of solutions are just very ugly hacks usually because they are highly timing sensitive. > Even if that patch is safe, this patch still helps with CONFIG_MMU=n case. Could you explain how? -- Michal Hocko SUSE Labs