From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f199.google.com (mail-pf0-f199.google.com [209.85.192.199]) by kanga.kvack.org (Postfix) with ESMTP id 493D96B025F for ; Thu, 20 Jul 2017 17:47:24 -0400 (EDT) Received: by mail-pf0-f199.google.com with SMTP id l28so40269823pfj.12 for ; Thu, 20 Jul 2017 14:47:24 -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 u5si2168173pgc.766.2017.07.20.14.47.22 for (version=TLS1 cipher=AES128-SHA bits=128/128); Thu, 20 Jul 2017 14:47:23 -0700 (PDT) Subject: Re: [PATCH] oom_reaper: close race without using oom_lock From: Tetsuo Handa 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> <20170720141138.GJ9058@dhcp22.suse.cz> In-Reply-To: <20170720141138.GJ9058@dhcp22.suse.cz> Message-Id: <201707210647.BDH57894.MQOtFFOJHLSOFV@I-love.SAKURA.ne.jp> Date: Fri, 21 Jul 2017 06:47:11 +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, hannes@cmpxchg.org, rientjes@google.com, linux-kernel@vger.kernel.org Michal Hocko wrote: > 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. Yes, OOM killer is highly timing sensitive. > > > Even if that patch is safe, this patch still helps with CONFIG_MMU=n case. > > Could you explain how? Nothing prevents sequence below. Process-1 Process-2 Takes oom_lock. Fails get_page_from_freelist(). Enters out_of_memory(). Gets SIGKILL. Gets TIF_MEMDIE. Leaves out_of_memory(). Releases oom_lock. Enters do_exit(). Calls __mmput(). Takes oom_lock. Fails get_page_from_freelist(). Releases some memory. Sets MMF_OOM_SKIP. Enters out_of_memory(). Selects next victim because there is no !MMF_OOM_SKIP mm. Sends SIGKILL needlessly. If we ignore MMF_OOM_SKIP once, we can avoid sequence above. Process-1 Process-2 Takes oom_lock. Fails get_page_from_freelist(). Enters out_of_memory(). Get SIGKILL. Get TIF_MEMDIE. Leaves out_of_memory(). Releases oom_lock. Enters do_exit(). Calls __mmput(). Takes oom_lock. Fails get_page_from_freelist(). Releases some memory. Sets MMF_OOM_SKIP. Enters out_of_memory(). Ignores MMF_OOM_SKIP mm once. Leaves out_of_memory(). Releases oom_lock. Succeeds get_page_from_freelist(). Strictly speaking, this patch is independent with OOM reaper. This patch increases possibility of succeeding get_page_from_freelist() without sending SIGKILL. Your patch is trying to drop it silently. Serializing setting of MMF_OOM_SKIP with oom_lock is one approach, and ignoring MMF_OOM_SKIP once without oom_lock is another approach. -- 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