From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4D322ECDFAA for ; Mon, 16 Jul 2018 07:04:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0BA172086E for ; Mon, 16 Jul 2018 07:04:44 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0BA172086E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=i-love.sakura.ne.jp Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729591AbeGPHah (ORCPT ); Mon, 16 Jul 2018 03:30:37 -0400 Received: from www262.sakura.ne.jp ([202.181.97.72]:24838 "EHLO www262.sakura.ne.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727540AbeGPHah (ORCPT ); Mon, 16 Jul 2018 03:30:37 -0400 Received: from fsav101.sakura.ne.jp (fsav101.sakura.ne.jp [27.133.134.228]) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTP id w6G74Vxd022208; Mon, 16 Jul 2018 16:04:31 +0900 (JST) (envelope-from penguin-kernel@i-love.sakura.ne.jp) Received: from www262.sakura.ne.jp (202.181.97.72) by fsav101.sakura.ne.jp (F-Secure/fsigk_smtp/530/fsav101.sakura.ne.jp); Mon, 16 Jul 2018 16:04:31 +0900 (JST) X-Virus-Status: clean(F-Secure/fsigk_smtp/530/fsav101.sakura.ne.jp) Received: from [192.168.1.8] (softbank126074194044.bbtec.net [126.74.194.44]) (authenticated bits=0) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTPSA id w6G74QhE022151 (version=TLSv1.2 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Mon, 16 Jul 2018 16:04:31 +0900 (JST) (envelope-from penguin-kernel@i-love.sakura.ne.jp) Subject: Re: [patch -mm] mm, oom: remove oom_lock from exit_mmap To: Michal Hocko Cc: David Rientjes , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20180713142612.GD19960@dhcp22.suse.cz> <44d26c25-6e09-49de-5e90-3c16115eb337@i-love.sakura.ne.jp> <20180716061317.GA17280@dhcp22.suse.cz> From: Tetsuo Handa Message-ID: <916d7e1d-66ea-00d9-c943-ef3d2e082584@i-love.sakura.ne.jp> Date: Mon, 16 Jul 2018 16:04:26 +0900 User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20180716061317.GA17280@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2018/07/16 15:13, Michal Hocko wrote: > On Sat 14-07-18 06:18:58, Tetsuo Handa wrote: >>> @@ -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. I'm saying that applying your patch does not work on linux-next.git because David's patch already did s/MMF_OOM_SKIP/MMF_UNSTABLE/ . >> >> 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. > How mm_is_oom_victim() helps? mm_is_oom_victim() is used by exit_mmap() whether current thread should call __oom_reap_task_mm(). I'm talking about below sequence (i.e. after returning from __oom_reap_task_mm()). 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. Sets MMF_OOM_SKIP. select_bad_process() selects new victim because MMF_OOM_SKIP is already set. Kills a new OOM victim without retrying last second allocation attempt. Leaves out_of_memory(). mutex_unlock(&oom_lock) in __alloc_pages_may_oom() is called. 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.