From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f72.google.com (mail-oi0-f72.google.com [209.85.218.72]) by kanga.kvack.org (Postfix) with ESMTP id 29F416B0003 for ; Mon, 16 Jul 2018 03:04:42 -0400 (EDT) Received: by mail-oi0-f72.google.com with SMTP id 14-v6so48785674ois.11 for ; Mon, 16 Jul 2018 00:04:42 -0700 (PDT) Received: from www262.sakura.ne.jp (www262.sakura.ne.jp. [202.181.97.72]) by mx.google.com with ESMTPS id d14-v6si12238065oig.71.2018.07.16.00.04.40 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 16 Jul 2018 00:04:40 -0700 (PDT) Subject: Re: [patch -mm] mm, oom: remove oom_lock from exit_mmap 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 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: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: David Rientjes , Andrew Morton , linux-mm@kvack.org, 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.