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[202.181.97.72]) by mx.google.com with ESMTPS id bj12si9984618plb.378.2019.07.01.06.56.22 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 01 Jul 2019 06:56:23 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of penguin-kernel@i-love.sakura.ne.jp designates 202.181.97.72 as permitted sender) client-ip=202.181.97.72; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of penguin-kernel@i-love.sakura.ne.jp designates 202.181.97.72 as permitted sender) smtp.mailfrom=penguin-kernel@i-love.sakura.ne.jp Received: from fsav403.sakura.ne.jp (fsav403.sakura.ne.jp [133.242.250.102]) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTP id x61DuG7r086597; Mon, 1 Jul 2019 22:56:16 +0900 (JST) (envelope-from penguin-kernel@i-love.sakura.ne.jp) Received: from www262.sakura.ne.jp (202.181.97.72) by fsav403.sakura.ne.jp (F-Secure/fsigk_smtp/530/fsav403.sakura.ne.jp); Mon, 01 Jul 2019 22:56:16 +0900 (JST) X-Virus-Status: clean(F-Secure/fsigk_smtp/530/fsav403.sakura.ne.jp) Received: from [192.168.1.8] (softbank126012062002.bbtec.net [126.12.62.2]) (authenticated bits=0) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTPSA id x61DuFMW086570 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NO); Mon, 1 Jul 2019 22:56:16 +0900 (JST) (envelope-from penguin-kernel@i-love.sakura.ne.jp) Subject: Re: [PATCH] mm: mempolicy: don't select exited threads as OOM victims To: Michal Hocko Cc: Shakeel Butt , linux-mm@kvack.org References: <1561807474-10317-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> <20190701111708.GP6376@dhcp22.suse.cz> <15099126-5d0f-51eb-7134-46c5c2db3bf0@i-love.sakura.ne.jp> <20190701131736.GX6376@dhcp22.suse.cz> <20190701134859.GZ6376@dhcp22.suse.cz> From: Tetsuo Handa Message-ID: Date: Mon, 1 Jul 2019 22:56:12 +0900 User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20190701134859.GZ6376@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2019/07/01 22:48, Michal Hocko wrote: > On Mon 01-07-19 22:38:58, Tetsuo Handa wrote: >> On 2019/07/01 22:17, Michal Hocko wrote: >>> On Mon 01-07-19 22:04:22, Tetsuo Handa wrote: >>>> But I realized that this patch was too optimistic. We need to wait for mm-less >>>> threads until MMF_OOM_SKIP is set if the process was already an OOM victim. >>> >>> If the process is an oom victim then _all_ threads are so as well >>> because that is the address space property. And we already do check that >>> before reaching oom_badness IIRC. So what is the actual problem you are >>> trying to solve here? >> >> I'm talking about behavioral change after tsk became an OOM victim. >> >> If tsk->signal->oom_mm != NULL, we have to wait for MMF_OOM_SKIP even if >> tsk->mm == NULL. Otherwise, the OOM killer selects next OOM victim as soon as >> oom_unkillable_task() returned true because has_intersects_mems_allowed() returned >> false because mempolicy_nodemask_intersects() returned false because all thread's >> mm became NULL (despite tsk->signal->oom_mm != NULL). > > OK, I finally got your point. It was not clear that you are referring to > the code _after_ the patch you are proposing. You are indeed right that > this would have a side effect that an additional victim could be > selected even though the current process hasn't terminated yet. Sigh, > another example how the whole thing is subtle so I retract my Ack and > request a real life example of where this matters before we think about > a proper fix and make the code even more complex. > Instead of checking for mm != NULL, can we move mpol_put_task_policy() from do_exit() to __put_task_struct() ? That change will (if it is safe to do) prevent exited threads from setting mempolicy = NULL (and confusing mempolicy_nodemask_intersects() due to mempolicy == NULL).