From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <owner-linux-mm@kvack.org>
Received: from mail-pg1-f200.google.com (mail-pg1-f200.google.com [209.85.215.200])
	by kanga.kvack.org (Postfix) with ESMTP id B82F96B792F
	for <linux-mm@kvack.org>; Thu,  6 Sep 2018 10:26:36 -0400 (EDT)
Received: by mail-pg1-f200.google.com with SMTP id d132-v6so5557828pgc.22
        for <linux-mm@kvack.org>; Thu, 06 Sep 2018 07:26:36 -0700 (PDT)
Received: from www262.sakura.ne.jp (www262.sakura.ne.jp. [202.181.97.72])
        by mx.google.com with ESMTPS id 97-v6si5202668plm.290.2018.09.06.07.26.35
        for <linux-mm@kvack.org>
        (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
        Thu, 06 Sep 2018 07:26:35 -0700 (PDT)
Subject: Re: [PATCH] mm, oom: Introduce time limit for dump_tasks duration.
References: <0252ad5d-46e6-0d7f-ef91-4e316657a83d@i-love.sakura.ne.jp>
 <CACT4Y+Yp6ZbusCWg5C1zaJpcS8=XnGPboKgWfyxVk1axQA2nbw@mail.gmail.com>
 <201809060553.w865rmpj036017@www262.sakura.ne.jp>
 <CACT4Y+YKJWJr-5rBQidt6nY7+VF=BAsvHyh+XTaf8spwNy3qPA@mail.gmail.com>
 <58aa0543-86d0-b2ad-7fb9-9bed7c6a1f6c@i-love.sakura.ne.jp>
 <20180906112306.GO14951@dhcp22.suse.cz>
 <1611e45d-235e-67e9-26e3-d0228255fa2f@i-love.sakura.ne.jp>
 <20180906115320.GS14951@dhcp22.suse.cz>
From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <7f50772a-f2ef-d16e-4d09-7f34f4bf9227@i-love.sakura.ne.jp>
Date: Thu, 6 Sep 2018 22:45:26 +0900
MIME-Version: 1.0
In-Reply-To: <20180906115320.GS14951@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: <linux-mm.kvack.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Dmitry Vyukov <dvyukov@google.com>, Andrew Morton <akpm@linux-foundation.org>, David Rientjes <rientjes@google.com>, syzbot <syzbot+f0fc7f62e88b1de99af3@syzkaller.appspotmail.com>, 'Dmitry Vyukov' via syzkaller-upstream-moderation <syzkaller-upstream-moderation@googlegroups.com>, linux-mm <linux-mm@kvack.org>

On 2018/09/06 20:53, Michal Hocko wrote:
> On Thu 06-09-18 20:40:34, Tetsuo Handa wrote:
>> On 2018/09/06 20:23, Michal Hocko wrote:
>>> On Thu 06-09-18 19:58:25, Tetsuo Handa wrote:
>>> [...]
>>>> >From 18876f287dd69a7c33f65c91cfcda3564233f55e Mon Sep 17 00:00:00 2001
>>>> From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
>>>> Date: Thu, 6 Sep 2018 19:53:18 +0900
>>>> Subject: [PATCH] mm, oom: Introduce time limit for dump_tasks duration.
>>>>
>>>> Since printk() is slow, printing one line takes nearly 0.01 second.
>>>> As a result, syzbot is stalling for 52 seconds trying to dump 5600
>>>> tasks at for_each_process() under RCU. Since such situation is almost
>>>> inflight fork bomb attack (the OOM killer will print similar tasks for
>>>> so many times), it makes little sense to print all candidate tasks.
>>>> Thus, this patch introduces 3 seconds limit for printing.
>>>>
>>>> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
>>>> Cc: Dmitry Vyukov <dvyukov@google.com>
>>>
>>> You really love timeout based solutions with randomly chosen timeouts,
>>> don't you. This is just ugly as hell. We already have means to disable
>>> tasks dumping (see /proc/sys/vm/oom_dump_tasks).
>>
>> I know /proc/sys/vm/oom_dump_tasks . Showing some entries while not always
>> printing all entries might be helpful.
> 
> Not really. It could be more confusing than helpful. The main purpose of
> the listing is to double check the list to understand the oom victim
> selection. If you have a partial list you simply cannot do that.

It serves as a safeguard for avoiding RCU stall warnings.

> 
> If the iteration takes too long and I can imagine it does with zillions
> of tasks then the proper way around it is either release the lock
> periodically after N tasks is processed or outright skip the whole thing
> if there are too many tasks. The first option is obviously tricky to
> prevent from duplicate entries or other artifacts.
> 

Can we add rcu_lock_break() like check_hung_uninterruptible_tasks() does?