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Subject: Re: [PATCH] mm, oom: Introduce time limit for dump_tasks duration.
References: <0252ad5d-46e6-0d7f-ef91-4e316657a83d@i-love.sakura.ne.jp>
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From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <1611e45d-235e-67e9-26e3-d0228255fa2f@i-love.sakura.ne.jp>
Date: Thu, 6 Sep 2018 20:40:34 +0900
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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 20:23, 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. For example, allow
/proc/sys/vm/oom_dump_tasks > 1 and use it as timeout in seconds.