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Subject: Re: [PATCH] proc, oom: do not report alien mms when setting
 oom_score_adj
To:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>
Cc:     David Rientjes <rientjes@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Yong-Taek Lee <ytk.lee@samsung.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>,
        Michal Hocko <mhocko@suse.com>
References: <20190212102129.26288-1-mhocko@kernel.org>
 <20190212125635.27742b5741e92a0d47690c53@linux-foundation.org>
From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <46b3262e-2a9a-da01-16de-14cd4d7eaa40@i-love.sakura.ne.jp>
Date:   Wed, 13 Feb 2019 06:07:26 +0900
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On 2019/02/13 5:56, Andrew Morton wrote:
> On Tue, 12 Feb 2019 11:21:29 +0100 Michal Hocko <mhocko@kernel.org> wrote:
> 
>> Tetsuo has reported that creating a thousands of processes sharing MM
>> without SIGHAND (aka alien threads) and setting
>> /proc/<pid>/oom_score_adj will swamp the kernel log and takes ages [1]
>> to finish. This is especially worrisome that all that printing is done
>> under RCU lock and this can potentially trigger RCU stall or softlockup
>> detector.
>>
>> The primary reason for the printk was to catch potential users who might
>> depend on the behavior prior to 44a70adec910 ("mm, oom_adj: make sure
>> processes sharing mm have same view of oom_score_adj") but after more
>> than 2 years without a single report I guess it is safe to simply remove
>> the printk altogether.
>>
>> The next step should be moving oom_score_adj over to the mm struct and
>> remove all the tasks crawling as suggested by [2]
>>
>> [1] http://lkml.kernel.org/r/97fce864-6f75-bca5-14bc-12c9f890e740@i-love.sakura.ne.jp
>> [2] http://lkml.kernel.org/r/20190117155159.GA4087@dhcp22.suse.cz
> 
> I think I'll put a cc:stable on this.  Deleting a might-trigger debug
> printk is safe and welcome.
> 

Putting cc:stable is fine. But I doubt the usefulness of this patch.
If nobody really depends on the behavior prior to 44a70adec910,
we should remove the pointless (otherwise racy) iteration itself.