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From: Michal Hocko <mhocko@kernel.org>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	David Rientjes <rientjes@google.com>,
	linux-mm@kvack.org, Yong-Taek Lee <ytk.lee@samsung.com>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] mm, oom: Tolerate processes sharing mm with different view of oom_score_adj.
Date: Fri, 1 Feb 2019 10:14:33 +0100	[thread overview]
Message-ID: <20190201091433.GH11599@dhcp22.suse.cz> (raw)
In-Reply-To: <5fd73d87-3e4b-f793-1976-b937955663e3@i-love.sakura.ne.jp>

On Fri 01-02-19 05:59:55, Tetsuo Handa wrote:
> On 2019/01/31 16:11, Michal Hocko wrote:
> > On Thu 31-01-19 07:49:35, Tetsuo Handa wrote:
> >> This patch reverts both commit 44a70adec910d692 ("mm, oom_adj: make sure
> >> processes sharing mm have same view of oom_score_adj") and commit
> >> 97fd49c2355ffded ("mm, oom: kill all tasks sharing the mm") in order to
> >> close a race and reduce the latency at __set_oom_adj(), and reduces the
> >> warning at __oom_kill_process() in order to minimize the latency.
> >>
> >> Commit 36324a990cf578b5 ("oom: clear TIF_MEMDIE after oom_reaper managed
> >> to unmap the address space") introduced the worst case mentioned in
> >> 44a70adec910d692. But since the OOM killer skips mm with MMF_OOM_SKIP set,
> >> only administrators can trigger the worst case.
> >>
> >> Since 44a70adec910d692 did not take latency into account, we can "hold RCU
> >> for minutes and trigger RCU stall warnings" by calling printk() on many
> >> thousands of thread groups. Also, current code becomes a DoS attack vector
> >> which will allow "stalling for more than one month in unkillable state"
> >> simply printk()ing same messages when many thousands of thread groups
> >> tried to iterate __set_oom_adj() on each other.
> >>
> >> I also noticed that 44a70adec910d692 is racy [1], and trying to fix the
> >> race will require a global lock which is too costly for rare events. And
> >> Michal Hocko is thinking to change the oom_score_adj implementation to per
> >> mm_struct (with shadowed score stored in per task_struct in order to
> >> support vfork() => __set_oom_adj() => execve() sequence) so that we don't
> >> need the global lock.
> >>
> >> If the worst case in 44a70adec910d692 happened, it is an administrator's
> >> request. Therefore, before changing the oom_score_adj implementation,
> >> let's eliminate the DoS attack vector first.
> > 
> > This is really ridiculous. I have already nacked the previous version
> > and provided two ways around. The simplest one is to drop the printk.
> > The second one is to move oom_score_adj to the mm struct. Could you
> > explain why do you still push for this?
> 
> Dropping printk() does not close the race.

But it does remove the source of a long operation from the RCU context.
If you are not willing to post such a trivial patch I will do so.

> You must propose an alternative patch if you dislike this patch.

I will eventually get there.
-- 
Michal Hocko
SUSE Labs

  reply	other threads:[~2019-02-01  9:14 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-16 10:55 [PATCH] " Tetsuo Handa
2019-01-16 11:09 ` Michal Hocko
2019-01-16 11:30   ` Tetsuo Handa
2019-01-16 12:19     ` Michal Hocko
2019-01-16 13:32       ` Tetsuo Handa
2019-01-16 13:41         ` Michal Hocko
2019-01-17 10:40           ` Tetsuo Handa
2019-01-17 15:51           ` Michal Hocko
2019-01-30 22:49             ` [PATCH v2] " Tetsuo Handa
2019-01-31  7:11               ` Michal Hocko
2019-01-31 20:59                 ` Tetsuo Handa
2019-02-01  9:14                   ` Michal Hocko [this message]
2019-02-02 11:06                     ` Tetsuo Handa
2019-02-11 15:07                       ` Michal Hocko

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