linux-kernel - Re: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM killer.

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Date:   Mon, 25 Jun 2018 15:03:40 +0200
From:   peter enderborg <peter.enderborg@...y.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
CC:     <linux-mm@...ck.org>, <rientjes@...gle.com>,
        <akpm@...ux-foundation.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM
 killer.

On 06/20/2018 01:55 PM, Michal Hocko wrote:
> On Wed 20-06-18 20:20:38, Tetsuo Handa wrote:
>> Sleeping with oom_lock held can cause AB-BA lockup bug because
>> __alloc_pages_may_oom() does not wait for oom_lock. Since
>> blocking_notifier_call_chain() in out_of_memory() might sleep, sleeping
>> with oom_lock held is currently an unavoidable problem.
> Could you be more specific about the potential deadlock? Sleeping while
> holding oom lock is certainly not nice but I do not see how that would
> result in a deadlock assuming that the sleeping context doesn't sleep on
> the memory allocation obviously.

It is a mutex you are supposed to be able to sleep.  It's even exported.

>> As a preparation for not to sleep with oom_lock held, this patch brings
>> OOM notifier callbacks to outside of OOM killer, with two small behavior
>> changes explained below.
> Can we just eliminate this ugliness and remove it altogether? We do not
> have that many notifiers. Is there anything fundamental that would
> prevent us from moving them to shrinkers instead?


@Hocko Do you remember the lowmemorykiller from android? Some things might not be the right thing for shrinkers.