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

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Message-ID: <20180625130756.GK28965@dhcp22.suse.cz>
Date:   Mon, 25 Jun 2018 15:07:56 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     peter enderborg <peter.enderborg@...y.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        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 Mon 25-06-18 15:03:40, peter enderborg wrote:
> 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.

What do you mean? oom_lock is certainly not exported for general use. It
is not local to oom_killer.c just because it is needed in other _mm_
code.
 
> >> 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.

Just that lmk did it wrong doesn't mean others have to follow.

-- 
Michal Hocko
SUSE Labs