Re: [PATCH v8] oom_kill.c: futex: Don't OOM reap the VMA containing the robust_list_head

From: Michal Hocko <mhocko@suse.com>
To: Nico Pache <npache@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Rafael Aquini <aquini@redhat.com>,
	Waiman Long <longman@redhat.com>, Baoquan He <bhe@redhat.com>,
	Christoph von Recklinghausen <crecklin@redhat.com>,
	Don Dutile <ddutile@redhat.com>,
	"Herton R . Krzesinski" <herton@redhat.com>,
	David Rientjes <rientjes@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Ingo Molnar <mingo@redhat.com>, Joel Savitz <jsavitz@redhat.com>,
	Darren Hart <dvhart@infradead.org>,
	stable@kernel.org
Subject: Re: [PATCH v8] oom_kill.c: futex: Don't OOM reap the VMA containing the robust_list_head
Date: Fri, 8 Apr 2022 13:48:08 +0200	[thread overview]
Message-ID: <YlAg+Pu7hIw6W+cc@dhcp22.suse.cz> (raw)
In-Reply-To: <465ab95b-3e71-5901-c184-812dc595af2f@redhat.com>

On Fri 08-04-22 07:26:07, Nico Pache wrote:
[...]
> Ok so if i understand that correctly, delaying can have some ugly effects and
> kinda breaks the initial purpose of the OOM reaper?

No, not really. The primary objective of the oom_reaper is to _guaratee_
a forward progress. It is not really meant to be an optimization to
respond to the oom killer faster. The reason the oom_reaper is kicked
off right away is because that was the simplest implementation.

> I personally don't like the delay approach. Especially if we have a better one
> we know is working, and that doesnt add regressions.

Well, I would say that handling futex case more gracefully would be
preferable but my understanding is that this is not all that easy. I am
far from being a futex expert so I will leave that up to Thomas and Peter.

On the other hand delaying oom_reaper is rather straightforward and I do
not think there is a big risk of regressions. Any QoS during OOM is
simply out of the window and the main purpose of the reaper will be
preserved with a timeout as well. I also do agree with Thomas that this
would cover 99% of cases.

> If someone can prove to me the private lock case, I'd be more willing to bite.
> 
> Thanks for all the OOM context :)

Welcome. The oom handling is a maze and it is really easy to miss all
the subtlety and conflicting requirements that are applied here.
-- 
Michal Hocko
SUSE Labs