From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f197.google.com (mail-pf0-f197.google.com [209.85.192.197]) by kanga.kvack.org (Postfix) with ESMTP id C749A6B0003 for ; Wed, 20 Jun 2018 09:07:50 -0400 (EDT) Received: by mail-pf0-f197.google.com with SMTP id b5-v6so1577521pfi.5 for ; Wed, 20 Jun 2018 06:07:50 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id 84-v6si2267234pfa.60.2018.06.20.06.07.49 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 20 Jun 2018 06:07:49 -0700 (PDT) Date: Wed, 20 Jun 2018 15:07:46 +0200 From: Michal Hocko Subject: Re: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM killer. Message-ID: <20180620130746.GN13685@dhcp22.suse.cz> References: <1529493638-6389-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> <20180620115531.GL13685@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Tetsuo Handa Cc: linux-mm@kvack.org, rientjes@google.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org On Wed 20-06-18 21:21:21, Tetsuo Handa wrote: > On 2018/06/20 20:55, 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. > > "A" is "owns oom_lock" and "B" is "owns CPU resources". It was demonstrated > at "mm,oom: Don't call schedule_timeout_killable() with oom_lock held." proposal. This is not a deadlock but merely a resource starvation AFAIU. > But since you don't accept preserving the short sleep which is a heuristic for > reducing the possibility of AB-BA lockup, the only way we would accept will be > wait for the owner of oom_lock (e.g. by s/mutex_trylock/mutex_lock/ or whatever) > which is free of heuristic and free of AB-BA lockup. > > > > >> 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? > > > > For long term, it would be possible. But not within this patch. For example, > I think that virtio_balloon wants to release memory only when we have no > choice but OOM kill. If virtio_balloon trivially releases memory, it will > increase the risk of killing the entire guest by OOM-killer from the host > side. I would _prefer_ to think long term here. The sleep inside the oom lock is not something real workload are seeing out there AFAICS. Adding quite some code to address such a case doesn't justify the inclusion IMHO. -- Michal Hocko SUSE Labs