From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754013AbcLFTZt (ORCPT ); Tue, 6 Dec 2016 14:25:49 -0500 Received: from mail-wj0-f196.google.com ([209.85.210.196]:35300 "EHLO mail-wj0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751982AbcLFTZs (ORCPT ); Tue, 6 Dec 2016 14:25:48 -0500 Date: Tue, 6 Dec 2016 20:25:45 +0100 From: Michal Hocko To: Vlastimil Babka Cc: Tetsuo Handa , akpm@linux-foundation.org, hannes@cmpxchg.org, mgorman@suse.de, rientjes@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] mm, oom: do not enfore OOM killer for __GFP_NOFAIL automatically Message-ID: <20161206192544.GB10273@dhcp22.suse.cz> References: <20161201152517.27698-1-mhocko@kernel.org> <20161201152517.27698-3-mhocko@kernel.org> <201612052245.HDB21880.OHJMOOQFFSVLtF@I-love.SAKURA.ne.jp> <20161205141009.GJ30758@dhcp22.suse.cz> <201612061938.DDD73970.QFHOFJStFOLVOM@I-love.SAKURA.ne.jp> <01a495b8-36f6-28f5-5a55-089f4860747d@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <01a495b8-36f6-28f5-5a55-089f4860747d@suse.cz> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 06-12-16 12:03:02, Vlastimil Babka wrote: > On 12/06/2016 11:38 AM, Tetsuo Handa wrote: > >> > >> So we are somewhere in the middle between pre-mature and pointless > >> system disruption (GFP_NOFS with a lots of metadata or lowmem request) > >> where the OOM killer even might not help and potential lockup which is > >> inevitable with the current design. Dunno about you but I would rather > >> go with the first option. To be honest I really fail to understand your > >> line of argumentation. We have this > >> do { > >> cond_resched(); > >> } while (!(page = alloc_page(GFP_NOFS))); > >> vs. > >> page = alloc_page(GFP_NOFS | __GFP_NOFAIL); > >> > >> the first one doesn't invoke OOM killer while the later does. This > >> discrepancy just cannot make any sense... The same is true for > >> > >> alloc_page(GFP_DMA) vs alloc_page(GFP_DMA|__GFP_NOFAIL) > >> > >> Now we can discuss whether it is a _good_ idea to not invoke OOM killer > >> for those exceptions but whatever we do __GFP_NOFAIL is not a way to > >> give such a subtle side effect. Or do you disagree even with that? > > > > "[PATCH 1/2] mm: consolidate GFP_NOFAIL checks in the allocator slowpath" > > silently changes __GFP_NOFAIL vs. __GFP_NORETRY priority. > > I guess that wasn't intended? I even didn't think about that possibility because it just doesn't make any sense. > > Currently, __GFP_NORETRY is stronger than __GFP_NOFAIL; __GFP_NOFAIL > > allocation requests fail without invoking the OOM killer when both > > __GFP_NORETRY and __GFP_NOFAIL are given. > > > > With [PATCH 1/2], __GFP_NOFAIL becomes stronger than __GFP_NORETRY; > > __GFP_NOFAIL allocation requests will loop forever without invoking > > the OOM killer when both __GFP_NORETRY and __GFP_NOFAIL are given. > > Does such combination of flag make sense? Should we warn about it, or > even silently remove __GFP_NORETRY in such case? No this combination doesn't make any sense. I seriously doubt we should even care about it and simply following the stronger requirement makes more sense from a semantic point of view. -- Michal Hocko SUSE Labs