From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755272AbcEDSTW (ORCPT ); Wed, 4 May 2016 14:19:22 -0400 Received: from mail-wm0-f65.google.com ([74.125.82.65]:36676 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752124AbcEDSTT (ORCPT ); Wed, 4 May 2016 14:19:19 -0400 Date: Wed, 4 May 2016 20:19:16 +0200 From: Michal Hocko To: Joonsoo Kim Cc: Joonsoo Kim , Andrew Morton , Linus Torvalds , Johannes Weiner , Mel Gorman , David Rientjes , Tetsuo Handa , Hillf Danton , Vlastimil Babka , Linux Memory Management List , LKML Subject: Re: [PATCH 12/14] mm, oom: protect !costly allocations some more Message-ID: <20160504181916.GB21490@dhcp22.suse.cz> References: <1461181647-8039-1-git-send-email-mhocko@kernel.org> <1461181647-8039-13-git-send-email-mhocko@kernel.org> <20160504060123.GB10899@js1304-P5Q-DELUXE> <20160504063112.GD10899@js1304-P5Q-DELUXE> <20160504085628.GE29978@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 04-05-16 23:57:50, Joonsoo Kim wrote: > 2016-05-04 17:56 GMT+09:00 Michal Hocko : > > On Wed 04-05-16 15:31:12, Joonsoo Kim wrote: > >> On Wed, May 04, 2016 at 03:01:24PM +0900, Joonsoo Kim wrote: > >> > On Wed, Apr 20, 2016 at 03:47:25PM -0400, Michal Hocko wrote: > > [...] > >> > > @@ -3408,6 +3456,17 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, > >> > > no_progress_loops)) > >> > > goto retry; > >> > > > >> > > + /* > >> > > + * It doesn't make any sense to retry for the compaction if the order-0 > >> > > + * reclaim is not able to make any progress because the current > >> > > + * implementation of the compaction depends on the sufficient amount > >> > > + * of free memory (see __compaction_suitable) > >> > > + */ > >> > > + if (did_some_progress > 0 && > >> > > + should_compact_retry(order, compact_result, > >> > > + &migration_mode, compaction_retries)) > >> > > >> > Checking did_some_progress on each round have subtle corner case. Think > >> > about following situation. > >> > > >> > round, compaction, did_some_progress, compaction > >> > 0, defer, 1 > >> > 0, defer, 1 > >> > 0, defer, 1 > >> > 0, defer, 1 > >> > 0, defer, 0 > >> > >> Oops...Example should be below one. > >> > >> 0, defer, 1 > >> 1, defer, 1 > >> 2, defer, 1 > >> 3, defer, 1 > >> 4, defer, 0 > > > > I am not sure I understand. The point of the check is that if the > > reclaim doesn't make _any_ progress then checking the result of the > > compaction after it didn't lead to a successful allocation just doesn't > > make any sense. > > Even if this round (#4) doesn't reclaim any pages, previous rounds > (#0, #1, #2, #3) would reclaim enough pages to succeed future > compaction attempt. Then the compaction shouldn't back off and I would consider it a compaction bug. I haven't see this happening though. Vlastimil is already working on patches which would simply guarantee that really important allocations will not defer. So unless I can see an example of a real issue with this I think it is just a theoretical issue which shouldn't block the patch as is. -- Michal Hocko SUSE Labs