Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932567AbcCHLMd (ORCPT ); Tue, 8 Mar 2016 06:12:33 -0500 Received: from mx2.suse.de ([195.135.220.15]:55129 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753560AbcCHLMZ (ORCPT ); Tue, 8 Mar 2016 06:12:25 -0500 Subject: Re: [PATCH] mm, oom: protect !costly allocations some more To: Michal Hocko References: <1450203586-10959-1-git-send-email-mhocko@kernel.org> <20160203132718.GI6757@dhcp22.suse.cz> <20160225092315.GD17573@dhcp22.suse.cz> <20160229210213.GX16930@dhcp22.suse.cz> <20160307160838.GB5028@dhcp22.suse.cz> <56DE9A68.2010301@suse.cz> <20160308094612.GB13542@dhcp22.suse.cz> <56DEA0CF.2070902@suse.cz> <20160308101016.GC13542@dhcp22.suse.cz> Cc: Hugh Dickins , Sergey Senozhatsky , Andrew Morton , Linus Torvalds , Johannes Weiner , Mel Gorman , David Rientjes , Tetsuo Handa , Hillf Danton , KAMEZAWA Hiroyuki , linux-mm@kvack.org, LKML , Joonsoo Kim From: Vlastimil Babka Message-ID: <56DEB394.40602@suse.cz> Date: Tue, 8 Mar 2016 12:12:20 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: <20160308101016.GC13542@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2793 Lines: 63 On 03/08/2016 11:10 AM, Michal Hocko wrote: > On Tue 08-03-16 10:52:15, Vlastimil Babka wrote: >> On 03/08/2016 10:46 AM, Michal Hocko wrote: > [...] >>>>> @@ -3294,6 +3289,18 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, >>>>> did_some_progress > 0, no_progress_loops)) >>>>> goto retry; >>>>> >>>>> + /* >>>>> + * !costly allocations are really important and we have to make sure >>>>> + * the compaction wasn't deferred or didn't bail out early due to locks >>>>> + * contention before we go OOM. >>>>> + */ >>>>> + if (order && order <= PAGE_ALLOC_COSTLY_ORDER) { >>>>> + if (compact_result <= COMPACT_CONTINUE) >>>> >>>> Same here. >>>> I was going to say that this didn't have effect on Sergey's test, but >>>> turns out it did :) >>> >>> This should work as expected because compact_result is unsigned long >>> and so this is the unsigned arithmetic. I can make >>> #define COMPACT_NONE -1UL >>> >>> to make the intention more obvious if you prefer, though. >> >> Well, what wasn't obvious to me is actually that here (unlike in the >> test above) it was actually intended that COMPACT_NONE doesn't result in >> a retry. But it makes sense, otherwise we would retry endlessly if >> reclaim couldn't form a higher-order page, right. > > Yeah, that was the whole point. An alternative would be moving the test > into should_compact_retry(order, compact_result, contended_compaction) > which would be CONFIG_COMPACTION specific so we can get rid of the > COMPACT_NONE altogether. Something like the following. We would lose the > always initialized compact_result but this would matter only for > order==0 and we check for that. Even gcc doesn't complain. Yeah I like this version better, you can add my Acked-By. Thanks. > A more important question is whether the criteria I have chosen are > reasonable and reasonably independent on the particular implementation > of the compaction. I still cannot convince myself about the convergence > here. Is it possible that the compaction would keep returning > compact_result <= COMPACT_CONTINUE while not making any progress at all? Theoretically, if reclaim/compaction suitability decisions and allocation attempts didn't match the watermark checks, including the alloc_flags and classzone_idx parameters. Possible scenarios: - reclaim thinks compaction has enough to proceed, but compaction thinks otherwise and returns COMPACT_SKIPPED - compaction thinks it succeeded and returns COMPACT_PARTIAL, but allocation attempt fails - and perhaps some other combinations > Sure we can see a case where somebody is stealing the compacted blocks > but that is very same with the order-0 where parallel mem eaters will > piggy back on the reclaimer and there is no upper boundary as well well. Yep.