From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751870AbcDZMFC (ORCPT ); Tue, 26 Apr 2016 08:05:02 -0400 Received: from mx2.suse.de ([195.135.220.15]:34708 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751953AbcDZMFA (ORCPT ); Tue, 26 Apr 2016 08:05:00 -0400 Subject: Re: [PATCH 13/28] mm, page_alloc: Remove redundant check for empty zonelist To: Mel Gorman , Andrew Morton References: <1460710760-32601-1-git-send-email-mgorman@techsingularity.net> <1460711275-1130-1-git-send-email-mgorman@techsingularity.net> Cc: Jesper Dangaard Brouer , Linux-MM , LKML From: Vlastimil Babka Message-ID: <571F5963.1000504@suse.cz> Date: Tue, 26 Apr 2016 14:04:51 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1 MIME-Version: 1.0 In-Reply-To: <1460711275-1130-1-git-send-email-mgorman@techsingularity.net> Content-Type: text/plain; charset=iso-8859-2; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/15/2016 11:07 AM, Mel Gorman wrote: > A check is made for an empty zonelist early in the page allocator fast path > but it's unnecessary. When get_page_from_freelist() is called, it'll return > NULL immediately. Removing the first check is slower for machines with > memoryless nodes but that is a corner case that can live with the overhead. > > Signed-off-by: Mel Gorman > --- > mm/page_alloc.c | 11 ----------- > 1 file changed, 11 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index df03ccc7f07c..21aaef6ddd7a 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -3374,14 +3374,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, > if (should_fail_alloc_page(gfp_mask, order)) > return NULL; > > - /* > - * Check the zones suitable for the gfp_mask contain at least one > - * valid zone. It's possible to have an empty zonelist as a result > - * of __GFP_THISNODE and a memoryless node > - */ > - if (unlikely(!zonelist->_zonerefs->zone)) > - return NULL; > - > if (IS_ENABLED(CONFIG_CMA) && ac.migratetype == MIGRATE_MOVABLE) > alloc_flags |= ALLOC_CMA; > > @@ -3394,8 +3386,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, > /* The preferred zone is used for statistics later */ > preferred_zoneref = first_zones_zonelist(ac.zonelist, ac.high_zoneidx, > ac.nodemask, &ac.preferred_zone); > - if (!ac.preferred_zone) > - goto out; Is this part really safe? Besides changelog doesn't mention preferred_zone. What if somebody attempts e.g. a DMA allocation with ac.nodemask being set to cpuset_current_mems_allowed and initially only containing nodes without ZONE_DMA. Then ac.preferred_zone is NULL, yet we proceed to get_page_from_freelist(). Meanwhile cpuset_current_mems_allowed gets changed so in fact it does contains a suitable node, so we manage to get inside for_each_zone_zonelist_nodemask(). Then there's zone_local(ac->preferred_zone, zone), which will defererence the NULL ac->preferred_zone? > ac.classzone_idx = zonelist_zone_idx(preferred_zoneref); > > /* First allocation attempt */ > @@ -3418,7 +3408,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, > > trace_mm_page_alloc(page, order, alloc_mask, ac.migratetype); > > -out: > /* > * When updating a task's mems_allowed, it is possible to race with > * parallel threads in such a way that an allocation can fail while >