Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752850AbdFUJnw (ORCPT ); Wed, 21 Jun 2017 05:43:52 -0400 Received: from mx2.suse.de ([195.135.220.15]:54557 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751069AbdFUJnu (ORCPT ); Wed, 21 Jun 2017 05:43:50 -0400 Date: Wed, 21 Jun 2017 11:43:45 +0200 From: Michal Hocko To: Rasmus Villemoes Cc: Vlastimil Babka , Hao Lee , akpm@linux-foundation.org, mgorman@techsingularity.net, hannes@cmpxchg.org, iamjoonsoo.kim@lge.com, minchan@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: remove a redundant condition in the for loop Message-ID: <20170621094344.GC22051@dhcp22.suse.cz> References: <20170619135418.8580-1-haolee.swjtu@gmail.com> <87y3snajd2.fsf@rasmusvillemoes.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87y3snajd2.fsf@rasmusvillemoes.dk> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1719 Lines: 47 On Mon 19-06-17 21:05:29, Rasmus Villemoes wrote: > On Mon, Jun 19 2017, Vlastimil Babka wrote: > > > On 06/19/2017 03:54 PM, Hao Lee wrote: > >> The variable current_order decreases from MAX_ORDER-1 to order, so the > >> condition current_order <= MAX_ORDER-1 is always true. > >> > >> Signed-off-by: Hao Lee > > > > Sounds right. > > > > Acked-by: Vlastimil Babka > > current_order and order are both unsigned, and if order==0, > current_order >= order is always true, and we may decrement > current_order past 0 making it UINT_MAX... A comment would be in order, > though. Yes, not the first time this has been brought up https://lkml.org/lkml/2016/6/20/493. I guess a comment is long overdue. Or just get rid of the unsigned trap which would be probably more clean. > >> --- > >> mm/page_alloc.c | 5 ++--- > >> 1 file changed, 2 insertions(+), 3 deletions(-) > >> > >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c > >> index 2302f25..9120c2b 100644 > >> --- a/mm/page_alloc.c > >> +++ b/mm/page_alloc.c > >> @@ -2215,9 +2215,8 @@ __rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype) > >> bool can_steal; > >> > >> /* Find the largest possible block of pages in the other list */ > >> - for (current_order = MAX_ORDER-1; > >> - current_order >= order && current_order <= MAX_ORDER-1; > >> - --current_order) { > >> + for (current_order = MAX_ORDER-1; current_order >= order; > >> + --current_order) { > >> area = &(zone->free_area[current_order]); > >> fallback_mt = find_suitable_fallback(area, current_order, > >> start_migratetype, false, &can_steal); > >> -- Michal Hocko SUSE Labs