Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752399AbdFWNLb (ORCPT ); Fri, 23 Jun 2017 09:11:31 -0400 Received: from mx2.suse.de ([195.135.220.15]:33730 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751182AbdFWNLa (ORCPT ); Fri, 23 Jun 2017 09:11:30 -0400 Subject: Re: [PATCH] mm/page_alloc.c: eliminate unsigned confusion in __rmqueue_fallback To: Rasmus Villemoes , Michal Hocko , Andrew Morton , Hillf Danton , Johannes Weiner , Tetsuo Handa , Vinayak Menon , Xishi Qiu Cc: Hao Lee , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20170621094344.GC22051@dhcp22.suse.cz> <20170621185529.2265-1-linux@rasmusvillemoes.dk> From: Vlastimil Babka Message-ID: Date: Fri, 23 Jun 2017 15:10:45 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.1 MIME-Version: 1.0 In-Reply-To: <20170621185529.2265-1-linux@rasmusvillemoes.dk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2167 Lines: 55 On 06/21/2017 08:55 PM, Rasmus Villemoes wrote: > Since current_order starts as MAX_ORDER-1 and is then only > decremented, the second half of the loop condition seems > superfluous. However, if order is 0, we may decrement current_order > past 0, making it UINT_MAX. This is obviously too subtle ([1], [2]). > > Since we need to add some comment anyway, change the two variables to > signed, making the counting-down for loop look more familiar, and > apparently also making gcc generate slightly smaller code. > > [1] https://lkml.org/lkml/2016/6/20/493 > [2] https://lkml.org/lkml/2017/6/19/345 > > Signed-off-by: Rasmus Villemoes Acked-by: Vlastimil Babka > --- > Michal, something like this, perhaps? > > mm/page_alloc.c | 10 +++++++--- > 1 file changed, 7 insertions(+), 3 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 2302f250d6b1..e656f4da9772 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -2204,19 +2204,23 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac, > * list of requested migratetype, possibly along with other pages from the same > * block, depending on fragmentation avoidance heuristics. Returns true if > * fallback was found so that __rmqueue_smallest() can grab it. > + * > + * The use of signed ints for order and current_order is a deliberate > + * deviation from the rest of this file, to make the for loop > + * condition simpler. > */ > static inline bool > -__rmqueue_fallback(struct zone *zone, unsigned int order, int start_migratetype) > +__rmqueue_fallback(struct zone *zone, int order, int start_migratetype) > { > struct free_area *area; > - unsigned int current_order; > + int current_order; > struct page *page; > int fallback_mt; > 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 >= order; > --current_order) { > area = &(zone->free_area[current_order]); > fallback_mt = find_suitable_fallback(area, current_order, >