Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751656AbdFUSz4 (ORCPT ); Wed, 21 Jun 2017 14:55:56 -0400 Received: from mail-wr0-f173.google.com ([209.85.128.173]:33238 "EHLO mail-wr0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751028AbdFUSzy (ORCPT ); Wed, 21 Jun 2017 14:55:54 -0400 From: Rasmus Villemoes To: Michal Hocko , Andrew Morton , Vlastimil Babka , Hillf Danton , Johannes Weiner , Tetsuo Handa , Vinayak Menon , Xishi Qiu Cc: Hao Lee , Rasmus Villemoes , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH] mm/page_alloc.c: eliminate unsigned confusion in __rmqueue_fallback Date: Wed, 21 Jun 2017 20:55:28 +0200 Message-Id: <20170621185529.2265-1-linux@rasmusvillemoes.dk> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20170621094344.GC22051@dhcp22.suse.cz> References: <20170621094344.GC22051@dhcp22.suse.cz> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1982 Lines: 52 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 --- 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, -- 2.11.0