From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932493Ab1LEQOp (ORCPT ); Mon, 5 Dec 2011 11:14:45 -0500 Received: from cantor2.suse.de ([195.135.220.15]:38282 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932447Ab1LEQOo (ORCPT ); Mon, 5 Dec 2011 11:14:44 -0500 Date: Mon, 5 Dec 2011 17:14:43 +0100 From: Michal Hocko To: Wang Sheng-Hui Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Mel Gorman , Andrew Morton Subject: Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing free_pages? Message-ID: <20111205161443.GA20663@tiehlicka.suse.cz> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 25-11-11 09:21:35, Wang Sheng-Hui wrote: > In line 1459, we have "free_pages -= (1 << order) + 1;". > Suppose allocating one 0-order page, here we'll get > free_pages -= 1 + 1 > I wonder why there is a "+ 1"? Good spot. Check the patch bellow. --- >>From 38a1cf351b111e8791d2db538c8b0b912f5df8b8 Mon Sep 17 00:00:00 2001 From: Michal Hocko Date: Mon, 5 Dec 2011 17:04:23 +0100 Subject: [PATCH] mm: fix off-by-two in __zone_watermark_ok 88f5acf8 [mm: page allocator: adjust the per-cpu counter threshold when memory is low] changed the form how free_pages is calculated but it forgot that we used to do free_pages - ((1 << order) - 1) so we ended up with off-by-two when calculating free_pages. Spotted-by: Wang Sheng-Hui Signed-off-by: Michal Hocko --- mm/page_alloc.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 9dd443d..8a2f1b6 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1457,7 +1457,7 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark, long min = mark; int o; - free_pages -= (1 << order) + 1; + free_pages -= (1 << order) - 1; if (alloc_flags & ALLOC_HIGH) min -= min / 2; if (alloc_flags & ALLOC_HARDER) -- 1.7.7.3 -- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic