From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx174.postini.com [74.125.245.174]) by kanga.kvack.org (Postfix) with SMTP id 7CC446B004F for ; Thu, 8 Dec 2011 03:05:50 -0500 (EST) Date: Thu, 8 Dec 2011 09:05:47 +0100 From: Michal Hocko Subject: Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing free_pages? Message-ID: <20111208080547.GA5631@tiehlicka.suse.cz> References: <20111205161443.GA20663@tiehlicka.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Wang Sheng-Hui Cc: linux-mm@kvack.org, Mel Gorman , Andrew Morton On Thu 08-12-11 10:38:39, Wang Sheng-Hui wrote: > Sorry, Michal. > > 2011/12/6 Michal Hocko > > > 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; > > > > I don't understand why there is additional "-1". > Use 0-order allocation as example: > 0-order page ---- one 4K page > free_pages should subtract 1. Here, free_pages will subtract 0? Check out all the conditions for free_pages... -- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org