From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755966Ab3AGWbm (ORCPT ); Mon, 7 Jan 2013 17:31:42 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:38857 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755920Ab3AGWbk (ORCPT ); Mon, 7 Jan 2013 17:31:40 -0500 Date: Mon, 7 Jan 2013 14:31:28 -0800 From: Andrew Morton To: Laura Abbott Cc: Mel Gorman , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org Subject: Re: [RESEND][PATCH v3] mm: Use aligned zone start for pfn_to_bitidx calculation Message-Id: <20130107143128.face9220.akpm@linux-foundation.org> In-Reply-To: <1357414111-20736-1-git-send-email-lauraa@codeaurora.org> References: <1357414111-20736-1-git-send-email-lauraa@codeaurora.org> X-Mailer: Sylpheed 3.0.2 (GTK+ 2.20.1; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 5 Jan 2013 11:28:31 -0800 Laura Abbott wrote: > The current calculation in pfn_to_bitidx assumes that > (pfn - zone->zone_start_pfn) >> pageblock_order will return the > same bit for all pfn in a pageblock. If zone_start_pfn is not > aligned to pageblock_nr_pages, this may not always be correct. > > Consider the following with pageblock order = 10, zone start 2MB: > > pfn | pfn - zone start | (pfn - zone start) >> page block order > ---------------------------------------------------------------- > 0x26000 | 0x25e00 | 0x97 > 0x26100 | 0x25f00 | 0x97 > 0x26200 | 0x26000 | 0x98 > 0x26300 | 0x26100 | 0x98 > > This means that calling {get,set}_pageblock_migratetype on a single > page will not set the migratetype for the full block. Fix this by > rounding down zone_start_pfn when doing the bitidx calculation. What are the user-visible effects of this bug?