From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752987AbaAQSk5 (ORCPT ); Fri, 17 Jan 2014 13:40:57 -0500 Received: from mx1.redhat.com ([209.132.183.28]:50389 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752231AbaAQSkz (ORCPT ); Fri, 17 Jan 2014 13:40:55 -0500 Date: Fri, 17 Jan 2014 16:40:42 -0200 From: Rafael Aquini To: Mel Gorman Cc: Andrew Morton , Minchan Kim , Xishi Qiu , riel@redhat.com, linux-mm@kvack.org, LKML Subject: Re: [PATCH] mm: Improve documentation of page_order Message-ID: <20140117184041.GA7449@localhost.localdomain> References: <520B0B75.4030708@huawei.com> <20130814085711.GK2296@suse.de> <20130814155205.GA2706@gmail.com> <20130814132602.814a88e991e29c5b93bbe22c@linux-foundation.org> <20130814222241.GQ2296@suse.de> <20140117143221.GA24851@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140117143221.GA24851@suse.de> 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, Jan 17, 2014 at 02:32:21PM +0000, Mel Gorman wrote: > Developers occasionally try and optimise PFN scanners by using page_order > but miss that in general it requires zone->lock. This has happened twice for > compaction.c and rejected both times. This patch clarifies the documentation > of page_order and adds a note to compaction.c why page_order is not used. > > Signed-off-by: Mel Gorman > --- > mm/compaction.c | 5 ++++- > mm/internal.h | 8 +++++--- > 2 files changed, 9 insertions(+), 4 deletions(-) > > diff --git a/mm/compaction.c b/mm/compaction.c > index f58bcd0..f91d26b 100644 > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -522,7 +522,10 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, > if (!isolation_suitable(cc, page)) > goto next_pageblock; > > - /* Skip if free */ > + /* > + * Skip if free. page_order cannot be used without zone->lock > + * as nothing prevents parallel allocations or buddy merging. > + */ > if (PageBuddy(page)) > continue; > > diff --git a/mm/internal.h b/mm/internal.h > index 684f7aa..09cd8be 100644 > --- a/mm/internal.h > +++ b/mm/internal.h > @@ -144,9 +144,11 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, > #endif > > /* > - * function for dealing with page's order in buddy system. > - * zone->lock is already acquired when we use these. > - * So, we don't need atomic page->flags operations here. > + * This functions returns the order of a free page in the buddy system. > + * In general, page_zone(page)->lock must be held by the caller to prevent > + * the page being allocated in parallel and returning garbage as the order. > + * If the caller does not hold page_zone(page), they must guarantee that > + * the page cannot be allocated or merged in parallel. > */ > static inline unsigned long page_order(struct page *page) > { Acked-by: Rafael Aquini