Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753488AbaAQUAI (ORCPT ); Fri, 17 Jan 2014 15:00:08 -0500 Received: from cantor2.suse.de ([195.135.220.15]:43728 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753435AbaAQUAE (ORCPT ); Fri, 17 Jan 2014 15:00:04 -0500 Date: Fri, 17 Jan 2014 19:59:57 +0000 From: Mel Gorman To: Laura Abbott Cc: Andrew Morton , Minchan Kim , Xishi Qiu , riel@redhat.com, aquini@redhat.com, linux-mm@kvack.org, LKML Subject: Re: [PATCH] mm: Improve documentation of page_order Message-ID: <20140117195957.GG4963@suse.de> 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> <52D97C3E.2080709@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <52D97C3E.2080709@codeaurora.org> 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 10:53:50AM -0800, Laura Abbott wrote: > On 1/17/2014 6:32 AM, 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 > page_zone(page)->lock here? *sigh* Yes, thanks. -- Mel Gorman SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/