Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754425AbaAULFX (ORCPT ); Tue, 21 Jan 2014 06:05:23 -0500 Received: from cantor2.suse.de ([195.135.220.15]:42064 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754023AbaAULFU (ORCPT ); Tue, 21 Jan 2014 06:05:20 -0500 Date: Tue, 21 Jan 2014 11:05:15 +0000 From: Mel Gorman To: Andrew Morton Cc: Minchan Kim , Xishi Qiu , Laura Abbott , riel@redhat.com, aquini@redhat.com, linux-mm@kvack.org, LKML Subject: [PATCH] mm: Improve documentation of page_order v2 Message-ID: <20140121110515.GA14687@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> <20140117195957.GG4963@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20140117195957.GG4963@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 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. [lauraa@codeaurora.org: Corrected a page_zone(page)->lock reference] Signed-off-by: Mel Gorman Acked-by: Rafael Aquini Acked-by: Minchan Kim --- 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)->lock, they must guarantee that the + * page cannot be allocated or merged in parallel. */ static inline unsigned long page_order(struct page *page) { -- 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/