From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f41.google.com (mail-pa0-f41.google.com [209.85.220.41]) by kanga.kvack.org (Postfix) with ESMTP id 70BAF6B0031 for ; Fri, 17 Jan 2014 13:53:53 -0500 (EST) Received: by mail-pa0-f41.google.com with SMTP id fa1so2149136pad.14 for ; Fri, 17 Jan 2014 10:53:53 -0800 (PST) Received: from smtp.codeaurora.org (smtp.codeaurora.org. [198.145.11.231]) by mx.google.com with ESMTPS id s7si10915635pae.156.2014.01.17.10.53.51 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 17 Jan 2014 10:53:52 -0800 (PST) Message-ID: <52D97C3E.2080709@codeaurora.org> Date: Fri, 17 Jan 2014 10:53:50 -0800 From: Laura Abbott MIME-Version: 1.0 Subject: Re: [PATCH] mm: Improve documentation of page_order 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> In-Reply-To: <20140117143221.GA24851@suse.de> Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman , Andrew Morton Cc: Minchan Kim , Xishi Qiu , riel@redhat.com, aquini@redhat.com, linux-mm@kvack.org, LKML 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? > + * the page cannot be allocated or merged in parallel. > */ > static inline unsigned long page_order(struct page *page) > { > > -- > 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/ . > Don't email: email@kvack.org > -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation -- 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/ . Don't email: email@kvack.org