Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752019Ab3HOEYz (ORCPT ); Thu, 15 Aug 2013 00:24:55 -0400 Received: from mail-pa0-f50.google.com ([209.85.220.50]:55601 "EHLO mail-pa0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751043Ab3HOEYy (ORCPT ); Thu, 15 Aug 2013 00:24:54 -0400 Date: Thu, 15 Aug 2013 13:24:43 +0900 From: Minchan Kim To: Xishi Qiu Cc: Mel Gorman , Andrew Morton , riel@redhat.com, aquini@redhat.com, linux-mm@kvack.org, LKML Subject: Re: [PATCH] mm: skip the page buddy block instead of one page Message-ID: <20130815042434.GA3139@gmail.com> References: <520B0B75.4030708@huawei.com> <20130814085711.GK2296@suse.de> <20130814155205.GA2706@gmail.com> <20130814161642.GM2296@suse.de> <20130814163921.GC2706@gmail.com> <20130814180012.GO2296@suse.de> <520C3DD2.8010905@huawei.com> <20130815024427.GA2718@gmail.com> <520C4EFF.8040305@huawei.com> <20130815041736.GA2592@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130815041736.GA2592@gmail.com> 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 Content-Length: 3556 Lines: 114 On Thu, Aug 15, 2013 at 01:17:55PM +0900, Minchan Kim wrote: > Hello, > > On Thu, Aug 15, 2013 at 11:46:07AM +0800, Xishi Qiu wrote: > > On 2013/8/15 10:44, Minchan Kim wrote: > > > > > Hi Xishi, > > > > > > On Thu, Aug 15, 2013 at 10:32:50AM +0800, Xishi Qiu wrote: > > >> On 2013/8/15 2:00, Mel Gorman wrote: > > >> > > >>>>> Even if the page is still page buddy, there is no guarantee that it's > > >>>>> the same page order as the first read. It could have be currently > > >>>>> merging with adjacent buddies for example. There is also a really > > >>>>> small race that a page was freed, allocated with some number stuffed > > >>>>> into page->private and freed again before the second PageBuddy check. > > >>>>> It's a bit of a hand grenade. How much of a performance benefit is there > > >>>> > > >>>> 1. Just worst case is skipping pageblock_nr_pages > > >>> > > >>> No, the worst case is that page_order returns a number that is > > >>> completely garbage and low_pfn goes off the end of the zone > > >>> > > >>>> 2. Race is really small > > >>>> 3. Higher order page allocation customer always have graceful fallback. > > >>>> > > >> > > >> Hi Minchan, > > >> I think in this case, we may get the wrong value from page_order(page). > > >> > > >> 1. page is in page buddy > > >> > > >>> if (PageBuddy(page)) { > > >> > > >> 2. someone allocated the page, and set page->private to another value > > >> > > >>> int nr_pages = (1 << page_order(page)) - 1; > > >> > > >> 3. someone freed the page > > >> > > >>> if (PageBuddy(page)) { > > >> > > >> 4. we will skip wrong pages > > > > > > So, what's the result by that? > > > As I said, it's just skipping (pageblock_nr_pages -1) at worst case > > > > Hi Minchan, > > I mean if the private is set to a large number, it will skip 2^private > > pages, not (pageblock_nr_pages -1). I find somewhere will use page->private, > > such as fs. Here is the comment about parivate. > > /* Mapping-private opaque data: > > * usually used for buffer_heads > > * if PagePrivate set; used for > > * swp_entry_t if PageSwapCache; > > * indicates order in the buddy > > * system if PG_buddy is set. > > */ > > Please read full thread in detail. > > Mel suggested following as > > if (PageBuddy(page)) { > int nr_pages = (1 << page_order(page)) - 1; > if (PageBuddy(page)) { > nr_pages = min(nr_pages, MAX_ORDER_NR_PAGES - 1); > low_pfn += nr_pages; > continue; > } > } > > min(nr_pages, xxx) removes your concern but I think Mel's version > isn't right. It should be aligned with pageblock boundary so I > suggested following. > > if (PageBuddy(page)) { > #ifdef CONFIG_MEMORY_ISOLATION > unsigned long order = page_order(page); > if (PageBuddy(page)) { > low_pfn += (1 << order) - 1; > low_pfn = min(low_pfn, end_pfn); > } > #endif > continue; > } > > so worst case is (pageblock_nr_pages - 1). > but we don't need to add CONFIG_MEMORY_ISOLATION so my suggestion > is following as. > > if (PageBuddy(page)) { > unsigned long order = page_order(page); > if (PageBuddy(page)) { > low_pfn += (1 << order) - 1; > low_pfn = min(low_pfn, end_pfn); Maybe it should be low_pfn = min(low_pfn, end_pfn - 1). > } > continue; > } > > -- Kind regards, Minchan Kim -- 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/