From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx186.postini.com [74.125.245.186]) by kanga.kvack.org (Postfix) with SMTP id F10C76B0032 for ; Wed, 14 Aug 2013 22:33:28 -0400 (EDT) Message-ID: <520C3DD2.8010905@huawei.com> Date: Thu, 15 Aug 2013 10:32:50 +0800 From: Xishi Qiu MIME-Version: 1.0 Subject: Re: [PATCH] mm: skip the page buddy block instead of one page 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> In-Reply-To: <20130814180012.GO2296@suse.de> Content-Type: text/plain; charset="ISO-8859-15" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman , Minchan Kim Cc: Andrew Morton , riel@redhat.com, aquini@redhat.com, linux-mm@kvack.org, LKML , Xishi Qiu 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 > nr_pages = min(nr_pages, MAX_ORDER_NR_PAGES - 1); > low_pfn += nr_pages; > continue; > } > } > > It's still race-prone meaning that it really should be backed by some > performance data justifying it. > -- 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