From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755085AbaFJH00 (ORCPT ); Tue, 10 Jun 2014 03:26:26 -0400 Received: from cantor2.suse.de ([195.135.220.15]:51054 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751462AbaFJH0X (ORCPT ); Tue, 10 Jun 2014 03:26:23 -0400 Message-ID: <5396B31B.6080706@suse.cz> Date: Tue, 10 Jun 2014 09:26:19 +0200 From: Vlastimil Babka User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 MIME-Version: 1.0 To: David Rientjes CC: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Greg Thelen , Minchan Kim , Mel Gorman , Joonsoo Kim , Michal Nazarewicz , Naoya Horiguchi , Christoph Lameter , Rik van Riel Subject: Re: [RFC PATCH 4/6] mm, compaction: skip buddy pages by their order in the migrate scanner References: <1401898310-14525-1-git-send-email-vbabka@suse.cz> <1401898310-14525-4-git-send-email-vbabka@suse.cz> <5390374E.5080708@suse.cz> <53916BB0.3070001@suse.cz> <53959C11.2000305@suse.cz> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/10/2014 12:25 AM, David Rientjes wrote: > On Mon, 9 Jun 2014, Vlastimil Babka wrote: > >>>>> Sorry, I meant ACCESS_ONCE(page_private(page)) in the migration scanner >>>> >>>> Hm but that's breaking the abstraction of page_order(). I don't know if >>>> it's >>>> worse to create a new variant of page_order() or to do this. BTW, seems >>>> like >>>> next_active_pageblock() in memory-hotplug.c should use this variant too. >>>> >>> >>> The compiler seems free to disregard the access of a volatile object above >>> because the return value of the inline function is unsigned long. What's >>> the difference between unsigned long order = page_order_unsafe(page) and >>> unsigned long order = (unsigned long)ACCESS_ONCE(page_private(page)) and >> >> I think there's none functionally, but one is abstraction layer violation and >> the other imply the context of usage as you say (but is that so uncommon?). >> >>> the compiler being able to reaccess page_private() because the result is >>> no longer volatile qualified? >> >> You think it will reaccess? That would defeat all current ACCESS_ONCE usages, >> no? >> > > I think the compiler is allowed to turn this into > > if (ACCESS_ONCE(page_private(page)) > 0 && > ACCESS_ONCE(page_private(page)) < MAX_ORDER) > low_pfn += (1UL << ACCESS_ONCE(page_private(page))) - 1; > > since the inline function has a return value of unsigned long but gcc may > not do this. I think > > /* > * Big fat comment describing why we're using ACCESS_ONCE(), that > * we're ok to race, and that this is meaningful only because of > * the previous PageBuddy() check. > */ > unsigned long pageblock_order = ACCESS_ONCE(page_private(page)); > > is better. I've talked about it with a gcc guy and (although he didn't actually see the code so it might be due to me not explaining it perfectly), the compiler will inline page_order_unsafe() so that there's effectively. unsigned long freepage_order = ACCESS_ONCE(page_private(page)); and now it cannot just replace all freepage_order occurences with new page_private() accesses. So thanks to the inlining, the volatile qualification propagates to where it matters. It makes sense to me, but if it's according to standard or gcc specific, I don't know.