From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f48.google.com (mail-oi0-f48.google.com [209.85.218.48]) by kanga.kvack.org (Postfix) with ESMTP id F26E26B007E for ; Wed, 30 Mar 2016 21:39:58 -0400 (EDT) Received: by mail-oi0-f48.google.com with SMTP id d205so40793055oia.0 for ; Wed, 30 Mar 2016 18:39:58 -0700 (PDT) Received: from szxga01-in.huawei.com (szxga01-in.huawei.com. [58.251.152.64]) by mx.google.com with ESMTPS id w7si4046545oia.138.2016.03.30.18.39.54 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 30 Mar 2016 18:39:58 -0700 (PDT) Subject: Re: [PATCH] Revert "mm/page_alloc: protect pcp->batch accesses with ACCESS_ONCE" References: <1459333327-89720-1-git-send-email-hekuang@huawei.com> <20160330103839.GA4773@techsingularity.net> <56FBAFA0.3010604@huawei.com> <20160330111044.GA4324@dhcp22.suse.cz> <56FC7A02.1080201@huawei.com> From: Zefan Li Message-ID: <56FC7FD2.9000203@huawei.com> Date: Thu, 31 Mar 2016 09:39:30 +0800 MIME-Version: 1.0 In-Reply-To: <56FC7A02.1080201@huawei.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Hekuang , Michal Hocko Cc: Mel Gorman , akpm@linux-foundation.org, vbabka@suse.cz, rientjes@google.com, cody@linux.vnet.ibm.com, gilad@benyossef.com, kosaki.motohiro@gmail.com, mgorman@suse.de, penberg@kernel.org, wangnan0@huawei.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 2016/3/31 9:14, Hekuang wrote: > Hi > > a?? 2016/3/30 19:10, Michal Hocko a??e??: >> On Wed 30-03-16 18:51:12, Hekuang wrote: >>> hi >>> >>> a?? 2016/3/30 18:38, Mel Gorman a??e??: >>>> On Wed, Mar 30, 2016 at 10:22:07AM +0000, He Kuang wrote: >>>>> This reverts commit 998d39cb236fe464af86a3492a24d2f67ee1efc2. >>>>> >>>>> When local irq is disabled, a percpu variable does not change, so we can >>>>> remove the access macros and let the compiler optimize the code safely. >>>>> >>>> batch can be changed from other contexts. Why is this safe? >>>> >>> I've mistakenly thought that per_cpu variable can only be accessed by that >>> cpu. >> git blame would point you to 998d39cb236f ("mm/page_alloc: protect >> pcp->batch accesses with ACCESS_ONCE"). I haven't looked into the code >> deeply to confirm this is still the case but it would be a good lead >> that this is not that simple. ACCESS_ONCE resp. {READ,WRITE}_ONCE are >> usually quite subtle so I would encourage you or anybody else who try to >> remove them to study the code and the history deeper before removing >> them. >> > Thank you for responding, I've read that commit and related articles and not sending > mail casually, though you may think it's a stupid patch. I'm a beginner and I think > sending mails to maillist is a effective way to learn kernel, And, sure i'll be more careful and > well prepared next time :) > pcp->batch can be changed in a different cpu. You may read percpu_pagelist_fraction_sysctl_handler() to see how that can happen. -- 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