From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BA94DC432C3 for ; Thu, 14 Nov 2019 06:01:14 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 9CDAB20659 for ; Thu, 14 Nov 2019 06:01:14 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9CDAB20659 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id CE01B6B0003; Thu, 14 Nov 2019 01:01:13 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id C90576B0005; Thu, 14 Nov 2019 01:01:13 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BA6BE6B0006; Thu, 14 Nov 2019 01:01:13 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0072.hostedemail.com [216.40.44.72]) by kanga.kvack.org (Postfix) with ESMTP id A5A876B0003 for ; Thu, 14 Nov 2019 01:01:13 -0500 (EST) Received: from smtpin26.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with SMTP id 626566137 for ; Thu, 14 Nov 2019 06:01:13 +0000 (UTC) X-FDA: 76153835226.26.dust55_27a52ed01c533 X-HE-Tag: dust55_27a52ed01c533 X-Filterd-Recvd-Size: 3286 Received: from out30-42.freemail.mail.aliyun.com (out30-42.freemail.mail.aliyun.com [115.124.30.42]) by imf47.hostedemail.com (Postfix) with ESMTP for ; Thu, 14 Nov 2019 06:01:12 +0000 (UTC) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R151e4;CH=green;DM=||false|;DS=|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e07486;MF=alex.shi@linux.alibaba.com;NM=1;PH=DS;RN=29;SR=0;TI=SMTPD_---0Ti1eLE7_1573711266; Received: from IT-FVFX43SYHV2H.local(mailfrom:alex.shi@linux.alibaba.com fp:SMTPD_---0Ti1eLE7_1573711266) by smtp.aliyun-inc.com(127.0.0.1); Thu, 14 Nov 2019 14:01:07 +0800 Subject: Re: [PATCH v2 4/8] mm/lru: only change the lru_lock iff page's lruvec is different To: Matthew Wilcox Cc: cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, mgorman@techsingularity.net, tj@kernel.org, hughd@google.com, khlebnikov@yandex-team.ru, daniel.m.jordan@oracle.com, yang.shi@linux.alibaba.com, Johannes Weiner , Michal Hocko , Vladimir Davydov , Roman Gushchin , Shakeel Butt , Chris Down , Thomas Gleixner , Vlastimil Babka , Andrey Ryabinin , swkhack , "Potyra, Stefan" , Jason Gunthorpe , Mauro Carvalho Chehab , Peng Fan , Nikolay Borisov , Ira Weiny , Kirill Tkhai , Yafang Shao References: <1573567588-47048-1-git-send-email-alex.shi@linux.alibaba.com> <1573567588-47048-5-git-send-email-alex.shi@linux.alibaba.com> <20191112143624.GA7934@bombadil.infradead.org> <297ad71c-081c-f7e1-d640-8720a0eeeeba@linux.alibaba.com> <20191113134502.GD7934@bombadil.infradead.org> From: Alex Shi Message-ID: Date: Thu, 14 Nov 2019 14:01:06 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:60.0) Gecko/20100101 Thunderbird/60.9.1 MIME-Version: 1.0 In-Reply-To: <20191113134502.GD7934@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: =E5=9C=A8 2019/11/13 =E4=B8=8B=E5=8D=889:45, Matthew Wilcox =E5=86=99=E9=81= =93: >>> Why not simply: >>> >>> rcu_read_lock(); >>> lruvec =3D mem_cgroup_page_lruvec(page, pgdat); >>> rcu_read_unlock(); >>> >>> if (locked_lruvec =3D=3D lruvec) >> The rcu_read_unlock here is for guarding the locked_lruvec/lruvec comp= arsion. >> Otherwise memcg/lruvec maybe changed, like, from memcg migration etc. = I guess. > How does holding the RCU lock guard the comparison? You're comparing t= wo > pointers for equality. Nothing any other CPU can do at this point will > change the results of that comparison. >=20 Hi Matthew, Thanks for reply! The reason of guarding lruvec compasion here is a bit undistinct, in fact= , it guards the page's memcg from free/migrate etc, since the lruvec is k= ind of binding to each of memcg. The detailed explanation could be found = in lock_page_memcg(). Thanks Alex