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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 40FE1C4338F for ; Sun, 8 Aug 2021 02:55:08 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 8F44B60F5C for ; Sun, 8 Aug 2021 02:55:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 8F44B60F5C 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=kvack.org Received: by kanga.kvack.org (Postfix) id C8FDB8D0001; Sat, 7 Aug 2021 22:55:06 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C40046B0071; Sat, 7 Aug 2021 22:55:06 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B55268D0001; Sat, 7 Aug 2021 22:55:06 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0028.hostedemail.com [216.40.44.28]) by kanga.kvack.org (Postfix) with ESMTP id 9DBC86B006C for ; Sat, 7 Aug 2021 22:55:06 -0400 (EDT) Received: from smtpin20.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 3DFE3181AF5D7 for ; Sun, 8 Aug 2021 02:55:06 +0000 (UTC) X-FDA: 78450396612.20.D2A1A5B Received: from out30-56.freemail.mail.aliyun.com (out30-56.freemail.mail.aliyun.com [115.124.30.56]) by imf11.hostedemail.com (Postfix) with ESMTP id 38098F000220 for ; Sun, 8 Aug 2021 02:55:02 +0000 (UTC) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R161e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04395;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=4;SR=0;TI=SMTPD_---0UiG8VD6_1628391298; Received: from 30.39.238.167(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0UiG8VD6_1628391298) by smtp.aliyun-inc.com(127.0.0.1); Sun, 08 Aug 2021 10:54:59 +0800 Subject: Re: [PATCH 1/5] mm: migrate: Move the page count validation to the proper place To: Matthew Wilcox Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1f7e1d083864fbb17a20a9c8349d2e8b427e20a3.1628174413.git.baolin.wang@linux.alibaba.com> From: Baolin Wang Message-ID: <36956352-246a-b3c2-3ade-2a6c22e2cd5a@linux.alibaba.com> Date: Sun, 8 Aug 2021 10:55:30 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=gbk; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 38098F000220 Authentication-Results: imf11.hostedemail.com; dkim=none; spf=pass (imf11.hostedemail.com: domain of baolin.wang@linux.alibaba.com designates 115.124.30.56 as permitted sender) smtp.mailfrom=baolin.wang@linux.alibaba.com; dmarc=pass (policy=none) header.from=alibaba.com X-Stat-Signature: wsq6pch5w46j8cycc1ajouaebiwy67g8 X-HE-Tag: 1628391302-657866 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: Hi, > On Fri, Aug 06, 2021 at 11:07:18AM +0800, Baolin Wang wrote: >> Hi Matthew, >> >>> On Thu, Aug 05, 2021 at 11:05:56PM +0800, Baolin Wang wrote: >>>> We've got the expected count for anonymous page or file page by >>>> expected_page_refs() at the beginning of migrate_page_move_mapping(), >>>> thus we should move the page count validation a little forward to >>>> reduce duplicated code. >>> >>> Please add an explanation to the changelog for why it's safe to pull >>> this out from under the i_pages lock. >> >> Sure. In folio_migrate_mapping(), we are sure that the migration page was >> isolated from lru list and locked, so I think there are no race to get the >> page count without i_pages lock. Please correct me if I missed something >> else. Thanks. > > Unless the page has been removed from i_pages, this isn't a correct > explanation. Even if it has been removed from i_pages, unless an > RCU grace period has passed, another CPU may still be able to inc the > refcount on it (temporarily). The same is true for the page tables, > by the way; if someone is using get_user_pages_fast(), they may still > be able to see the page. I don't think this is an issue, cause now we've established a migration pte for this migration page under page lock. If the user want to get page by get_user_pages_fast(), it will wait for the page miggration finished by migration_entry_wait(). So I still think there is no need to check the migration page count under the i_pages lock.