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 2652FC4338F for ; Sun, 8 Aug 2021 02:55:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0326160F50 for ; Sun, 8 Aug 2021 02:55:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230086AbhHHCzV (ORCPT ); Sat, 7 Aug 2021 22:55:21 -0400 Received: from out30-57.freemail.mail.aliyun.com ([115.124.30.57]:44603 "EHLO out30-57.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229882AbhHHCzT (ORCPT ); Sat, 7 Aug 2021 22:55:19 -0400 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 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.