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,URIBL_BLOCKED,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 7FE27C433DB for ; Tue, 30 Mar 2021 03:16:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4345261968 for ; Tue, 30 Mar 2021 03:16:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230526AbhC3DQR (ORCPT ); Mon, 29 Mar 2021 23:16:17 -0400 Received: from szxga05-in.huawei.com ([45.249.212.191]:15034 "EHLO szxga05-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229483AbhC3DPz (ORCPT ); Mon, 29 Mar 2021 23:15:55 -0400 Received: from DGGEMS401-HUB.china.huawei.com (unknown [172.30.72.58]) by szxga05-in.huawei.com (SkyGuard) with ESMTP id 4F8ZJr117bzPmZW; Tue, 30 Mar 2021 11:13:16 +0800 (CST) Received: from [10.174.179.86] (10.174.179.86) by DGGEMS401-HUB.china.huawei.com (10.3.19.201) with Microsoft SMTP Server id 14.3.498.0; Tue, 30 Mar 2021 11:15:52 +0800 Subject: Re: [Question] Is there a race window between swapoff vs synchronous swap_readpage To: "Huang, Ying" CC: Linux-MM , linux-kernel , Andrew Morton , Matthew Wilcox , Yu Zhao , "Shakeel Butt" , Alex Shi , "Minchan Kim" References: <364d7ce9-ccb7-fa04-7067-44a96be87060@huawei.com> <8735wdbdy4.fsf@yhuang6-desk1.ccr.corp.intel.com> From: Miaohe Lin Message-ID: <0cb765aa-1783-cd62-c4a4-b3fbc620532d@huawei.com> Date: Tue, 30 Mar 2021 11:15:52 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <8735wdbdy4.fsf@yhuang6-desk1.ccr.corp.intel.com> Content-Type: text/plain; charset="windows-1252" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.179.86] X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/3/30 9:57, Huang, Ying wrote: > Hi, Miaohe, > > Miaohe Lin writes: > >> Hi all, >> I am investigating the swap code, and I found the below possible race window: >> >> CPU 1 CPU 2 >> ----- ----- >> do_swap_page >> skip swapcache case (synchronous swap_readpage) >> alloc_page_vma >> swapoff >> release swap_file, bdev, or ... >> swap_readpage >> check sis->flags is ok >> access swap_file, bdev or ...[oops!] >> si->flags = 0 >> >> The swapcache case is ok because swapoff will wait on the page_lock of swapcache page. >> Is this will really happen or Am I miss something ? >> Any reply would be really grateful. Thanks! :) > > This appears possible. Even for swapcache case, we can't guarantee the Many thanks for reply! > swap entry gotten from the page table is always valid too. The The page table may change at any time. And we may thus do some useless work. But the pte_same() check could handle these races correctly if these do not result in oops. > underlying swap device can be swapped off at the same time. So we use > get/put_swap_device() for that. Maybe we need similar stuff here. Using get/put_swap_device() to guard against swapoff for swap_readpage() sounds really bad as swap_readpage() may take really long time. Also such race may not be really hurtful because swapoff is usually done when system shutdown only. I can not figure some simple and stable stuff out to fix this. Any suggestions or could anyone help get rid of such race? Anyway, thanks again! > > Best Regards, > Huang, Ying > . >