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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=unavailable 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 3E38AC433B4 for ; Wed, 14 Apr 2021 10:18:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 20556613B1 for ; Wed, 14 Apr 2021 10:18:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233622AbhDNKSt (ORCPT ); Wed, 14 Apr 2021 06:18:49 -0400 Received: from out30-132.freemail.mail.aliyun.com ([115.124.30.132]:54881 "EHLO out30-132.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233507AbhDNKSq (ORCPT ); Wed, 14 Apr 2021 06:18:46 -0400 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=e01e04420;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=4;SR=0;TI=SMTPD_---0UVXj3bU_1618395500; Received: from 30.21.164.69(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0UVXj3bU_1618395500) by smtp.aliyun-inc.com(127.0.0.1); Wed, 14 Apr 2021 18:18:20 +0800 Subject: Re: [PATCH v2 1/2] fuse: Fix possible deadlock when writing back dirty pages To: Miklos Szeredi Cc: Peng Tao , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org References: <807bb470f90bae5dcd80a29020d38f6b5dd6ef8e.1616826872.git.baolin.wang@linux.alibaba.com> <0fdb09fa-9b0f-1115-2540-6016ce664370@linux.alibaba.com> From: Baolin Wang Message-ID: <495c1637-8d63-6620-ca76-e77f61ae11cf@linux.alibaba.com> Date: Wed, 14 Apr 2021 18:18:34 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 在 2021/4/14 17:47, Miklos Szeredi 写道: > On Wed, Apr 14, 2021 at 11:22 AM Baolin Wang > wrote: >> >> >> >> 在 2021/4/14 17:02, Miklos Szeredi 写道: >>> On Wed, Apr 14, 2021 at 10:42 AM Baolin Wang >>> wrote: >>> >>>> Sorry I missed this patch before, and I've tested this patch, it seems >>>> can solve the deadlock issue I met before. >>> >>> Great, thanks for testing. >>> >>>> But look at this patch in detail, I think this patch only reduced the >>>> deadlock window, but did not remove the possible deadlock scenario >>>> completely like I explained in the commit log. >>>> >>>> Since the fuse_fill_write_pages() can still lock the partitail page in >>>> your patch, and will be wait for the partitail page waritehack is >>>> completed if writeback is set in fuse_send_write_pages(). >>>> >>>> But at the same time, a writeback worker thread may be waiting for >>>> trying to lock the partitail page to write a bunch of dirty pages by >>>> fuse_writepages(). >>> >>> As you say, fuse_fill_write_pages() will lock a partial page. This >>> page cannot become dirty, only after being read completely, which >>> first requires the page lock. So dirtying this page can only happen >>> after the writeback of the fragment was completed. >> >> What I mean is the writeback worker had looked up the dirty pages in >> write_cache_pages() and stored them into a temporary pagevec, then try >> to lock dirty page one by one and write them. >> >> For example, suppose it looked up 2 dirty pages (named page 1 and page >> 2), and writed down page 1 by fuse_writepages_fill(), unlocked page 1. >> Then try to lock page 2. >> >> At the same time, suppose the fuse_fill_write_pages() will write the >> same page 1 and partitail page 2, and it will lock partital page 2 and >> wait for the page 1's writeback is completed. But page 1's writeback can >> not be completed, since the writeback worker is waiting for locking page >> 2, which was already locked by fuse_fill_write_pages(). > > How would page2 become not uptodate, when it was already collected by > write_cache_pages()? I.e. page2 is a dirty page, hence it must be > uptodate, and fuse_writepages_fill() will not keep it locked. Read your patch carefully again, now I realized you are right, and your patch can solve the deadlock issue I met. Please feel free to add my tested-by tag for your patch. Thanks. Tested-by: Baolin Wang