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=-5.5 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 D9D0CC433E0 for ; Fri, 8 Jan 2021 02:43:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A0AA323603 for ; Fri, 8 Jan 2021 02:43:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727293AbhAHCnd (ORCPT ); Thu, 7 Jan 2021 21:43:33 -0500 Received: from out30-132.freemail.mail.aliyun.com ([115.124.30.132]:36092 "EHLO out30-132.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726993AbhAHCnb (ORCPT ); Thu, 7 Jan 2021 21:43:31 -0500 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=e01e04420;MF=wenyang@linux.alibaba.com;NM=1;PH=DS;RN=5;SR=0;TI=SMTPD_---0UL1ZENl_1610073767; Received: from IT-C02W23QPG8WN.local(mailfrom:wenyang@linux.alibaba.com fp:SMTPD_---0UL1ZENl_1610073767) by smtp.aliyun-inc.com(127.0.0.1); Fri, 08 Jan 2021 10:42:47 +0800 Subject: Re: [PATCH v2 4.9 00/10] fix a race in release_task when flushing the dentry To: Greg Kroah-Hartman , Christian Brauner Cc: Sasha Levin , Xunlei Pang , linux-kernel@vger.kernel.org References: <20210107075222.62623-1-wenyang@linux.alibaba.com> From: Wen Yang Message-ID: <82fb683a-bc9d-2083-f657-116f3e96d785@linux.alibaba.com> Date: Fri, 8 Jan 2021 10:42:47 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.1.0 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 2021/1/8 上午2:28, Greg Kroah-Hartman 写道: > On Fri, Jan 08, 2021 at 12:21:38AM +0800, Wen Yang wrote: >> >> >> 在 2021/1/7 下午8:17, Greg Kroah-Hartman 写道: >>> On Thu, Jan 07, 2021 at 03:52:12PM +0800, Wen Yang wrote: >>>> The dentries such as /proc//ns/ have the DCACHE_OP_DELETE flag, they >>>> should be deleted when the process exits. >>>> >>>> Suppose the following race appears： >>>> >>>> release_task dput >>>> -> proc_flush_task >>>> -> dentry->d_op->d_delete(dentry) >>>> -> __exit_signal >>>> -> dentry->d_lockref.count-- and return. >>>> >>>> In the proc_flush_task(), if another process is using this dentry, it will >>>> not be deleted. At the same time, in dput(), d_op->d_delete() can be executed >>>> before __exit_signal(pid has not been hashed), d_delete returns false, so >>>> this dentry still cannot be deleted. >>>> >>>> This dentry will always be cached (although its count is 0 and the >>>> DCACHE_OP_DELETE flag is set), its parent denry will also be cached too, and >>>> these dentries can only be deleted when drop_caches is manually triggered. >>>> >>>> This will result in wasted memory. What's more troublesome is that these >>>> dentries reference pid, according to the commit f333c700c610 ("pidns: Add a >>>> limit on the number of pid namespaces"), if the pid cannot be released, it >>>> may result in the inability to create a new pid_ns. >>>> >>>> This issue was introduced by 60347f6716aa ("pid namespaces: prepare >>>> proc_flust_task() to flush entries from multiple proc trees"), exposed by >>>> f333c700c610 ("pidns: Add a limit on the number of pid namespaces"), and then >>>> fixed by 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc"). >>> >>> Why are you just submitting a series for 4.9 and 4.19, what about 4.14? >>> We can't have users move to a newer kernel and then experience old bugs, >>> right? >>> >> Okay, the patches corresponding to 4.14 will be ready later. > > Note for some reason you didn't cc: the stable list for these patches :( > >>> But the larger question is why are you backporting a whole new feature >>> here? Why is CLONE_PIDFD needed? That feels really wrong... >>> >> >> The reason for backporting CLONE_PIDFD is because 7bc3e6e55acf ("proc: Use a >> list of inodes to flush from proc") relies on wait_pidfd.lock. There are >> indeed many associated modifications here. We are also testing it. Please >> check the code more. > > Is the only "issue" here wasted memory? Will it eventually be freed > anyway even if you do not echo to the proc file to flush caches? > > You mention the inability to create a new pid for a specific namespace, > is that really a problem? Shouldn't the code handle such issues > normally? What breaks without these changes? > > I think at this point, it might just time for you to move to a newer > kernel release, as adding a whole new userspace feature for this feels > really really odd. > > What is preventing you from doing that today? What holds you to older > kernels that will not allow you to move forward? > We have encountered this problem in the cloud server environment. Users will frequently create and delete containers, and the corresponding pid_ns will accumulate, eventually making it impossible to create a new container. https://bugzilla.kernel.org/show_bug.cgi?id=208613 The kernels (4.9/4.19) used on a large scale in our current production environment (almost tens of thousands of machines) may need to be fixed. Thanks. -- Best wishes, Wen