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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 D43B4C432BE for ; Wed, 11 Aug 2021 20:48:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B2C9D61077 for ; Wed, 11 Aug 2021 20:48:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231466AbhHKUs1 (ORCPT ); Wed, 11 Aug 2021 16:48:27 -0400 Received: from cloud48395.mywhc.ca ([173.209.37.211]:47944 "EHLO cloud48395.mywhc.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229589AbhHKUs0 (ORCPT ); Wed, 11 Aug 2021 16:48:26 -0400 Received: from modemcable064.203-130-66.mc.videotron.ca ([66.130.203.64]:54440 helo=[192.168.1.179]) by cloud48395.mywhc.ca with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1mDv8q-000667-4E; Wed, 11 Aug 2021 16:48:00 -0400 Message-ID: Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps From: Olivier Langlois To: Tony Battersby , "Eric W. Biederman" , Oleg Nesterov Cc: Linus Torvalds , Linux Kernel Mailing List , linux-fsdevel , io-uring , Alexander Viro , Jens Axboe , "Pavel Begunkov>" Date: Wed, 11 Aug 2021 16:47:58 -0400 In-Reply-To: <8af373ec-9609-35a4-f185-f9bdc63d39b7@cybernetics.com> References: <198e912402486f66214146d4eabad8cb3f010a8e.camel@trillion01.com> <87eeda7nqe.fsf@disp2133> <87pmwt6biw.fsf@disp2133> <87czst5yxh.fsf_-_@disp2133> <87y2bh4jg5.fsf@disp2133> <87sg1p4h0g.fsf_-_@disp2133> <20210614141032.GA13677@redhat.com> <87pmwmn5m0.fsf@disp2133> <4d93d0600e4a9590a48d320c5a7dd4c54d66f095.camel@trillion01.com> <8af373ec-9609-35a4-f185-f9bdc63d39b7@cybernetics.com> Organization: Trillion01 Inc Content-Type: text/plain; charset="ISO-8859-1" User-Agent: Evolution 3.40.3 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - cloud48395.mywhc.ca X-AntiAbuse: Original Domain - vger.kernel.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - trillion01.com X-Get-Message-Sender-Via: cloud48395.mywhc.ca: authenticated_id: olivier@trillion01.com X-Authenticated-Sender: cloud48395.mywhc.ca: olivier@trillion01.com X-Source: X-Source-Args: X-Source-Dir: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2021-08-10 at 17:48 -0400, Tony Battersby wrote: > > > I just ran into this problem also - coredumps from an io_uring > program > to a pipe are truncated.  But I am using kernel 5.10.57, which does > NOT > have commit 12db8b690010 ("entry: Add support for TIF_NOTIFY_SIGNAL") > or > commit 06af8679449d ("coredump: Limit what can interrupt > coredumps").  > Kernel 5.4 works though, so I bisected the problem to commit > f38c7e3abfba ("io_uring: ensure async buffered read-retry is setup > properly") in kernel 5.9.  Note that my io_uring program uses only > async > buffered reads, which may be why this particular commit makes a > difference to my program. > > My io_uring program is a multi-purpose long-running program with many > threads.  Most threads don't use io_uring but a few of them do.  > Normally, my core dumps are piped to a program so that they can be > compressed before being written to disk, but I can also test writing > the > core dumps directly to disk.  This is what I have found: > > *) Unpatched 5.10.57: if a thread that doesn't use io_uring triggers > a > coredump, the core file is written correctly, whether it is written > to > disk or piped to a program, even if another thread is using io_uring > at > the same time. > > *) Unpatched 5.10.57: if a thread that uses io_uring triggers a > coredump, the core file is truncated, whether written directly to > disk > or piped to a program. > > *) 5.10.57+backport 06af8679449d: if a thread that uses io_uring > triggers a coredump, and the core is written directly to disk, then > it > is written correctly. > > *) 5.10.57+backport 06af8679449d: if a thread that uses io_uring > triggers a coredump, and the core is piped to a program, then it is > truncated. > > *) 5.10.57+revert f38c7e3abfba: core dumps are written correctly, > whether written directly to disk or piped to a program. > > Tony Battersby > Cybernetics > Tony, this is super interesting details. I'm leaving for few days so I will not be able to look into it until I am back but here is my interpretation of your findings: f38c7e3abfba makes it more likely that your task ends up in a fd read wait queue. Previously the io_uring req queuing was failing and returning EAGAIN. Now it ends up using io uring fast poll. When the core dump gets written through a pipe, pipe_write must block waiting for some event. If the task gets waken up by the io_uring wait queue entry instead, it must somehow make pipe_write fails. So the problem must be a mix of TIF_NOTIFY_SIGNAL and the fact that io_uring wait queue entries aren't cleaned up while doing the core dump. I have a new modif to try out. I'll hopefully be able to submit a patch to fix that once I come back (I cannot do it now or else, I'll never leave ;-))