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=-6.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 A06CFC433DB for ; Thu, 7 Jan 2021 07:12:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5626F23101 for ; Thu, 7 Jan 2021 07:12:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726477AbhAGHMi (ORCPT ); Thu, 7 Jan 2021 02:12:38 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:41381 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725763AbhAGHMh (ORCPT ); Thu, 7 Jan 2021 02:12:37 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1610003471; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=jzTZnQ9+T2I2w5hDhpjlR9ds0jT9ucs1opY1L7Giz0Q=; b=bRfWVtEdaeNz3BRQCe2wVX/4y8ZDotCawtJ017VGSR1hYxNKx+KXScE/3FzBpa2Ea+GY5K jc+289vOlQJkcK2oqNn7U7eOkIAIbYJXfOuOU3KwopcCqsUO7hErFqfZO0uQLUq/Z61O35 JnBhoKtje6AWqPTCJcDpKQ/Qz/NXe38= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-253-p3hlNXLoPBCVKdcRGL00yw-1; Thu, 07 Jan 2021 02:11:08 -0500 X-MC-Unique: p3hlNXLoPBCVKdcRGL00yw-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 879C91800D42; Thu, 7 Jan 2021 07:11:07 +0000 (UTC) Received: from T590 (ovpn-13-146.pek2.redhat.com [10.72.13.146]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 33B8271CB8; Thu, 7 Jan 2021 07:11:00 +0000 (UTC) Date: Thu, 7 Jan 2021 15:10:55 +0800 From: Ming Lei To: Mauricio Faria de Oliveira Cc: Jens Axboe , linux-block@vger.kernel.org, Gabriel Krisman Bertazi , Eric Desrochers Subject: Re: [PATCH v2] loop: fix I/O error on fsync() in detached loop devices Message-ID: <20210107071055.GA3900112@T590> References: <20210105135419.68715-1-mfo@canonical.com> <20210106090758.GB3845805@T590> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Wed, Jan 06, 2021 at 08:33:50PM -0300, Mauricio Faria de Oliveira wrote: > On Wed, Jan 6, 2021 at 6:08 AM Ming Lei wrote: > > > > On Tue, Jan 05, 2021 at 10:54:19AM -0300, Mauricio Faria de Oliveira wrote: > > > There's an I/O error on fsync() in a detached loop device > > > if it has been previously attached. > > > > > > The issue is write cache is enabled in the attach path in > > > loop_configure() but it isn't disabled in the detach path; > > > thus it remains enabled in the block device regardless of > > > whether it is attached or not. > > > > > > Now fsync() can get an I/O request that will just be failed > > > later in loop_queue_rq() as device's state is not 'Lo_bound'. > > > > > > So, disable write cache in the detach path. > > > > > > Test-case: > > > > > > # DEV=/dev/loop7 > > > > > > # IMG=/tmp/image > > > # truncate --size 1M $IMG > > > > > > # losetup $DEV $IMG > > > # losetup -d $DEV > > > > > > Before: > > > > > > # strace -e fsync parted -s $DEV print 2>&1 | grep fsync > > > fsync(3) = -1 EIO (Input/output error) > > > Warning: Error fsyncing/closing /dev/loop7: Input/output error > > > [ 982.529929] blk_update_request: I/O error, dev loop7, sector 0 op 0x1:(WRITE) flags 0x800 phys_seg 0 prio class 0 > > > > > > After: > > > > > > # strace -e fsync parted -s $DEV print 2>&1 | grep fsync > > > fsync(3) = 0 > > > > But IO on detached loop should have been failed, right? The magic is > > that submit_bio_checks() filters FLUSH request for queues which doesn't > > support writeback cache, and always fake a normal completion. > > > > Hey Ming, thanks for taking a look at this. > > Well, it depends -- currently read() works (without I/O errors) and > write() fails (ENOSPC). > Example tests are provided below. read() actually returns 0 because of the following code in blkdev_read_iter(): if (pos >= size) return 0; > > And that's consistent before and after attach/detach; so, I thought > fsync() should follow. > > > I understand that the issue is that user becomes confused with this observation > > because no such failure if they run 'parted -s /dev/loop0 print' on one detached > > loop disk if it is never attached. > > > > That is indeed one of the issues. There's also a monitoring/alerting > perspective that > would benefit; e.g., sosreport runs parted, it's run on data > collection for support cases. > Now, that I/O error message is thrown in the logs, and some mon/alert > tools might not > yet have filters to ignore (detached) loop devices, and alert. It'd be > nice to deflect that. IMO, if loop is detached, any IO should have been failed. However, read/flush is just a bit special: - blkdev_read_iter() always return 0 if the read is beyond the device size(0) - submit_bio(FLUSH) return successfully if the queue doesn't support writeback cache. > > It's not a common issue, to be honest; but the consistency point > seemed fair to me, > as essentially the current code doesn't deinitialize something it > previously initialized, > and the block device is left running with that enabled regardless. OK, looks it is fine to disable writeback cache in __loop_clr_fd(). BTW, just wondering why don't you disable WC unconditionally in __loop_clr_fd() or clear it in the following way because WC can be changed via sysfs? if (test_bit(QUEUE_FLAG_WC, &q->queue_flags)) blk_queue_write_cache(q, false, false); Thanks, Ming