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=-0.5 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 25581CA9EA0 for ; Fri, 25 Oct 2019 10:09:52 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id E7B0020663 for ; Fri, 25 Oct 2019 10:09:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="hUNl3yQN" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E7B0020663 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:58246 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iNwXW-0008Vk-Qk for qemu-devel@archiver.kernel.org; Fri, 25 Oct 2019 06:09:50 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60794) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iNwNE-0006Wg-V5 for qemu-devel@nongnu.org; Fri, 25 Oct 2019 05:59:14 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iNwND-0004hl-4O for qemu-devel@nongnu.org; Fri, 25 Oct 2019 05:59:12 -0400 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:44810 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iNwNC-0004as-VP for qemu-devel@nongnu.org; Fri, 25 Oct 2019 05:59:11 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1571997541; 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: content-transfer-encoding:content-transfer-encoding; bh=hlA0R1g5WXsAHk7gDWlmXOz/G3RhG48NKi6/mhZyIeI=; b=hUNl3yQNsT6DAB+OrIsw/GWyeVg+97Z+RSPEtPpud5mTSKMnzptqJ0j7VyMgzLfRj62Pin ieqX4BI+8sCZbiwk1WUTAVV/HZYrBkt4H/3l07HW7FuDj/5/GpG2jZoqrTfDNzsKm4HvGs nYV8wu6uEWSo/aZ49aTh8FR/d5Q+V2M= 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-123-54bGxlm0M2OFKSfSwawPhQ-1; Fri, 25 Oct 2019 05:58:57 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5DC1080183D; Fri, 25 Oct 2019 09:58:56 +0000 (UTC) Received: from localhost (ovpn-117-205.ams2.redhat.com [10.36.117.205]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7739960BEC; Fri, 25 Oct 2019 09:58:52 +0000 (UTC) From: Max Reitz To: qemu-block@nongnu.org Subject: [RFC 0/3] block/file-posix: Work around XFS bug Date: Fri, 25 Oct 2019 11:58:46 +0200 Message-Id: <20191025095849.25283-1-mreitz@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-MC-Unique: 54bGxlm0M2OFKSfSwawPhQ-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Anton Nefedov , Alberto Garcia , qemu-devel@nongnu.org, Max Reitz , Vladimir Sementsov-Ogievskiy , Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Hi, It seems to me that there is a bug in Linux=E2=80=99s XFS kernel driver, as I=E2=80=99ve explained here: https://lists.nongnu.org/archive/html/qemu-block/2019-10/msg01429.html In combination with our commit c8bb23cbdbe32f, this may lead to guest data corruption when using qcow2 images on XFS with aio=3Dnative. We can=E2=80=99t wait until the XFS kernel driver is fixed, we should work around the problem ourselves. This is an RFC for two reasons: (1) I don=E2=80=99t know whether this is the right way to address the issue= , (2) Ideally, we should detect whether the XFS kernel driver is fixed and if so stop applying the workaround. I don=E2=80=99t know how we would go about this, so this series doesn= =E2=80=99t do it. (Hence it=E2=80=99s an RFC.) (3) Perhaps it=E2=80=99s a bit of a layering violation to let the file-posi= x driver access and modify a BdrvTrackedRequest object. As for how we can address the issue, I see three ways: (1) The one presented in this series: On XFS with aio=3Dnative, we extend tracked requests for post-EOF fallocate() calls (i.e., write-zero operations) to reach until infinity (INT64_MAX in practice), mark them serializing and wait for other conflicting requests. Advantages: + Limits the impact to very specific cases (And that means it wouldn=E2=80=99t hurt too much to keep this workar= ound even when the XFS driver has been fixed) + Works around the bug where it happens, namely in file-posix Disadvantages: - A bit complex - A bit of a layering violation (should file-posix have access to tracked requests?) (2) Always skip qcow2=E2=80=99s handle_alloc_space() on XFS. The XFS bug o= nly becomes visible due to that function: I don=E2=80=99t think qcow2 write= s zeroes in any other I/O path, and raw images are fixed in size so post-EOF writes won=E2=80=99t happen. Advantages: + Maybe simpler, depending on how difficult it is to handle the layering violation + Also fixes the performance problem of handle_alloc_space() being slow on ppc64+XFS. Disadvantages: - Huge layering violation because qcow2 would need to know whether the image is stored on XFS or not. - We=E2=80=99d definitely want to skip this workaround when the XFS dri= ver has been fixed, so we need some method to find out whether it has (3) Drop handle_alloc_space(), i.e. revert c8bb23cbdbe32f. To my knowledge I=E2=80=99m the only one who has provided any benchmark= s for this commit, and even then I was a bit skeptical because it performs well in some cases and bad in others. I concluded that it=E2=80=99s probably worth it because the =E2=80=9Csome cases=E2=80=9D are more lik= ely to occur. Now we have this problem of corruption here (granted due to a bug in the XFS driver), and another report of massively degraded performance on ppc64 (https://bugzilla.redhat.com/show_bug.cgi?id=3D1745823 =E2=80=93 sorry,= a private BZ; I hate that :-/ The report is about 40 % worse performance for an in-guest fio write benchmark.) So I have to ask the question about what the justification for keeping c8bb23cbdbe32f is. How much does performance increase with it actually? (On non-(ppc64+XFS) machines, obviously) Advantages: + Trivial + No layering violations + We wouldn=E2=80=99t need to keep track of whether the kernel bug has = been fixed or not + Fixes the ppc64+XFS performance problem Disadvantages: - Reverts cluster allocation performance to pre-c8bb23cbdbe32f levels, whatever that means So this is the main reason this is an RFC: What should we do? Is (1) really the best choice? In any case, I=E2=80=99ve ran the test case I showed in https://lists.nongnu.org/archive/html/qemu-block/2019-10/msg01282.html more than ten times with this series applied and the installation succeeded every time. (Without this series, it fails like every other time.) Max Reitz (3): block: Make wait/mark serialising requests public block/file-posix: Detect XFS with CONFIG_FALLOCATE block/file-posix: Let post-EOF fallocate serialize include/block/block_int.h | 3 +++ block/file-posix.c | 46 +++++++++++++++++++++++++++++++++++++-- block/io.c | 24 ++++++++++---------- 3 files changed, 59 insertions(+), 14 deletions(-) --=20 2.21.0