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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, 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 945C7C43603 for ; Fri, 6 Dec 2019 19:51:03 +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 5E86524673 for ; Fri, 6 Dec 2019 19:51:03 +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="dZBbXCmB" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5E86524673 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]:44828 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1idJd0-0001MF-Go for qemu-devel@archiver.kernel.org; Fri, 06 Dec 2019 14:51:02 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:53699) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1idJbI-0007zx-AX for qemu-devel@nongnu.org; Fri, 06 Dec 2019 14:49:17 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1idJbE-0005aq-Bi for qemu-devel@nongnu.org; Fri, 06 Dec 2019 14:49:14 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:23499 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1idJb7-0005Wh-4Z for qemu-devel@nongnu.org; Fri, 06 Dec 2019 14:49:06 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1575661742; 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: in-reply-to:in-reply-to:references:references; bh=9mglkLeSMgatG2QuCuv8J26Cg/iVIUGIcRnZXgy5kHc=; b=dZBbXCmBxqPkvNOWWdSKTG2Z5K9XJqH0oHxREn55IBAk8R7haoA/1AJQMEfqJi5CbwWLsG HpmxWy/lygM6HwrhZ0hLQobZoNQMzq26p0VToxxhVJcc0a1CKSTs+TQTBOoKypwdc4xvSW MFpR5DI1+ILvCkVwT/O2feoFVMNURHU= 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-332-wkclNKJuNu2BvfumVAoTEw-1; Fri, 06 Dec 2019 14:49:00 -0500 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C002F134C21; Fri, 6 Dec 2019 19:48:59 +0000 (UTC) Received: from [10.3.116.171] (ovpn-116-171.phx2.redhat.com [10.3.116.171]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D806619C4F; Fri, 6 Dec 2019 19:48:53 +0000 (UTC) Subject: Re: [PATCH for-4.2?] block/qcow2-bitmap: fix crash bug in qcow2_co_remove_persistent_dirty_bitmap To: John Snow , Vladimir Sementsov-Ogievskiy , "qemu-block@nongnu.org" References: <20191205193049.30666-1-vsementsov@virtuozzo.com> <22af83d1-91e2-ab71-2dc4-349b5b24d1d8@redhat.com> <990da2e0-8223-b257-254d-a27659ef1d24@redhat.com> <6a74ae0a-e377-0f70-c347-0307580d7981@virtuozzo.com> <36655f7a-7ea5-e80a-ebfd-5b19c90622c0@redhat.com> <8c7c5f50-1899-3457-e1bc-77d8fee87de7@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: Date: Fri, 6 Dec 2019 13:48:53 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: <8c7c5f50-1899-3457-e1bc-77d8fee87de7@redhat.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-MC-Unique: wkclNKJuNu2BvfumVAoTEw-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=WINDOWS-1252; format=flowed 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: 205.139.110.120 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: "kwolf@redhat.com" , Peter Maydell , Peter Krempa , "libvirt-list@redhat.com" , "qemu-devel@nongnu.org" , "mreitz@redhat.com" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 12/6/19 1:02 PM, John Snow wrote: >>> I'm afraid that the only thing is not remove persistent bitmaps, which >>> were never synced to the image. So, instead the sequence above, we need >>> >>> >>> 1. create persistent bitmap A >>> 2. shutdown vm >>> 3. start vm >>> 4. create persistent bitmap B >>> 5. remember, that we want to remove bitmap B after vm shutdown >>> ... >>> =A0=A0 some other operations >>> ... >>> 6. vm shutdown >>> 7. start vm in stopped mode, and remove all bitmaps marked for removing >>> 8. stop vm >>> >>> But, I think that in real circumstances, vm shutdown is rare thing... Part of me wonders if we would have detected this MUCH sooner if I had=20 gotten my wish of having the qcow2 metadata updated on creation of any=20 persistent bitmap (not actually writing out the bitmap itself, just=20 updating the bitmap table to mark that there is a new persistent=20 inconsistent bitmap), so that a) qemu-img info -U can see the persistent=20 bitmap's existence, and b) an unexpected abrupt crash of qemu does not=20 lose the existence of the bitmap. At the time I raised the question,=20 the push-back at the time was a desire to minimize writes to the qcow2=20 metadata at all costs, warranting our current extreme code contortions=20 to keep persistent bitmaps were kept in memory only until VM shutdown.=20 But had we been doing it, we would spot problems like this without=20 having to do VM shutdown, and our code might actually be simpler than=20 our current contortions. Maybe we should still revisit that decision=20 (of course, that question is independent of this patch, and therefore=20 5.0 material at earliest). --=20 Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org