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=-5.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 3AF43C432C0 for ; Wed, 20 Nov 2019 21:18:36 +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 018A5208A1 for ; Wed, 20 Nov 2019 21:18:36 +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="OhId1dDn" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 018A5208A1 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]:34516 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iXXMx-0005k0-6l for qemu-devel@archiver.kernel.org; Wed, 20 Nov 2019 16:18:35 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:37136) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iXXKa-0004Yv-NI for qemu-devel@nongnu.org; Wed, 20 Nov 2019 16:16:10 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iXXKZ-0004jw-0F for qemu-devel@nongnu.org; Wed, 20 Nov 2019 16:16:08 -0500 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:24291 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 1iXXKY-0004jG-Ox for qemu-devel@nongnu.org; Wed, 20 Nov 2019 16:16:06 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1574284564; 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=3kzzQhYXIpNh5/pnQrvsKPBCvjnMXthrE5pZzjnKo7I=; b=OhId1dDnj9ofHHDgX7aIDtNJyj2aYjOCSMtfFzXxLLyrbM0YE6kazFZU8/OxDj/dcsZUfn 5jo8hBpxHFKkZ7DumfHp1M1/DptKXd5WDqoMaxe/Vx0i2J5Af/6cnjvgzDHLLNflkfpMA+ 6LtuZ4syOpAyNMSPCiRZDm9nsFsNxEg= 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-211-acCxfPLHPRuirtCuTRF-gA-1; Wed, 20 Nov 2019 16:16:02 -0500 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7B2B11925765; Wed, 20 Nov 2019 21:16:01 +0000 (UTC) Received: from [10.3.116.221] (ovpn-116-221.phx2.redhat.com [10.3.116.221]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6EFA551C88; Wed, 20 Nov 2019 21:15:58 +0000 (UTC) Subject: Re: [PATCH v2 2/6] block: truncate: Don't make backing file data visible To: Kevin Wolf , qemu-block@nongnu.org References: <20191120184501.28159-1-kwolf@redhat.com> <20191120184501.28159-3-kwolf@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <96c2c681-851d-4b83-2acd-3952fa850e5f@redhat.com> Date: Wed, 20 Nov 2019 15:15:57 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <20191120184501.28159-3-kwolf@redhat.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-MC-Unique: acCxfPLHPRuirtCuTRF-gA-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=UTF-8; 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: 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: vsementsov@virtuozzo.com, qemu-devel@nongnu.org, stefanha@redhat.com, mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 11/20/19 12:44 PM, Kevin Wolf wrote: > When extending the size of an image that has a backing file larger than > its old size, make sure that the backing file data doesn't become > visible in the guest, but the added area is properly zeroed out. >=20 > Consider the following scenario where the overlay is shorter than its > backing file: >=20 > base.qcow2: AAAAAAAA > overlay.qcow2: BBBB >=20 > When resizing (extending) overlay.qcow2, the new blocks should not stay > unallocated and make the additional As from base.qcow2 visible like > before this patch, but zeros should be read. >=20 > A similar case happens with the various variants of a commit job when an > intermediate file is short (- for unallocated): >=20 > base.qcow2: A-A-AAAA > mid.qcow2: BB-B > top.qcow2: C--C--C- >=20 > After commit top.qcow2 to mid.qcow2, the following happens: >=20 > mid.qcow2: CB-C00C0 (correct result) > mid.qcow2: CB-C--C- (before this fix) >=20 > Without the fix, blocks that previously read as zeros on top.qcow2 > suddenly turn into A. >=20 > Signed-off-by: Kevin Wolf > --- > block/io.c | 32 ++++++++++++++++++++++++++++++++ > 1 file changed, 32 insertions(+) >=20 > + if (new_bytes && bs->backing && prealloc =3D=3D PREALLOC_MODE_OFF) { > + int64_t backing_len; > + > + backing_len =3D bdrv_getlength(backing_bs(bs)); > + if (backing_len < 0) { > + ret =3D backing_len; > + goto out; > + } > + > + if (backing_len > old_size) { > + ret =3D bdrv_co_do_pwrite_zeroes( > + bs, old_size, MIN(new_bytes, backing_len - old_size)= , > + BDRV_REQ_ZERO_WRITE | BDRV_REQ_MAY_UNMAP); > + if (ret < 0) { > + goto out; > + } > + } > + } Note that if writing zeroes is not fast, and it turns out that we copy a=20 lot of data rather than unallocated sections from the image being=20 committed, that this can actually slow things down (doing a bulk=20 pre-zero doubles up data I/O unless it is fast, which is why we added=20 BDRV_REQ_NO_FALLBACK to avoid slow pre-zeroing). However, the=20 complication of zeroing only the unallocated clusters rather than a bulk=20 pre-zeroing for something that is an unlikely corner case (how often do=20 you create an overlay shorter than the backing file?) is not worth the=20 extra code maintenance (unlike in the 'qemu-img convert' case where it=20 was worth the optimization). So I'm fine with how you fixed it here. Reviewed-by: Eric Blake --=20 Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org