From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-ECDSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6D304606A32F for ; Tue, 3 Nov 2026 16:07:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=mimecast20190719 header.b=h+4Aowok Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1xOH2V-0003ym-RI; Tue, 03 Nov 2026 11:07:11 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xOH2S-0003y2-GJ for qemu-devel@nongnu.org; Tue, 03 Nov 2026 11:07:09 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xOH2Q-0007JB-Fy for qemu-devel@nongnu.org; Tue, 03 Nov 2026 11:07:07 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1793722024; 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=X/xH7owUOWlecAyymfxk65DI0LsFF/g3YYyBZycan2Q=; b=h+4AowokdDNXloRY3w4WmWhUiZu96SJLLv9nWXE7D/RQzb/hC1BS2FyOLEW7QmAaG2s0FG iyhRpC19r6K0b2PI7jDFLD2csVaHF2jr0C+OQaPHJq+poqxIVvBml0m6AjIQ6f+IMAB5Ud f6rp+uobxgwdQZIvE3Zh1Kq4gjOsJoA= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-562-BIW03FS9NnK4ToB53y5Xmg-1; Tue, 03 Nov 2026 11:07:02 -0500 X-MC-Unique: BIW03FS9NnK4ToB53y5Xmg-1 X-Mimecast-MFC-AGG-ID: BIW03FS9NnK4ToB53y5Xmg_1793722021 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 111C71977421; Tue, 3 Nov 2026 16:06:45 +0000 (UTC) Received: from redhat.com (unknown [10.44.48.52]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 0A7FC30002CF; Tue, 3 Nov 2026 16:06:42 +0000 (UTC) Date: Tue, 3 Nov 2026 17:06:40 +0100 From: Kevin Wolf To: Jean-Louis Dupond Cc: Andrey Drobyshev , qemu-block@nongnu.org, qemu-devel@nongnu.org, hreitz@redhat.com, den@openvz.org Subject: Re: [PATCH 2/3] qcow2: fix snapshot data corruption with discard-no-unref=on Message-ID: References: <20261027104441.830212-1-andrey.drobyshev@virtuozzo.com> <20261027104441.830212-3-andrey.drobyshev@virtuozzo.com> <713e8c49-5a9c-4d42-9038-8b6f9cc70260@dupond.be> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <713e8c49-5a9c-4d42-9038-8b6f9cc70260@dupond.be> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 5 X-Spam_score: 0.5 X-Spam_bar: / X-Spam_report: (0.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Archive: List-Post: List-Help: List-Subscribe: , Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 Am 29.10.2026 um 15:58 hat Jean-Louis Dupond geschrieben: > > On 27/10/2026 11:44, Andrey Drobyshev wrote: > > Whenever option discard-no-unref=on is enabled, we bypass refcount > > machinery and unconditionally punch hole in the data on 'discard' and > > 'write -z -u' paths. However, when this same cluster is also referenced > > by an internal snapshot, such operations lead to problems. The active L2 > > table is copied before it is changed, so the snapshot metadata stays > > correct, but the host range under it is punched out and the snapshot reads > > zeroes there afterwards. qemu-img check finds nothing wrong. > > > > Fix this by guarding the 'keep_reference' flag by the presence of the > > QCOW_OFLAG_COPIED bit. I.e. only proceed with unconditional data > > punching if we own this cluster exclusively. > > > > Fixes: 42a2890a76f4 ("qcow2: add discard-no-unref option") > > Signed-off-by: Andrey Drobyshev > > --- > > block/qcow2-cluster.c | 6 ++++-- > > 1 file changed, 4 insertions(+), 2 deletions(-) > > > > diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c > > index e02fae6a0c6..cc473c7c796 100644 > > --- a/block/qcow2-cluster.c > > +++ b/block/qcow2-cluster.c > > @@ -1953,6 +1953,7 @@ discard_in_l2_slice(BlockDriverState *bs, uint64_t offset, uint64_t nb_clusters, > > QCow2ClusterType cluster_type = > > qcow2_get_cluster_type(bs, old_l2_entry); > > bool keep_reference = (cluster_type != QCOW2_CLUSTER_COMPRESSED) && > > + (old_l2_entry & QCOW_OFLAG_COPIED) && > > !full_discard && > > (s->discard_no_unref && > > type == QCOW2_DISCARD_REQUEST); > > Wouldn't it be better to gate only qcow2_discard_cluster when the cluster is > QCOW_OFLAG_COPIED? > Cause with the above change, we fully clear/remove the offset of the > cluster, causing fragmentation again in the image afterwards. > And that was just the goal for discard-no-unref :) I thought of this, too, but merely because it seems to be a closer match for the expectation the name "discard-no-unref" sets. In practice, I don't expect that you'd see any advantage from it. Writing again to this cluster will cause COW, so you'll get fragmentation either way. Kevin