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 Received: from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 516B5CD6E55 for ; Mon, 1 Jun 2026 16:38:32 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wU5eR-00030M-4Q; Mon, 01 Jun 2026 12:38:07 -0400 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 1wU5eP-0002xN-5H for qemu-devel@nongnu.org; Mon, 01 Jun 2026 12:38:05 -0400 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 1wU5eN-0005st-Hb for qemu-devel@nongnu.org; Mon, 01 Jun 2026 12:38:04 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1780331882; 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=At6atDiDT6k/rmPgC/cs5BeBhQG7KB7OEEHtMFlfYoE=; b=Bkq3BnZUlteLfMfk2AjZrnt+ns78Pev7PU28B4/iaaoZdzRBVQcHgRaNKvdlCXphtUtrTU ihKb6mX9Nr7UVSG5UDk/o57Tl+EYIhuamvaOBRO4os3rhulVCEoo4xnnCypmLuorcJk+ZZ mGeR6cz2SDrB8LTsH36yQSTUW2X9798= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-54-zpntn5ZUP3GDSU1114HdQA-1; Mon, 01 Jun 2026 12:38:01 -0400 X-MC-Unique: zpntn5ZUP3GDSU1114HdQA-1 X-Mimecast-MFC-AGG-ID: zpntn5ZUP3GDSU1114HdQA_1780331880 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 758E918002CF; Mon, 1 Jun 2026 16:38:00 +0000 (UTC) Received: from redhat.com (unknown [10.44.33.161]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 0465119560A3; Mon, 1 Jun 2026 16:37:57 +0000 (UTC) Date: Mon, 1 Jun 2026 18:37:55 +0200 From: Kevin Wolf To: Thomas Lamprecht Cc: qemu-block@nongnu.org, f.ebner@proxmox.com, hreitz@redhat.com, qemu-devel@nongnu.org, qemu-stable@nongnu.org Subject: Re: [PATCH v2] qcow2: Fix data loss on zero write with detect-zeroes=unmap Message-ID: References: <20260522151318.238064-1-t.lamprecht@proxmox.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260522151318.238064-1-t.lamprecht@proxmox.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 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: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham 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-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 22.05.2026 um 17:13 hat Thomas Lamprecht geschrieben: > Commit b8bfb1478d ("qcow2: Fix corruption on discard during write with > COW") added a wait_for_dependencies() at the start of > qcow2_subcluster_zeroize(). That fixes the inconsistency it set out to > fix, but turns the lock-protected pre-check in the caller, > qcow2_co_pwrite_zeroes(), into a stale one: the wait yields s->lock, > so an in-flight allocating write whose QCowL2Meta is already on > s->cluster_allocs (but whose L2 entry is not yet linked) gets to link > its entry during the yield. When the zeroize wakes, the cluster is now > NORMAL, and with BDRV_REQ_MAY_UNMAP the free path in zero_in_l2_slice() > unmaps the just-written cluster, silently dropping the data write's > payload. > > This is reachable with detect-zeroes=unmap (the default for VirtIO > disks with discard on in Proxmox VE), under which the block layer > auto-promotes all-zero buffers to BDRV_REQ_ZERO_WRITE | > BDRV_REQ_MAY_UNMAP. A memory-constrained Debian guest running 'apt > full-upgrade' on such a disk reproduces it as random SIGSEGVs: > swapped-out code pages come back as zero. > > Wait for in-flight dependencies before the lock-protected check in > qcow2_co_pwrite_zeroes(). If a write linked its L2 entry during the > wait, the type check now fails and the block layer falls back to a > bounce-buffered zero write that only touches the requested subrange, > preserving the racing write's data. Promote wait_for_dependencies() to > qcow2_wait_for_dependencies() so qcow2.c can call it. > > Fixes: b8bfb1478d ("qcow2: Fix corruption on discard during write with COW") > Cc: qemu-stable@nongnu.org > Tested-by: Fiona Ebner > Reviewed-by: Fiona Ebner > Signed-off-by: Thomas Lamprecht > --- > > Changes v1 -> v2: > * improve comments (thx @Fiona) > * add Fiona's R-b/T-b > > block/qcow2-cluster.c | 10 +++++----- > block/qcow2.c | 10 ++++++++-- > block/qcow2.h | 4 ++++ > tests/qemu-iotests/046 | 23 +++++++++++++++++++++++ > tests/qemu-iotests/046.out | 10 ++++++++++ > 5 files changed, 50 insertions(+), 7 deletions(-) > > diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c > index 8b1e80bd0b..e02fae6a0c 100644 > --- a/block/qcow2-cluster.c > +++ b/block/qcow2-cluster.c > @@ -1474,9 +1474,9 @@ static int coroutine_fn handle_dependencies(BlockDriverState *bs, > return 0; > } > > -static void coroutine_mixed_fn wait_for_dependencies(BlockDriverState *bs, > - uint64_t guest_offset, > - uint64_t bytes) > +void coroutine_mixed_fn qcow2_wait_for_dependencies(BlockDriverState *bs, > + uint64_t guest_offset, > + uint64_t bytes) > { > BDRVQcow2State *s = bs->opaque; > QCowL2Meta *m = NULL; > @@ -2035,7 +2035,7 @@ int qcow2_cluster_discard(BlockDriverState *bs, uint64_t offset, > * We don't need to allocate a QCowL2Meta for the discard operation because > * s->lock is held for the duration of the whole operation. > */ > - wait_for_dependencies(bs, offset, bytes); > + qcow2_wait_for_dependencies(bs, offset, bytes); > > /* Caller must pass aligned values, except at image end */ > assert(QEMU_IS_ALIGNED(offset, s->cluster_size)); > @@ -2204,7 +2204,7 @@ int coroutine_fn qcow2_subcluster_zeroize(BlockDriverState *bs, uint64_t offset, > * We don't need to allocate a QCowL2Meta for the zeroize operation because > * s->lock is held for the duration of the whole operation. > */ > - wait_for_dependencies(bs, offset, bytes); > + qcow2_wait_for_dependencies(bs, offset, bytes); > > /* If we have to stay in sync with an external data file, zero out > * s->data_file first. */ > diff --git a/block/qcow2.c b/block/qcow2.c > index 81fd299b4c..96efdd4503 100644 > --- a/block/qcow2.c > +++ b/block/qcow2.c > @@ -4234,10 +4234,16 @@ qcow2_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset, int64_t bytes, > } > > qemu_co_mutex_lock(&s->lock); > - /* We can have new write after previous check */ > offset -= head; > bytes = s->subcluster_size; > - nr = s->subcluster_size; I'm not sure why you changed this line. Isn't the new version of it doing exactly the same thing as before? This makes the diff a little more confusing than it should be. > + /* > + * Wait for in-flight allocating writes first: otherwise the type > + * check below could pass on UNALLOCATED while a yet-to-link_l2 write > + * completes during qcow2_subcluster_zeroize()'s own wait, letting the > + * resumed MAY_UNMAP discard the just-written data. > + */ > + qcow2_wait_for_dependencies(bs, offset, bytes); > + nr = bytes; > ret = qcow2_get_host_offset(bs, offset, &nr, &off, &type); > if (ret < 0 || > (type != QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN && The rest looks good to me. Maybe not perfect because we're calling qcow2_wait_for_dependencies() twice now and rely on the second instance not doing anything any more, but I guess that's okay. So if you agree, I'd just change the assignment of nr above back and apply this. Kevin