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From 9ba321e18a357c1a3a238ceee301bbb174f96eee Mon Sep 17 00:00:00 2001
From: Kevin Wolf <>
Date: Fri, 13 Mar 2020 12:34:34 +0000
Subject: [PATCH 14/20] block: Relax restrictions for blockdev-snapshot

RH-Author: Kevin Wolf <>
Message-id: <>
Patchwork-id: 94285
O-Subject: [RHEL-AV-8.2.0 qemu-kvm PATCH v2 08/13] block: Relax restrictions for blockdev-snapshot
Bugzilla: 1790482 1805143
RH-Acked-by: John Snow <>
RH-Acked-by: Daniel P. Berrange <>
RH-Acked-by: Peter Krempa <>

blockdev-snapshot returned an error if the overlay was already in use,
which it defined as having any BlockBackend parent. This is in fact both
too strict (some parents can tolerate the change of visible data caused
by attaching a backing file) and too loose (some non-BlockBackend
parents may not be happy with it).

One important use case that is prevented by the too strict check is live
storage migration with blockdev-mirror. Here, the target node is
usually opened without a backing file so that the active layer is
mirrored while its backing chain can be copied in the background.

The backing chain should be attached to the mirror target node when
finalising the job, just before switching the users of the source node
to the new copy (at which point the mirror job still has a reference to
the node). drive-mirror did this automatically, but with blockdev-mirror
this is the job of the QMP client, so it needs a way to do this.

blockdev-snapshot is the obvious way, so this patch makes it work in
this scenario. The new condition is that no parent uses CONSISTENT_READ
permissions. This will ensure that the operation will still be blocked
when the node is attached to the guest device, so blockdev-snapshot
remains safe.

(For the sake of completeness, x-blockdev-reopen can be used to achieve
the same, however it is a big hammer, performs the graph change
completely unchecked and is still experimental. So even with the option
of using x-blockdev-reopen, there are reasons why blockdev-snapshot
should be able to perform this operation.)

Signed-off-by: Kevin Wolf <>
Message-Id: <>
Reviewed-by: Peter Krempa <>
Tested-by: Peter Krempa <>
Signed-off-by: Kevin Wolf <>
(cherry picked from commit d29d3d1f80b3947fb26e7139645c83de66d146a9)
Signed-off-by: Kevin Wolf <>
Signed-off-by: Danilo C. L. de Paula <>
 blockdev.c                 | 14 ++++++++------
 tests/qemu-iotests/085.out |  4 ++--
 2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/blockdev.c b/blockdev.c
index 4cd9a58..7918533 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1536,6 +1536,7 @@ static void external_snapshot_prepare(BlkActionState *common,
     TransactionAction *action = common->action;
     AioContext *aio_context;
     AioContext *old_context;
+    uint64_t perm, shared;
     int ret;
     /* 'blockdev-snapshot' and 'blockdev-snapshot-sync' have similar
@@ -1656,16 +1657,17 @@ static void external_snapshot_prepare(BlkActionState *common,
         goto out;
-    if (bdrv_has_blk(state->new_bs)) {
+    /*
+     * Allow attaching a backing file to an overlay that's already in use only
+     * if the parents don't assume that they are already seeing a valid image.
+     * (Specifically, allow it as a mirror target, which is write-only access.)
+     */
+    bdrv_get_cumulative_perm(state->new_bs, &perm, &shared);
+    if (perm & BLK_PERM_CONSISTENT_READ) {
         error_setg(errp, "The overlay is already in use");
         goto out;
-    if (bdrv_op_is_blocked(state->new_bs, BLOCK_OP_TYPE_EXTERNAL_SNAPSHOT,
-                           errp)) {
-        goto out;
-    }
     if (state->new_bs->backing != NULL) {
         error_setg(errp, "The overlay already has a backing image");
         goto out;
diff --git a/tests/qemu-iotests/085.out b/tests/qemu-iotests/085.out
index bb50227..487d920 100644
--- a/tests/qemu-iotests/085.out
+++ b/tests/qemu-iotests/085.out
@@ -82,7 +82,7 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/
 === Invalid command - cannot create a snapshot using a file BDS ===
 { 'execute': 'blockdev-snapshot', 'arguments': { 'node':'virtio0', 'overlay':'file_12' } }
-{"error": {"class": "GenericError", "desc": "The overlay does not support backing images"}}
+{"error": {"class": "GenericError", "desc": "The overlay is already in use"}}
 === Invalid command - snapshot node used as active layer ===
@@ -96,7 +96,7 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/
 === Invalid command - snapshot node used as backing hd ===
 { 'execute': 'blockdev-snapshot', 'arguments': { 'node': 'virtio0', 'overlay':'snap_11' } }
-{"error": {"class": "GenericError", "desc": "Node 'snap_11' is busy: node is used as backing hd of 'snap_12'"}}
+{"error": {"class": "GenericError", "desc": "The overlay is already in use"}}
 === Invalid command - snapshot node has a backing image ===