[PATCH v5 0/5] replication: Bugfix and properly attach children

[PATCH v5 0/5] replication: Bugfix and properly attach children

[Lukas Straub]

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Hello Everyone,
A while ago Kevin noticed that the replication driver doesn't properly attach
the children it wants to use. Instead, it directly copies the BdrvChilds from
it's backing file, which is wrong. Ths Patchset fixes the problem, fixes a
potential crash in replication_co_writev due to missing permissions and removes
a workaround that was put in place back then.

Tested with full COLO-migration setup in my COLO testsuite.

Regards,
Lukas Straub

Changes:

-v5:
    -Assert that children are writable where it's needed

-v4:
    -minor style fixes
    -clarify why children areguaranteed to be writable in
     "replication: Remove workaround"
    -Added Reviewed-by tags

-v3:
    -Split up into multiple patches
    -Remove s->active_disk
    -Clarify child permissions in commit message

-v2: Test for BDRV_CHILD_PRIMARY in replication_child_perm, since
     bs->file might not be set yet. (Vladimir)


Lukas Straub (5):
  replication: Remove s->active_disk
  replication: Reduce usage of s->hidden_disk and s->secondary_disk
  replication: Properly attach children
  replication: Assert that children are writable
  replication: Remove workaround

 block/replication.c | 121 ++++++++++++++++++++++++++++----------------
 1 file changed, 78 insertions(+), 43 deletions(-)

--
2.20.1

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[PATCH v5 1/5] replication: Remove s->active_disk

[Lukas Straub]

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s->active_disk is bs->file. Remove it and use local variables instead.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/replication.c | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/block/replication.c b/block/replication.c
index 774e15df16..9ad2dfdc69 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -35,7 +35,6 @@ typedef enum {
 typedef struct BDRVReplicationState {
     ReplicationMode mode;
     ReplicationStage stage;
-    BdrvChild *active_disk;
     BlockJob *commit_job;
     BdrvChild *hidden_disk;
     BdrvChild *secondary_disk;
@@ -307,8 +306,10 @@ out:
     return ret;
 }

-static void secondary_do_checkpoint(BDRVReplicationState *s, Error **errp)
+static void secondary_do_checkpoint(BlockDriverState *bs, Error **errp)
 {
+    BDRVReplicationState *s = bs->opaque;
+    BdrvChild *active_disk = bs->file;
     Error *local_err = NULL;
     int ret;

@@ -323,13 +324,13 @@ static void secondary_do_checkpoint(BDRVReplicationState *s, Error **errp)
         return;
     }

-    if (!s->active_disk->bs->drv) {
+    if (!active_disk->bs->drv) {
         error_setg(errp, "Active disk %s is ejected",
-                   s->active_disk->bs->node_name);
+                   active_disk->bs->node_name);
         return;
     }

-    ret = bdrv_make_empty(s->active_disk, errp);
+    ret = bdrv_make_empty(active_disk, errp);
     if (ret < 0) {
         return;
     }
@@ -458,6 +459,7 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
     BlockDriverState *bs = rs->opaque;
     BDRVReplicationState *s;
     BlockDriverState *top_bs;
+    BdrvChild *active_disk;
     int64_t active_length, hidden_length, disk_length;
     AioContext *aio_context;
     Error *local_err = NULL;
@@ -495,15 +497,14 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
     case REPLICATION_MODE_PRIMARY:
         break;
     case REPLICATION_MODE_SECONDARY:
-        s->active_disk = bs->file;
-        if (!s->active_disk || !s->active_disk->bs ||
-                                    !s->active_disk->bs->backing) {
+        active_disk = bs->file;
+        if (!active_disk || !active_disk->bs || !active_disk->bs->backing) {
             error_setg(errp, "Active disk doesn't have backing file");
             aio_context_release(aio_context);
             return;
         }

-        s->hidden_disk = s->active_disk->bs->backing;
+        s->hidden_disk = active_disk->bs->backing;
         if (!s->hidden_disk->bs || !s->hidden_disk->bs->backing) {
             error_setg(errp, "Hidden disk doesn't have backing file");
             aio_context_release(aio_context);
@@ -518,7 +519,7 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
         }

         /* verify the length */
-        active_length = bdrv_getlength(s->active_disk->bs);
+        active_length = bdrv_getlength(active_disk->bs);
         hidden_length = bdrv_getlength(s->hidden_disk->bs);
         disk_length = bdrv_getlength(s->secondary_disk->bs);
         if (active_length < 0 || hidden_length < 0 || disk_length < 0 ||
@@ -530,9 +531,9 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
         }

         /* Must be true, or the bdrv_getlength() calls would have failed */
-        assert(s->active_disk->bs->drv && s->hidden_disk->bs->drv);
+        assert(active_disk->bs->drv && s->hidden_disk->bs->drv);

-        if (!s->active_disk->bs->drv->bdrv_make_empty ||
+        if (!active_disk->bs->drv->bdrv_make_empty ||
             !s->hidden_disk->bs->drv->bdrv_make_empty) {
             error_setg(errp,
                        "Active disk or hidden disk doesn't support make_empty");
@@ -586,7 +587,7 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
     s->stage = BLOCK_REPLICATION_RUNNING;

     if (s->mode == REPLICATION_MODE_SECONDARY) {
-        secondary_do_checkpoint(s, errp);
+        secondary_do_checkpoint(bs, errp);
     }

     s->error = 0;
@@ -615,7 +616,7 @@ static void replication_do_checkpoint(ReplicationState *rs, Error **errp)
     }

     if (s->mode == REPLICATION_MODE_SECONDARY) {
-        secondary_do_checkpoint(s, errp);
+        secondary_do_checkpoint(bs, errp);
     }
     aio_context_release(aio_context);
 }
@@ -652,7 +653,6 @@ static void replication_done(void *opaque, int ret)
     if (ret == 0) {
         s->stage = BLOCK_REPLICATION_DONE;

-        s->active_disk = NULL;
         s->secondary_disk = NULL;
         s->hidden_disk = NULL;
         s->error = 0;
@@ -705,7 +705,7 @@ static void replication_stop(ReplicationState *rs, bool failover, Error **errp)
         }

         if (!failover) {
-            secondary_do_checkpoint(s, errp);
+            secondary_do_checkpoint(bs, errp);
             s->stage = BLOCK_REPLICATION_DONE;
             aio_context_release(aio_context);
             return;
@@ -713,7 +713,7 @@ static void replication_stop(ReplicationState *rs, bool failover, Error **errp)

         s->stage = BLOCK_REPLICATION_FAILOVER;
         s->commit_job = commit_active_start(
-                            NULL, s->active_disk->bs, s->secondary_disk->bs,
+                            NULL, bs->file->bs, s->secondary_disk->bs,
                             JOB_INTERNAL, 0, BLOCKDEV_ON_ERROR_REPORT,
                             NULL, replication_done, bs, true, errp);
         break;
--
2.20.1


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[PATCH v5 2/5] replication: Reduce usage of s->hidden_disk and s->secondary_disk

[Lukas Straub]

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In preparation for the next patch, initialize s->hidden_disk and
s->secondary_disk later and replace access to them with local variables
in the places where they aren't initialized yet.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/replication.c | 45 ++++++++++++++++++++++++++++-----------------
 1 file changed, 28 insertions(+), 17 deletions(-)

diff --git a/block/replication.c b/block/replication.c
index 9ad2dfdc69..25bbdf5d4b 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -366,27 +366,35 @@ static void reopen_backing_file(BlockDriverState *bs, bool writable,
                                 Error **errp)
 {
     BDRVReplicationState *s = bs->opaque;
+    BdrvChild *hidden_disk, *secondary_disk;
     BlockReopenQueue *reopen_queue = NULL;

+    /*
+     * s->hidden_disk and s->secondary_disk may not be set yet, as they will
+     * only be set after the children are writable.
+     */
+    hidden_disk = bs->file->bs->backing;
+    secondary_disk = hidden_disk->bs->backing;
+
     if (writable) {
-        s->orig_hidden_read_only = bdrv_is_read_only(s->hidden_disk->bs);
-        s->orig_secondary_read_only = bdrv_is_read_only(s->secondary_disk->bs);
+        s->orig_hidden_read_only = bdrv_is_read_only(hidden_disk->bs);
+        s->orig_secondary_read_only = bdrv_is_read_only(secondary_disk->bs);
     }

-    bdrv_subtree_drained_begin(s->hidden_disk->bs);
-    bdrv_subtree_drained_begin(s->secondary_disk->bs);
+    bdrv_subtree_drained_begin(hidden_disk->bs);
+    bdrv_subtree_drained_begin(secondary_disk->bs);

     if (s->orig_hidden_read_only) {
         QDict *opts = qdict_new();
         qdict_put_bool(opts, BDRV_OPT_READ_ONLY, !writable);
-        reopen_queue = bdrv_reopen_queue(reopen_queue, s->hidden_disk->bs,
+        reopen_queue = bdrv_reopen_queue(reopen_queue, hidden_disk->bs,
                                          opts, true);
     }

     if (s->orig_secondary_read_only) {
         QDict *opts = qdict_new();
         qdict_put_bool(opts, BDRV_OPT_READ_ONLY, !writable);
-        reopen_queue = bdrv_reopen_queue(reopen_queue, s->secondary_disk->bs,
+        reopen_queue = bdrv_reopen_queue(reopen_queue, secondary_disk->bs,
                                          opts, true);
     }

@@ -401,8 +409,8 @@ static void reopen_backing_file(BlockDriverState *bs, bool writable,
         }
     }

-    bdrv_subtree_drained_end(s->hidden_disk->bs);
-    bdrv_subtree_drained_end(s->secondary_disk->bs);
+    bdrv_subtree_drained_end(hidden_disk->bs);
+    bdrv_subtree_drained_end(secondary_disk->bs);
 }

 static void backup_job_cleanup(BlockDriverState *bs)
@@ -459,7 +467,7 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
     BlockDriverState *bs = rs->opaque;
     BDRVReplicationState *s;
     BlockDriverState *top_bs;
-    BdrvChild *active_disk;
+    BdrvChild *active_disk, *hidden_disk, *secondary_disk;
     int64_t active_length, hidden_length, disk_length;
     AioContext *aio_context;
     Error *local_err = NULL;
@@ -504,15 +512,15 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
             return;
         }

-        s->hidden_disk = active_disk->bs->backing;
-        if (!s->hidden_disk->bs || !s->hidden_disk->bs->backing) {
+        hidden_disk = active_disk->bs->backing;
+        if (!hidden_disk->bs || !hidden_disk->bs->backing) {
             error_setg(errp, "Hidden disk doesn't have backing file");
             aio_context_release(aio_context);
             return;
         }

-        s->secondary_disk = s->hidden_disk->bs->backing;
-        if (!s->secondary_disk->bs || !bdrv_has_blk(s->secondary_disk->bs)) {
+        secondary_disk = hidden_disk->bs->backing;
+        if (!secondary_disk->bs || !bdrv_has_blk(secondary_disk->bs)) {
             error_setg(errp, "The secondary disk doesn't have block backend");
             aio_context_release(aio_context);
             return;
@@ -520,8 +528,8 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,

         /* verify the length */
         active_length = bdrv_getlength(active_disk->bs);
-        hidden_length = bdrv_getlength(s->hidden_disk->bs);
-        disk_length = bdrv_getlength(s->secondary_disk->bs);
+        hidden_length = bdrv_getlength(hidden_disk->bs);
+        disk_length = bdrv_getlength(secondary_disk->bs);
         if (active_length < 0 || hidden_length < 0 || disk_length < 0 ||
             active_length != hidden_length || hidden_length != disk_length) {
             error_setg(errp, "Active disk, hidden disk, secondary disk's length"
@@ -531,10 +539,10 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
         }

         /* Must be true, or the bdrv_getlength() calls would have failed */
-        assert(active_disk->bs->drv && s->hidden_disk->bs->drv);
+        assert(active_disk->bs->drv && hidden_disk->bs->drv);

         if (!active_disk->bs->drv->bdrv_make_empty ||
-            !s->hidden_disk->bs->drv->bdrv_make_empty) {
+            !hidden_disk->bs->drv->bdrv_make_empty) {
             error_setg(errp,
                        "Active disk or hidden disk doesn't support make_empty");
             aio_context_release(aio_context);
@@ -549,6 +557,9 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
             return;
         }

+        s->hidden_disk = hidden_disk;
+        s->secondary_disk = secondary_disk;
+
         /* start backup job now */
         error_setg(&s->blocker,
                    "Block device is in use by internal backup job");
--
2.20.1


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[PATCH v5 3/5] replication: Properly attach children

[Lukas Straub]

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The replication driver needs access to the children block-nodes of
it's child so it can issue bdrv_make_empty() and bdrv_co_pwritev()
to manage the replication. However, it does this by directly copying
the BdrvChilds, which is wrong.

Fix this by properly attaching the block-nodes with
bdrv_attach_child() and requesting the required permissions.

This ultimatively fixes a potential crash in replication_co_writev(),
because it may write to s->secondary_disk if it is in state
BLOCK_REPLICATION_FAILOVER_FAILED, without requesting write
permissions first. And now the workaround in
secondary_do_checkpoint() can be removed.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/replication.c | 30 +++++++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/block/replication.c b/block/replication.c
index 25bbdf5d4b..b74192f795 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -165,7 +165,12 @@ static void replication_child_perm(BlockDriverState *bs, BdrvChild *c,
                                    uint64_t perm, uint64_t shared,
                                    uint64_t *nperm, uint64_t *nshared)
 {
-    *nperm = BLK_PERM_CONSISTENT_READ;
+    if (role & BDRV_CHILD_PRIMARY) {
+        *nperm = BLK_PERM_CONSISTENT_READ;
+    } else {
+        *nperm = 0;
+    }
+
     if ((bs->open_flags & (BDRV_O_INACTIVE | BDRV_O_RDWR)) == BDRV_O_RDWR) {
         *nperm |= BLK_PERM_WRITE;
     }
@@ -557,8 +562,25 @@ static void replication_start(ReplicationState *rs, ReplicationMode mode,
             return;
         }

-        s->hidden_disk = hidden_disk;
-        s->secondary_disk = secondary_disk;
+        bdrv_ref(hidden_disk->bs);
+        s->hidden_disk = bdrv_attach_child(bs, hidden_disk->bs, "hidden disk",
+                                           &child_of_bds, BDRV_CHILD_DATA,
+                                           &local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+            aio_context_release(aio_context);
+            return;
+        }
+
+        bdrv_ref(secondary_disk->bs);
+        s->secondary_disk = bdrv_attach_child(bs, secondary_disk->bs,
+                                              "secondary disk", &child_of_bds,
+                                              BDRV_CHILD_DATA, &local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+            aio_context_release(aio_context);
+            return;
+        }

         /* start backup job now */
         error_setg(&s->blocker,
@@ -664,7 +686,9 @@ static void replication_done(void *opaque, int ret)
     if (ret == 0) {
         s->stage = BLOCK_REPLICATION_DONE;

+        bdrv_unref_child(bs, s->secondary_disk);
         s->secondary_disk = NULL;
+        bdrv_unref_child(bs, s->hidden_disk);
         s->hidden_disk = NULL;
         s->error = 0;
     } else {
--
2.20.1


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[PATCH v5 4/5] replication: Assert that children are writable

[Lukas Straub]

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Assert that the children are writable where it's needed.
While there is no test-case for the
BLOCK_REPLICATION_FAILOVER_FAILED state, this at least ensures that
s->secondary_disk is always writable in case replication might go
into that state.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
---
 block/replication.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/block/replication.c b/block/replication.c
index b74192f795..772bb63374 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -261,6 +261,13 @@ static coroutine_fn int replication_co_writev(BlockDriverState *bs,
     int64_t n;

     assert(!flags);
+    assert(top->perm & BLK_PERM_WRITE);
+    if (s->mode == REPLICATION_MODE_SECONDARY &&
+        s->stage != BLOCK_REPLICATION_NONE &&
+        s->stage != BLOCK_REPLICATION_DONE) {
+        assert(base->perm & BLK_PERM_WRITE);
+    }
+
     ret = replication_get_io_status(s);
     if (ret < 0) {
         goto out;
@@ -318,6 +325,9 @@ static void secondary_do_checkpoint(BlockDriverState *bs, Error **errp)
     Error *local_err = NULL;
     int ret;

+    assert(active_disk->perm & BLK_PERM_WRITE);
+    assert(s->hidden_disk->perm & BLK_PERM_WRITE);
+
     if (!s->backup_job) {
         error_setg(errp, "Backup job was cancelled unexpectedly");
         return;
--
2.20.1


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Re: [PATCH v5 4/5] replication: Assert that children are writable

[Vladimir Sementsov-Ogievskiy]

12.07.2021 14:54, Lukas Straub wrote:
> Assert that the children are writable where it's needed.
> While there is no test-case for the
> BLOCK_REPLICATION_FAILOVER_FAILED state, this at least ensures that
> s->secondary_disk is always writable in case replication might go
> into that state.
> 
> Signed-off-by: Lukas Straub <lukasstraub2@web.de>
> ---
>   block/replication.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/block/replication.c b/block/replication.c
> index b74192f795..772bb63374 100644
> --- a/block/replication.c
> +++ b/block/replication.c
> @@ -261,6 +261,13 @@ static coroutine_fn int replication_co_writev(BlockDriverState *bs,
>       int64_t n;
> 
>       assert(!flags);
> +    assert(top->perm & BLK_PERM_WRITE);
> +    if (s->mode == REPLICATION_MODE_SECONDARY &&
> +        s->stage != BLOCK_REPLICATION_NONE &&
> +        s->stage != BLOCK_REPLICATION_DONE) {
> +        assert(base->perm & BLK_PERM_WRITE);
> +    }
> +

write has assertions in generic code so actually we don't need this.

Also using this additional conditions is not obvious. Better is assert about base without extra conditiions exactly before while loop.

>       ret = replication_get_io_status(s);
>       if (ret < 0) {
>           goto out;
> @@ -318,6 +325,9 @@ static void secondary_do_checkpoint(BlockDriverState *bs, Error **errp)
>       Error *local_err = NULL;
>       int ret;
> 
> +    assert(active_disk->perm & BLK_PERM_WRITE);
> +    assert(s->hidden_disk->perm & BLK_PERM_WRITE);

Oops, bdrv_make_empty also has this assertion inside. It also is satisfied by BLK_PERM_WRITE_UNCHANGED, but we don't work with it here anyway. So we don't need it.

> +
>       if (!s->backup_job) {
>           error_setg(errp, "Backup job was cancelled unexpectedly");
>           return;
> --
> 2.20.1
> 

Sorry, seems my suggestion to add assertions was bad, as we already have them in generic code.

-- 
Best regards,
Vladimir

[PATCH v5 5/5] replication: Remove workaround

[Lukas Straub]

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Remove the workaround introduced in commit
6ecbc6c52672db5c13805735ca02784879ce8285
"replication: Avoid blk_make_empty() on read-only child".

It is not needed anymore since s->hidden_disk is guaranteed to be
writable when secondary_do_checkpoint() runs. Because replication_start(),
_do_checkpoint() and _stop() are only called by COLO migration code
and COLO-migration activates all disks via bdrv_invalidate_cache_all()
before it calls these functions.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
---
 block/replication.c | 12 +-----------
 1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/block/replication.c b/block/replication.c
index 772bb63374..1e9dc4d309 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -356,17 +356,7 @@ static void secondary_do_checkpoint(BlockDriverState *bs, Error **errp)
         return;
     }

-    BlockBackend *blk = blk_new(qemu_get_current_aio_context(),
-                                BLK_PERM_WRITE, BLK_PERM_ALL);
-    blk_insert_bs(blk, s->hidden_disk->bs, &local_err);
-    if (local_err) {
-        error_propagate(errp, local_err);
-        blk_unref(blk);
-        return;
-    }
-
-    ret = blk_make_empty(blk, errp);
-    blk_unref(blk);
+    ret = bdrv_make_empty(s->hidden_disk, errp);
     if (ret < 0) {
         return;
     }
--
2.20.1

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Re: [PATCH v5 5/5] replication: Remove workaround

[Vladimir Sementsov-Ogievskiy]

12.07.2021 14:54, Lukas Straub wrote:
> Remove the workaround introduced in commit
> 6ecbc6c52672db5c13805735ca02784879ce8285
> "replication: Avoid blk_make_empty() on read-only child".
> 
> It is not needed anymore since s->hidden_disk is guaranteed to be
> writable when secondary_do_checkpoint() runs. Because replication_start(),
> _do_checkpoint() and _stop() are only called by COLO migration code
> and COLO-migration activates all disks via bdrv_invalidate_cache_all()
> before it calls these functions.

In replication_child_perm()
we have

  if ((bs->open_flags & (BDRV_O_INACTIVE | BDRV_O_RDWR)) == BDRV_O_RDWR) {
      *nperm |= BLK_PERM_WRITE;
  }

That's probably possible

1. configure a block graph like described in replicatio doc, but all disks opened read-only. so we don't have WRITE permission

2. start replication

3. crash on trying to make disk empty in do_checkpoint with no WRITE permission

Still, I think if it possible, we'll crash on first bdrv_make_empty of active disk, and that's preexisting.

So:

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

> 
> Signed-off-by: Lukas Straub <lukasstraub2@web.de>
> ---
>   block/replication.c | 12 +-----------
>   1 file changed, 1 insertion(+), 11 deletions(-)
> 
> diff --git a/block/replication.c b/block/replication.c
> index 772bb63374..1e9dc4d309 100644
> --- a/block/replication.c
> +++ b/block/replication.c
> @@ -356,17 +356,7 @@ static void secondary_do_checkpoint(BlockDriverState *bs, Error **errp)
>           return;
>       }
> 
> -    BlockBackend *blk = blk_new(qemu_get_current_aio_context(),
> -                                BLK_PERM_WRITE, BLK_PERM_ALL);
> -    blk_insert_bs(blk, s->hidden_disk->bs, &local_err);
> -    if (local_err) {
> -        error_propagate(errp, local_err);
> -        blk_unref(blk);
> -        return;
> -    }
> -
> -    ret = blk_make_empty(blk, errp);
> -    blk_unref(blk);
> +    ret = bdrv_make_empty(s->hidden_disk, errp);
>       if (ret < 0) {
>           return;
>       }
> --
> 2.20.1
> 



-- 
Best regards,
Vladimir