Re: [PATCH 2/2] nbd: Add new qemu:joint-allocation metadata context

[Nir Soffer]

On Wed, Jun 9, 2021 at 9:01 PM Eric Blake <eblake@redhat.com> wrote:
>
> When trying to reconstruct a qcow2 chain using information provided
> over NBD, ovirt had been relying on an unsafe assumption that any
> portion of the qcow2 file advertised as sparse would defer to the
> backing image; this worked with what qemu 5.2 reports for a qcow2 BSD
> loaded with "backing":null.  However, in 6.0, commit 0da9856851 (nbd:
> server: Report holes for raw images) also had a side-effect of
> reporting unallocated zero clusters in qcow2 files as sparse.  This
> change is correct from the NBD spec perspective (advertising bits has
> always been optional based on how much information the server has
> available, and should only be used to optimize behavior when a bit is
> set, while not assuming semantics merely because a bit is clear), but
> means that a qcow2 file that uses an unallocated zero cluster to
> override a backing file now shows up as sparse over NBD, and causes
> ovirt to fail to reproduce that cluster (ie. ovirt was assuming it
> only had to write clusters where the bit was clear, and the 6.0
> behavior change shows the flaw in that assumption).
>
> The correct fix is for ovirt to additionally use the
> qemu:allocation-depth metadata context added in 5.2: after all, the
> actual determination for what is needed to recreate a qcow2 file is
> not whether a cluster is sparse, but whether the allocation-depth
> shows the cluster to be local.  But reproducing an image is more
> efficient when handling known-zero clusters, which means that ovirt
> has to track both base:allocation and qemu:allocation-depth metadata
> contexts simultaneously.  While NBD_CMD_BLOCK_STATUS is just fine
> sending back information for two contexts in parallel, it comes with
> some bookkeeping overhead at the client side: the two contexts need
> not report the same length of replies, and it involves more network
> traffic.
>
> So, as a convenience, we can provide yet another metadata context,
> "qemu:joint-allocation", which provides the bulk of the same
> information already available from using "base:allocation" and
> "qemu:allocation-depth" in parallel; the only difference is that an
> allocation depth larger than one is collapsed to a single bit, rather
> than remaining an integer representing actual depth.  By connecting to
> just this context, a client has less work to perform while still
> getting at all pieces of information needed to recreate a qcow2
> backing chain.

Providing extended allocation is awsome, and makes client life much
easier. But I'm not sure about the name, that comes from "joining"
"base:allocation" and "qemu:allocation-depth". This is correct when
thinking about qemu internals, but this is not really getting both, since
"qemu:allocation-depth" is reduced to local and backing.

>From a client point of view, I think this is best described as 
>"qemu:allocation"
which is an extension to NBD protocol, providing the same HOLE and ZERO
bits, and qemu specific info LOCAL, BACKING. Using different "namespace"
("qemu" vs "base") makes it clear that this is not the same.

We discussed in the past the option to expose also the dirty status of every
block in the response. Again this info is available using
"qemu:dirty-bitmap:xxx"
but just like allocation depth and base allocation, merging the results is hard
and if we could expose also the dirty bit, this can make clients life
even better.
In this case I'm not sure "qemu:allocation" is the best name, maybe something
more generic like "qemu:extents" or "qemu:block-status" is even better.

> With regards to exposing this new feature from qemu as NBD server, it
> is sufficient to reuse the existing 'qemu-nbd -A': since that already
> exposes allocation depth, it does not hurt to advertise two separate
> qemu:XXX metadata contexts at once for two different views of
> allocation depth.  And just because the server supports multiple
> contexts does not mean a client will want or need to connect to
> everything available.  On the other hand, the existing hack of using
> the qemu NBD client option of x-dirty-bitmap to select an alternative
> context from the client does NOT make it possible to read the extra
> information exposed by the new metadata context.  For now, you MUST
> use something like libnbd's 'nbdinfo --map=qemu:joint-allocation' in
> order to properly see all four bits in action:

Makes sense.

>     # Create a qcow2 image with a raw backing file:
>     $ qemu-img create base.raw $((4*64*1024))
>     $ qemu-img create -f qcow2 -b base.raw -F raw top.qcow2
>
>     # Write to first 3 clusters of base:
>     $ qemu-io -f raw -c "w -P 65 0 64k" -c "w -P 66 64k 64k" \
>       -c "w -P 67 128k 64k" base.raw
>
>     # Write to second and third clusters of top, hiding base:
>     $ qemu-io -f qcow2 -c "w -P 69 64k 64k" -c "w -z 128k 64k" top.qcow2

Looks familiar but nicer :-)

>     # Expose top.qcow2 without backing file over NBD
>     $ ./qemu-nbd -r -t -f qcow2 -A 'json:{"driver":"qcow2", "backing":null, \
>       "file":{"driver":"file", "filename":"top.qcow2"}}'
>     $ nbdinfo --map=qemu:joint-allocation nbd://localhost
>          0       65536    3
>      65536       65536    4
>     131072       65536    7
>     196608       65536    3

Using the libnbd patch this shows:

$ ./nbdinfo --map="qemu:joint-allocation" nbd://localhost
         0       65536    3  hole,zero,unallocated
     65536       65536    4  allocated,local
    131072       65536    7  hole,zero,local
    196608       65536    3  hole,zero,unallocated

Looks good.

We need to convert this output to:

    {"start": 0, "length": 65536, "zero": true, "hole": true},
    {"start": 65536, "length": 65536, "zero": false, "hole": false},
    {"start": 131072, "length": 65536, "zero": true, "hole": false},
    {"start": 196608, "length": 65536, "zero": true, "hole": true},

So it seems that we need to use this logic for holes when we inspect a
single qcow2 image:

    hole = not (flags & NBD_STATE_LOCAL)

And ignore the NBD_STATE_HOLE, which is about qcow2 internals.

This patch fixes the critical issue for oVirt, but in a way it returns
the previous
state when you could not report holes in raw images. With this patch holes
in raw image looks like:

    $ truncate -s 1g empty.raw
    $  ./qemu-nbd -r -t -f raw empty.raw --allocation-depth
    $ ./nbdinfo --map="qemu:joint-allocation" nbd://localhost
         0  1073741824    7  hole,zero,local

This is not a practical issue for oVirt, but it would be better to report:

    $ ./nbdinfo --map="qemu:joint-allocation" nbd://localhost
         0  1073741824    3  hole,zero,unallocated

This is the output for the empty qcow2 image.

And this also affects --allocation-depth, for raw empty image we get:

    $ ./nbdinfo --map="qemu:allocation-depth" nbd://localhost
         0  1073741824    1  local

But for empty qcow2 we get:

    $ ./nbdinfo --map="qemu:allocation-depth" nbd://localhost
         0  1073741824    0  unallocated

I think we have a bug reporting
BDRV_BLOCK_ALLOCATED when the BDRV_BLOCK_DATA bit is not set.

> [This was output from nbdinfo 1.8.0; a later version will also add a
> column to decode the bits into human-readable strings]
>
> Additionally, later qemu patches may try to improve qemu-img to
> automatically take advantage of additional NBD context information,
> without having to use x-dirty-bitmap.
>
> Reported-by: Nir Soffer <nsoffer@redhat.com>
> Resolves: https://bugzilla.redhat.com/1968693
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>  docs/interop/nbd.txt                          | 31 ++++++-
>  docs/tools/qemu-nbd.rst                       |  4 +-
>  qapi/block-export.json                        |  4 +-
>  include/block/nbd.h                           | 10 ++-
>  nbd/server.c                                  | 87 +++++++++++++++++--
>  .../tests/nbd-qemu-allocation.out             |  3 +-
>  6 files changed, 125 insertions(+), 14 deletions(-)
>
> diff --git a/docs/interop/nbd.txt b/docs/interop/nbd.txt
> index 10ce098a29bf..cc8ce2d5389f 100644
> --- a/docs/interop/nbd.txt
> +++ b/docs/interop/nbd.txt
> @@ -17,7 +17,7 @@ namespace "qemu".
>
>  == "qemu" namespace ==
>
> -The "qemu" namespace currently contains two available metadata context
> +The "qemu" namespace currently contains three available metadata context
>  types.  The first is related to exposing the contents of a dirty
>  bitmap alongside the associated disk contents.  That metadata context
>  is named with the following form:
> @@ -39,8 +39,32 @@ depth of which layer in a thin-provisioned backing chain 
> provided the
>  data (0 for unallocated, 1 for the active layer, 2 for the first
>  backing layer, and so forth).
>
> -For NBD_OPT_LIST_META_CONTEXT the following queries are supported
> -in addition to the specific "qemu:allocation-depth" and
> +The third is for convenience in querying the results of
> +base:allocation and qemu:allocation-depth in one go, under the
> +metadata context named
> +
> +    qemu:joint-allocation
> +
> +In this context, bits 0 and 1 reflect the same information as
> +base:allocation, and bits 2 and 3 provide a summary of
> +qemu:allocation-depth (although this context is not able to convey
> +how deep in the backing chain data comes from).
> +
> +    bit 0: NBD_STATE_HOLE, set when the extent is sparse
> +    bit 1: NBD_STATE_ZERO, set when the extent reads as zero
> +    bit 2: NBD_STATE_LOCAL, set when the extent contents come from the
> +           local layer
> +    bit 3: NBD_STATE_BACKING, set when the extent contents come from a
> +           backing layer of unspecified depth
> +
> +This context does not provide any unique information, but does make it
> +easier to reconstruct backing chains without having to piece together
> +what might other be disparate-length returns of two separate metadat
> +contexts.
> +
> +For NBD_OPT_LIST_META_CONTEXT the following queries are supported in
> +addition to the specific "qemu:allocation-depth",
> +"qemu:joint-allocation", and
>  "qemu:dirty-bitmap:<dirty-bitmap-export-name>":

Sending NBD_OPT_LIST_META_CONTEXT with "qemu:joint-allocation"
should tell if the feature is available, right?

If we don't find it we will fallback to "base:allocation".

>  * "qemu:" - returns list of all available metadata contexts in the
> @@ -68,3 +92,4 @@ NBD_CMD_BLOCK_STATUS for "qemu:dirty-bitmap:", NBD_CMD_CACHE
>  * 4.2: NBD_FLAG_CAN_MULTI_CONN for shareable read-only exports,
>  NBD_CMD_FLAG_FAST_ZERO
>  * 5.2: NBD_CMD_BLOCK_STATUS for "qemu:allocation-depth"
> +* 6.1: NBD_CMD_BLOCK_STATUS for "qemu:joint-allocation"
> diff --git a/docs/tools/qemu-nbd.rst b/docs/tools/qemu-nbd.rst
> index ee862fa0bc02..975ef5cedd14 100644
> --- a/docs/tools/qemu-nbd.rst
> +++ b/docs/tools/qemu-nbd.rst
> @@ -75,8 +75,8 @@ driver options if ``--image-opts`` is specified.
>  .. option:: -A, --allocation-depth
>
>    Expose allocation depth information via the
> -  ``qemu:allocation-depth`` metadata context accessible through
> -  NBD_OPT_SET_META_CONTEXT.
> +  ``qemu:allocation-depth`` and ``qemu:joint-allocation`` metadata
> +  contexts accessible through NBD_OPT_SET_META_CONTEXT.
>
>  .. option:: -B, --bitmap=NAME

It would be awesome if this would enable NBD_STATE_DIRTY bit
in the response, in he same way that -A enables NBD_STATE_LOCAL/BACKING

> diff --git a/qapi/block-export.json b/qapi/block-export.json
> index e819e70cac0d..1bd315b7958d 100644
> --- a/qapi/block-export.json
> +++ b/qapi/block-export.json
> @@ -92,8 +92,8 @@
>  #
>  # @allocation-depth: Also export the allocation depth map for @device, so
>  #                    the NBD client can use NBD_OPT_SET_META_CONTEXT with
> -#                    the metadata context name "qemu:allocation-depth" to
> -#                    inspect allocation details. (since 5.2)
> +#                    the metadata context name "qemu:allocation-depth" or
> +#                    "qemu:joint-allocation" to inspect allocation details.
>  #
>  # Since: 5.2
>  ##
> diff --git a/include/block/nbd.h b/include/block/nbd.h
> index 5f34d23bb037..7f411dc59e57 100644
> --- a/include/block/nbd.h
> +++ b/include/block/nbd.h
> @@ -1,5 +1,5 @@
>  /*
> - *  Copyright (C) 2016-2020 Red Hat, Inc.
> + *  Copyright (C) 2016-2021 Red Hat, Inc.
>   *  Copyright (C) 2005  Anthony Liguori <anthony@codemonkey.ws>
>   *
>   *  Network Block Device
> @@ -261,6 +261,14 @@ enum {
>
>  /* No flags needed for qemu:allocation-depth in NBD_REPLY_TYPE_BLOCK_STATUS 
> */
>
> +/*
> + * Extent flags for qemu:joint-allocation in NBD_REPLY_TYPE_BLOCK_STATUS,
> + * reusing the two flags already in base:allocation, and compressing the
> + * results of qemu:allocation-depth into two bits
> + */
> +#define NBD_STATE_LOCAL    (1 << 2)
> +#define NBD_STATE_BACKING  (1 << 3)
> +
>  static inline bool nbd_reply_type_is_error(int type)
>  {
>      return type & (1 << 15);
> diff --git a/nbd/server.c b/nbd/server.c
> index b60ebc3ab6ac..8b825ccaf2ac 100644
> --- a/nbd/server.c
> +++ b/nbd/server.c
> @@ -28,8 +28,9 @@
>
>  #define NBD_META_ID_BASE_ALLOCATION 0
>  #define NBD_META_ID_ALLOCATION_DEPTH 1
> +#define NBD_META_ID_JOINT_ALLOCATION 2
>  /* Dirty bitmaps use 'NBD_META_ID_DIRTY_BITMAP + i', so keep this id last. */
> -#define NBD_META_ID_DIRTY_BITMAP 2
> +#define NBD_META_ID_DIRTY_BITMAP 3
>
>  /*
>   * NBD_MAX_BLOCK_STATUS_EXTENTS: 1 MiB of extents data. An empirical
> @@ -97,6 +98,7 @@ struct NBDExport {
>      Notifier eject_notifier;
>
>      bool allocation_depth;
> +    bool joint_allocation;
>      BdrvDirtyBitmap **export_bitmaps;
>      size_t nr_export_bitmaps;
>  };
> @@ -111,6 +113,7 @@ typedef struct NBDExportMetaContexts {
>      size_t count; /* number of negotiated contexts */
>      bool base_allocation; /* export base:allocation context (block status) */
>      bool allocation_depth; /* export qemu:allocation-depth */
> +    bool joint_allocation; /* export qemu:joint-allocation */
>      bool *bitmaps; /*
>                      * export qemu:dirty-bitmap:<export bitmap name>,
>                      * sized by exp->nr_export_bitmaps
> @@ -862,9 +865,9 @@ static bool nbd_meta_base_query(NBDClient *client, 
> NBDExportMetaContexts *meta,
>
>  /* nbd_meta_qemu_query
>   *
> - * Handle queries to 'qemu' namespace. For now, only the qemu:dirty-bitmap:
> - * and qemu:allocation-depth contexts are available.  Return true if @query
> - * has been handled.
> + * Handle queries to 'qemu' namespace. For now, only the qemu:dirty-bitmap:,
> + * qemu:joint-allocation, and qemu:allocation-depth contexts are available.
> + * Return true if @query has been handled.
>   */
>  static bool nbd_meta_qemu_query(NBDClient *client, NBDExportMetaContexts 
> *meta,
>                                  const char *query)
> @@ -891,6 +894,12 @@ static bool nbd_meta_qemu_query(NBDClient *client, 
> NBDExportMetaContexts *meta,
>          return true;
>      }
>
> +    if (strcmp(query, "joint-allocation") == 0) {
> +        trace_nbd_negotiate_meta_query_parse("joint-allocation");
> +        meta->joint_allocation = meta->exp->joint_allocation;
> +        return true;
> +    }
> +
>      if (nbd_strshift(&query, "dirty-bitmap:")) {
>          trace_nbd_negotiate_meta_query_parse("dirty-bitmap:");
>          if (!*query) {
> @@ -1023,6 +1032,7 @@ static int nbd_negotiate_meta_queries(NBDClient *client,
>          /* enable all known contexts */
>          meta->base_allocation = true;
>          meta->allocation_depth = meta->exp->allocation_depth;
> +        meta->joint_allocation = meta->exp->joint_allocation;
>          memset(meta->bitmaps, 1, meta->exp->nr_export_bitmaps);
>      } else {
>          for (i = 0; i < nb_queries; ++i) {
> @@ -1053,6 +1063,16 @@ static int nbd_negotiate_meta_queries(NBDClient 
> *client,
>          count++;
>      }
>
> +    if (meta->joint_allocation) {
> +        ret = nbd_negotiate_send_meta_context(client, 
> "qemu:joint-allocation",
> +                                              NBD_META_ID_JOINT_ALLOCATION,
> +                                              errp);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +        count++;
> +    }
> +
>      for (i = 0; i < meta->exp->nr_export_bitmaps; i++) {
>          const char *bm_name;
>          g_autofree char *context = NULL;
> @@ -1743,7 +1763,8 @@ static int nbd_export_create(BlockExport *blk_exp, 
> BlockExportOptions *exp_args,
>          bdrv_dirty_bitmap_set_busy(exp->export_bitmaps[i], true);
>      }
>
> -    exp->allocation_depth = arg->allocation_depth;
> +    /* QMP allocation_depth flag controls export of both metadata contexts. 
> */
> +    exp->allocation_depth = exp->joint_allocation = arg->allocation_depth;
>
>      /*
>       * We need to inhibit request queuing in the block layer to ensure we can
> @@ -2164,6 +2185,47 @@ static int blockalloc_to_extents(BlockDriverState *bs, 
> uint64_t offset,
>      return 0;
>  }
>
> +static int blockjoint_to_extents(BlockDriverState *bs, uint64_t offset,
> +                                 uint64_t bytes, NBDExtentArray *ea)
> +{
> +    while (bytes) {
> +        uint32_t flags = 0;
> +        int64_t num, num1, num2;
> +        int ret1 = bdrv_block_status_above(bs, NULL, offset, bytes, &num1,
> +                                           NULL, NULL);
> +        int ret2 = bdrv_is_allocated_above(bs, NULL, false, offset, bytes,
> +                                           &num2);

Do we really need to make 2 calls?

> +
> +        if (ret1 < 0) {
> +            return ret1;
> +        }
> +        if (ret2 < 0) {
> +            return ret2;
> +        }
> +        num = MIN(num1, num2);
> +
> +        if (!(ret1 & BDRV_BLOCK_DATA)) {
> +            flags |= NBD_STATE_HOLE;
> +        }
> +        if (ret1 & BDRV_BLOCK_ZERO) {
> +            flags |= NBD_STATE_ZERO;
> +        }
> +        if (ret2 == 1) {
> +            flags |= NBD_STATE_LOCAL;
> +        } else if (ret2 > 1) {
> +            flags |= NBD_STATE_BACKING;
> +        }
> +        if (nbd_extent_array_add(ea, num, flags) < 0) {
> +            return 0;
> +        }
> +
> +        offset += num;
> +        bytes -= num;
> +    }
> +
> +    return 0;
> +}

I tried this quick hack on top of your patch:

diff --git a/block/io.c b/block/io.c
index 1a05f320d3..aaf01c9119 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2656,6 +2656,15 @@ int bdrv_block_status_above(BlockDriverState
*bs, BlockDriverState *base,
                                           pnum, map, file, NULL);
 }

+int bdrv_block_status_above_depth(BlockDriverState *bs, BlockDriverState *base,
+                                 int64_t offset, int64_t bytes, int64_t *pnum,
+                                 int64_t *map, BlockDriverState **file,
+                                 int *depth)
+{
+    return bdrv_common_block_status_above(bs, base, false, true, offset, bytes,
+                                          pnum, map, file, depth);
+}
+
 int bdrv_block_status(BlockDriverState *bs, int64_t offset, int64_t bytes,
                       int64_t *pnum, int64_t *map, BlockDriverState **file)
 {
diff --git a/include/block/block.h b/include/block/block.h
index 8e707a83b7..e3bbe26695 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -515,6 +515,10 @@ int bdrv_block_status(BlockDriverState *bs, int64_t offset,
 int bdrv_block_status_above(BlockDriverState *bs, BlockDriverState *base,
                             int64_t offset, int64_t bytes, int64_t *pnum,
                             int64_t *map, BlockDriverState **file);
+int bdrv_block_status_above_depth(BlockDriverState *bs, BlockDriverState *base,
+                                 int64_t offset, int64_t bytes, int64_t *pnum,
+                                 int64_t *map, BlockDriverState **file,
+                                 int *depth);
 int bdrv_is_allocated(BlockDriverState *bs, int64_t offset, int64_t bytes,
                       int64_t *pnum);
 int bdrv_is_allocated_above(BlockDriverState *top, BlockDriverState *base,
diff --git a/nbd/server.c b/nbd/server.c
index 8b825ccaf2..16dbd8ed51 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -2190,31 +2190,31 @@ static int
blockjoint_to_extents(BlockDriverState *bs, uint64_t offset,
 {
     while (bytes) {
         uint32_t flags = 0;
-        int64_t num, num1, num2;
-        int ret1 = bdrv_block_status_above(bs, NULL, offset, bytes, &num1,
-                                           NULL, NULL);
-        int ret2 = bdrv_is_allocated_above(bs, NULL, false, offset, bytes,
-                                           &num2);
-
-        if (ret1 < 0) {
-            return ret1;
-        }
-        if (ret2 < 0) {
-            return ret2;
+        int depth = 0;
+        int64_t num;
+        int ret = bdrv_block_status_above_depth(bs, NULL, offset, bytes, &num,
+                                                NULL, NULL, &depth);
+
+        if (ret < 0) {
+            return ret;
         }
-        num = MIN(num1, num2);

-        if (!(ret1 & BDRV_BLOCK_DATA)) {
+        if (!(ret & BDRV_BLOCK_DATA)) {
             flags |= NBD_STATE_HOLE;
         }
-        if (ret1 & BDRV_BLOCK_ZERO) {
+
+        if (ret & BDRV_BLOCK_ZERO) {
             flags |= NBD_STATE_ZERO;
         }
-        if (ret2 == 1) {
-            flags |= NBD_STATE_LOCAL;
-        } else if (ret2 > 1) {
-            flags |= NBD_STATE_BACKING;
+
+        if (ret & BDRV_BLOCK_ALLOCATED) {
+            if (depth == 1) {
+                flags |= NBD_STATE_LOCAL;
+            } else if (depth > 1) {
+                flags |= NBD_STATE_BACKING;
+            }
         }
+
         if (nbd_extent_array_add(ea, num, flags) < 0) {
             return 0;
         }

And seems to give the same results:

top.qcow2 without backing file:

    $ ./nbdinfo --map="qemu:joint-allocation" nbd://localhost
         0       65536    3  hole,zero,unallocated
     65536       65536    4  allocated,local
    131072       65536    7  hole,zero,local
    196608       65536    3  hole,zero,unallocated

empty qcow2 image:

    $ ./nbdinfo --map="qemu:joint-allocation" nbd://localhost
         0  1073741824    3  hole,zero,unallocated

empty raw image:

    $ ./nbdinfo --map="qemu:joint-allocation" nbd://localhost
         0  1073741824    7  hole,zero,local

> +
>  /*
>   * nbd_co_send_extents
>   *
> @@ -2205,6 +2267,8 @@ static int nbd_co_send_block_status(NBDClient *client, 
> uint64_t handle,
>
>      if (context_id == NBD_META_ID_BASE_ALLOCATION) {
>          ret = blockstatus_to_extents(bs, offset, length, ea);
> +    } else if (context_id == NBD_META_ID_JOINT_ALLOCATION) {
> +        ret = blockjoint_to_extents(bs, offset, length, ea);
>      } else {
>          ret = blockalloc_to_extents(bs, offset, length, ea);
>      }
> @@ -2574,6 +2638,19 @@ static coroutine_fn int nbd_handle_request(NBDClient 
> *client,
>                  }
>              }
>
> +            if (client->export_meta.joint_allocation) {
> +                ret = nbd_co_send_block_status(client, request->handle,
> +                                               blk_bs(exp->common.blk),
> +                                               request->from, request->len,
> +                                               dont_fragment,
> +                                               !--contexts_remaining,
> +                                               NBD_META_ID_JOINT_ALLOCATION,
> +                                               errp);
> +                if (ret < 0) {
> +                    return ret;
> +                }
> +            }
> +
>              for (i = 0; i < client->exp->nr_export_bitmaps; i++) {
>                  if (!client->export_meta.bitmaps[i]) {
>                      continue;
> diff --git a/tests/qemu-iotests/tests/nbd-qemu-allocation.out 
> b/tests/qemu-iotests/tests/nbd-qemu-allocation.out
> index c51022b2a38d..b9e469efc58e 100644
> --- a/tests/qemu-iotests/tests/nbd-qemu-allocation.out
> +++ b/tests/qemu-iotests/tests/nbd-qemu-allocation.out
> @@ -21,9 +21,10 @@ exports available: 1
>    min block: 1
>    opt block: 4096
>    max block: 33554432
> -  available meta contexts: 2
> +  available meta contexts: 3
>     base:allocation
>     qemu:allocation-depth
> +   qemu:joint-allocation
>  [{ "start": 0, "length": 3145728, "depth": 0, "zero": false, "data": true, 
> "offset": OFFSET},
>  { "start": 3145728, "length": 1048576, "depth": 0, "zero": true, "data": 
> false, "offset": OFFSET}]
>  [{ "start": 0, "length": 1048576, "depth": 0, "zero": true, "data": true, 
> "offset": OFFSET},
> --
> 2.31.1
>

Thanks for working on this!

Nir