Re: [Qemu-devel] [PATCH for-2.10] block: Skip implicit nodes in query-block/blockstats

[Markus Armbruster <armbru@redhat.com>]

Kevin Wolf <kwolf@redhat.com> writes:

> Commits 0db832f and 6cdbceb introduced the automatic insertion of filter
> nodes above the top layer of mirror and commit block jobs. The
> assumption made there was that since libvirt doesn't do node-level
> management of the block layer yet, it shouldn't be affected by added
> nodes.
>
> This is true as far as commands issued by libvirt are concerned. It only
> uses BlockBackend names to address nodes, so any operations it performs
> still operate on the root of the tree as intended.
>
> However, the assumption breaks down when you consider query commands,
> which return data for the wrong node now. These commands also return
> information on some child nodes (bs->file and/or bs->backing), which
> libvirt does make use of, and which refer to the wrong nodes, too.
>
> One of the consequences is that oVirt gets wrong information about the
> image size and stops the VM in response as long as a mirror or commit
> job is running:
>
> https://bugzilla.redhat.com/show_bug.cgi?id=1470634
>
> This patch fixes the problem by hiding the implict nodes created
> automatically by the mirror and commit block jobs in the output of
> query-block and BlockBackend-based query-blockstats as long as the user
> doesn't indicate that they are aware of those nodes by providing a node
> name for them in the QMP command to start the block job.
>
> The node-based commands query-named-block-nodes and query-blockstats
> with query-nodes=true still show all nodes, including implicit ones.

The only other query-FOO in block*.json is query-block-jobs.  Good.

> This ensures that users that are capable of node-level management can
> still access the full information; users that only know BlockBackends
> won't use these commands.

I think I can follow your reasoning, but I could use a concrete example.
A small reproducer with output before and after the patch, and an
explanation what exactly makes the output before the patch problematic.

> Cc: qemu-stable@nongnu.org
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/commit.c             |  3 +++
>  block/mirror.c             |  3 +++
>  block/qapi.c               | 13 ++++++++++++-
>  include/block/block_int.h  |  1 +
>  qapi/block-core.json       |  6 ++++--
>  tests/qemu-iotests/041     | 23 +++++++++++++++++++++++
>  tests/qemu-iotests/041.out |  4 ++--
>  7 files changed, 48 insertions(+), 5 deletions(-)
>
> diff --git a/block/commit.c b/block/commit.c
> index 5cc910f..c7857c3 100644
> --- a/block/commit.c
> +++ b/block/commit.c
> @@ -346,6 +346,9 @@ void commit_start(const char *job_id, BlockDriverState *bs,
>      if (commit_top_bs == NULL) {
>          goto fail;
>      }
> +    if (!filter_node_name) {
> +        commit_top_bs->implicit = true;
> +    }
>      commit_top_bs->total_sectors = top->total_sectors;
>      bdrv_set_aio_context(commit_top_bs, bdrv_get_aio_context(top));
>  
> diff --git a/block/mirror.c b/block/mirror.c
> index 8583b76..c9a6a3c 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -1168,6 +1168,9 @@ static void mirror_start_job(const char *job_id, BlockDriverState *bs,
>      if (mirror_top_bs == NULL) {
>          return;
>      }
> +    if (!filter_node_name) {
> +        mirror_top_bs->implicit = true;
> +    }
>      mirror_top_bs->total_sectors = bs->total_sectors;
>      bdrv_set_aio_context(mirror_top_bs, bdrv_get_aio_context(bs));
>  
> diff --git a/block/qapi.c b/block/qapi.c
> index 95b2e2d..0ed23b8 100644
> --- a/block/qapi.c
> +++ b/block/qapi.c
> @@ -324,6 +324,11 @@ static void bdrv_query_info(BlockBackend *blk, BlockInfo **p_info,
>      BlockDriverState *bs = blk_bs(blk);
>      char *qdev;
>  
> +    /* Skip automatically inserted nodes that the user isn't aware of */
> +    while (bs && bs->drv && bs->implicit) {
> +        bs = backing_bs(bs);
> +    }
> +
>      info->device = g_strdup(blk_name(blk));
>      info->type = g_strdup("unknown");
>      info->locked = blk_dev_is_medium_locked(blk);
> @@ -518,12 +523,18 @@ BlockStatsList *qmp_query_blockstats(bool has_query_nodes,
>          }
>      } else {
>          for (blk = blk_next(NULL); blk; blk = blk_next(blk)) {
> +            BlockDriverState *bs = blk_bs(blk);
>              BlockStatsList *info = g_malloc0(sizeof(*info));
>              AioContext *ctx = blk_get_aio_context(blk);
>              BlockStats *s;
>  
> +            /* Skip automatically inserted nodes that the user isn't aware of */
> +            while (bs && bs->drv && bs->implicit) {
> +                bs = backing_bs(bs);
> +            }
> +
>              aio_context_acquire(ctx);
> -            s = bdrv_query_bds_stats(blk_bs(blk), true);
> +            s = bdrv_query_bds_stats(bs, true);
>              s->has_device = true;
>              s->device = g_strdup(blk_name(blk));
>              bdrv_query_blk_stats(s->stats, blk);

The result types of query-block and query-blockstats are recursive.  How
can I convince myself that we're skipping everywhere we need to?

> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index 5c6b761..d4f4ea7 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -549,6 +549,7 @@ struct BlockDriverState {
>      bool sg;        /* if true, the device is a /dev/sg* */
>      bool probed;    /* if true, format was probed rather than specified */
>      bool force_share; /* if true, always allow all shared permissions */
> +    bool implicit;  /* if true, this filter node was automatically inserted */

Makes sense.  How can I convince myself that your patch marks them all?

>  
>      BlockDriver *drv; /* NULL means no media */
>      void *opaque;
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index ff8e2ba..006e048 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -520,7 +520,8 @@
>  #
>  # Get a list of BlockInfo for all virtual block devices.
>  #
> -# Returns: a list of @BlockInfo describing each virtual block device
> +# Returns: a list of @BlockInfo describing each virtual block device. Filter
> +# nodes that were created implicitly are skipped over.
>  #
>  # Since: 0.14.0
>  #
> @@ -780,7 +781,8 @@
>  #               information, but not "backing".
>  #               If false or omitted, the behavior is as before - query all the
>  #               device backends, recursively including their "parent" and
> -#               "backing". (Since 2.3)
> +#               "backing". Filter nodes that were created implicitly are
> +#               skipped over in this mode. (Since 2.3)
>  #
>  # Returns: A list of @BlockStats for each virtual block devices.
>  #
> diff --git a/tests/qemu-iotests/041 b/tests/qemu-iotests/041
> index 2f54986..b798cca 100755
> --- a/tests/qemu-iotests/041
> +++ b/tests/qemu-iotests/041
> @@ -169,6 +169,29 @@ class TestSingleDrive(iotests.QMPTestCase):
>          self.assertTrue(iotests.compare_images(test_img, target_img),
>                          'target image does not match source after mirroring')
>  
> +    # Tests that the insertion of the mirror_top filter node doesn't make a
> +    # difference to query-block
> +    def test_implicit_node(self):
> +        self.assert_no_active_block_jobs()
> +
> +        result = self.vm.qmp(self.qmp_cmd, device='drive0', sync='full',
> +                             target=self.qmp_target)
> +        self.assert_qmp(result, 'return', {})
> +
> +        result = self.vm.qmp('query-block')
> +        self.assert_qmp(result, 'return[0]/inserted/file', test_img)
> +        self.assert_qmp(result, 'return[0]/inserted/drv', iotests.imgfmt)
> +        self.assert_qmp(result, 'return[0]/inserted/backing_file', backing_img)
> +        self.assert_qmp(result, 'return[0]/inserted/backing_file_depth', 1)
> +
> +        self.cancel_and_wait(force=True)
> +        result = self.vm.qmp('query-block')
> +        self.assert_qmp(result, 'return[0]/inserted/file', test_img)
> +        self.assert_qmp(result, 'return[0]/inserted/drv', iotests.imgfmt)
> +        self.assert_qmp(result, 'return[0]/inserted/backing_file', backing_img)
> +        self.assert_qmp(result, 'return[0]/inserted/backing_file_depth', 1)
> +        self.vm.shutdown()
> +
>      def test_medium_not_found(self):
>          if iotests.qemu_default_machine != 'pc':
>              return
> diff --git a/tests/qemu-iotests/041.out b/tests/qemu-iotests/041.out
> index e30fd3b..c28b392 100644
> --- a/tests/qemu-iotests/041.out
> +++ b/tests/qemu-iotests/041.out
> @@ -1,5 +1,5 @@
> -...............................................................................
> +.....................................................................................
>  ----------------------------------------------------------------------
> -Ran 79 tests
> +Ran 85 tests
>  
>  OK

The patch makes sense to me.  The hard part is judging whether it's
complete.  I could use a bit of help with that.