On 09.09.20 14:33, Eric Blake wrote:
> If you have the chain 'base.qcow2 <- top.qcow2' and want to merge a
> bitmap from top into base, qemu-img was failing with:
> 
> qemu-img: Could not open 'top.qcow2': Could not open backing file: Failed to get shared "write" lock
> Is another process using the image [base.qcow2]?
> 
> The easiest fix is to not open the entire backing chain of the source
> image, so that we aren't worrying about competing BDS visiting the
> backing image as both a read-only backing of the source and the
> writeable destination.
> 
> Fixes: http://bugzilla.redhat.com/1877209
> Reported-by: Eyal Shenitzky <eshenitz@redhat.com>
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>  qemu-img.c                 |  3 +-
>  tests/qemu-iotests/291     | 12 ++++++++
>  tests/qemu-iotests/291.out | 56 ++++++++++++++++++++++++++++++++++++++
>  3 files changed, 70 insertions(+), 1 deletion(-)
> 
> diff --git a/qemu-img.c b/qemu-img.c
> index eb2fc1f86243..b15098a2f9b3 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -4755,7 +4755,8 @@ static int img_bitmap(int argc, char **argv)
>      }
>      bs = blk_bs(blk);
>      if (src_filename) {
> -        src = img_open(false, src_filename, src_fmt, 0, false, false, false);
> +        src = img_open(false, src_filename, src_fmt, BDRV_O_NO_BACKING,
> +                       false, false, false);

Why not do the same for the destination BB?

>          if (!src) {
>              goto out;
>          }
> diff --git a/tests/qemu-iotests/291 b/tests/qemu-iotests/291
> index 1e0bb76959bb..4f837b205655 100755
> --- a/tests/qemu-iotests/291
> +++ b/tests/qemu-iotests/291

[...]

> @@ -107,6 +116,9 @@ $QEMU_IMG map --output=json --image-opts \
>  nbd_server_start_unix_socket -r -f qcow2 -B b2 "$TEST_IMG"
>  $QEMU_IMG map --output=json --image-opts \
>      "$IMG,x-dirty-bitmap=qemu:dirty-bitmap:b2" | _filter_qemu_img_map
> +nbd_server_start_unix_socket -r -f qcow2 -B b3 "$TEST_IMG"

Why not look into $TEST_IMG.base to see specifically whether the bitmap
is there?

(I also am quite surprised that it’s even possible to export bitmaps
from backing nodes, but, well.)

Max