Comment 6 for bug 1761646

I am also seeing this behaviour on Ubuntu 16.04, linux-image-4.4.0-127-generic.

Swapping to kernel 4.13.0-43-generic solved it here too.

Steps:

$ qemu-img create -b example.qcow -f qcow2 /run/shm/temp1.qcow2

$ qemu-nbd -c /dev/nbd1 /run/shm/temp1.qcow2

$ mkdir /tmp/mnt1

$ sudo /bin/mount /dev/nbd1p2 /tmp/mnt1

mount: special device /dev/nbd1p2 does not exist

I can do whatever other diag is needed, please let me know how I can help.