Potential regression in 'qemu-img convert' to LVM

[Stefan Reiter <s.reiter@proxmox.com>]

Hi list,

following command fails since 5.1 (tested on kernel 5.4.60):

# qemu-img convert -p -f raw -O raw /dev/zvol/pool/disk-1 /dev/vg/disk-1
qemu-img: error while writing at byte 2157968896: Device or resource busy

(source is ZFS here, but doesn't matter in practice, it always fails the 
same; offset changes slightly but consistently hovers around 2^31)

strace shows the following:
fallocate(13, FALLOC_FL_KEEP_SIZE|FALLOC_FL_PUNCH_HOLE, 2157968896, 
4608) = -1 EBUSY (Device or resource busy)

Other fallocate calls leading up to this work fine.

This happens since commit edafc70c0c "qemu-img convert: Don't pre-zero 
images", before that all fallocates happened at the start. Reverting the 
commit and calling qemu-img exactly the same way on the same data works 
fine. Simply retrying the syscall on EBUSY (like EINTR) does *not* work, 
once it fails it keeps failing with the same error.

I couldn't find anything related to EBUSY on fallocate, and it only 
happens on LVM targets... Any idea or pointers where to look?

~ Stefan