btrfs qemu-ga - multiple mounts block fsfreeze

Having two mounts of the same device makes fsfreeze through qemu-ga impossible.
root@CmsrvMTA:/# mount -l | grep /dev/vda2
/dev/vda2 on / type btrfs (rw,relatime,space_cache,subvolid=257,subvol=/@)
/dev/vda2 on /home type btrfs (rw,relatime,space_cache,subvolid=258,subvol=/@home)

Having two mounts is rather common with btrfs, so the feature fsfreeze is unusable on these systems. 


Below more information about how we encountered this issue...

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Message 1:
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I use external snapshots to backup my guests. I use the 'quiesce' option to flush and frees the guest file system with the qemu guest agent.

With the exeption of one guest, this procedure works fine. On the 'unwilling' guest, I get this error message:
"ERROR 2016-05-25 00:51:19 | T25-bakVMSCmsrvVH2 | fout: internal error: unable to execute QEMU agent command 'guest-fsfreeze-freeze': failed to freeze /: Device or resource busy"

I don't think this is not some sort of time-out error, because activation of the fsfreeze and the error message happen immediately after each other:

$ grep qemu-ga syslog.1
May 25 00:51:19 CmsrvMTA qemu-ga: info: guest-fsfreeze called

This is the only entry of the qemu guest agent in syslog.

$ sudo virsh version
Compiled against library: libvirt 1.3.1
Using library: libvirt 1.3.1
Gebruikte API: QEMU 1.3.1
Draaiende hypervisor: QEMU 2.5.0

$ virsh qemu-agent-command CmsrvMTA '{"execute": "guest-info"}'
{"return":{"version":"2.5.0", ... ,{"enabled":true,"name":"guest-fstrim","success-response":true},{"enabled":true,"name":"guest-fsfreeze-thaw","success-response":true},{"enabled":true,"name":"guest-fsfreeze-status","success-response":true},{"enabled":true,"name":"guest-fsfreeze-freeze-list","success-response":true},{"enabled":true,"name":"guest-fsfreeze-freeze","success-response":true}, ... }

For making an external snapshot, I use this command:
$ virsh snapshot-create-as --domain CmsrvMTA sn1 --disk-only --atomic --quiesce --no-metadata --diskspec vda,file=/srv/poolVMS/CmsrvMTA.sn1

Piece of reply 1:
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I have encountered this before. Some operating systems
 may have bind-mounts that let a device appear multiple times in the mount list. Unfortunately the guest agent is not smart enough to consider a device that has been frozen as succesfull and keep going. This causes this specific error.

Piece of reply 2:
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Ok, that seems to be it.

I’ve got the ‘/’ and ‘/home’ on the same device formatted as btrfs on two separate sub volumes.