When umount says target is busy, but no process can be blamed

A short one: What to do if unmount is impossible with a

# umount /path/to/mount
umount: /path/to/mount: target is busy

but grepping the output of lsof for the said path yields nothing. In other words, the mount is busy, but no process can be blamed for accessing it (even as a home directory).

If this happens, odds are that it’s an NFS mount, held by some remote machine. The access might have been over long ago, but the mount is still considered busy. So the solution for this case is simple: Restart the NFS daemon. On Linux Mint 19 (and probably a lot of others) it’s simply

# systemctl restart nfs-server

and after this, umount is sucessful (hopefully…)