I’m not sure why it had never occurred to me before this morning but it’s trivial to umount just NFS mounted filesystems by combing umount‘s -t and -a flags. This is particularly useful for fixing hung NFS mounts.

Attempt to nicely unmount all NFS mounts:

umount -t nfs -a

Unmount all NFS mounts, even if they have open file descriptors:

umount -t nfs -l -a

Mount all NFS mounts declared in /etc/fstab (or your platforms equivalent):

umount -t nfs -l -a