In some email I received from Simon Burge, sie wrote:
[...]
> I've just now got a basic empty filesystem framework working (ie, no vops
> supported except for mount, umount and statfs, and mount doesn't do
> anything like creating a filesystem).  At the moment I can't unmount the
> filesystem:
> 
> 	wincen:vfs/miscfs/swapfs 1# modload obj.i386/swapfs.o 
> 	Module loaded as ID 0
> 	wincen:vfs/miscfs/swapfs 2# mount_swapfs -s 131072 swapfs /mnt
> 	wincen:vfs/miscfs/swapfs 3# df -i
> 	Filesystem  1K-blocks  Used  Avail Capacity iused   ifree  %iused  Mounted on
> 	...
> 	swapfs          65404     0  65404     0%       0  523232     0%   /mnt
[...]

What does it look like if you do:
mkdir /mnt2
mount_swapfs -s 131072 swapfs /mnt
mount_swapfs -s 131072 swapfs /mnt2
df -it swapfs

Darren