[bityard]

A filesystem in the kernel isn't userspace.

[TheDong]

Isn't "mount -t reiserfs" userspace though? Presumably removing reiserfs from the kernel would also break that use of the 'mount' syscall.

[djur]

The syscall would return ENODEV, just like any other case where a user attempts to mount a filesystem type that isn't compiled for the running kernel.