It's obviously not possible to mmap() an object beyond the 4G limit.
The first thing sys_mmap() does is to truncate the offset to
a vm_offset_t. (all kinds of offsets, into local vm space or into
external objects, are of that type)
It seems not too hard to solve this by replacing the vm_offset_t
by off_t at the appropriate places.
Is it worth trying or are there deeper reasons that there is
no separation between vm offsets and external ones?

(I want to use it to mmap() portions of the (64bit) SCI
address space on a i386.)

best regards
Matthias Drochner