Recursive case in uma?

[Evan Geller]

Hey, I'm writing a small hobby operating system and I'm looking to the UMA
for inspiration. The part I don't understand is the recursive case in
uma_zone_slab(). I'm curious where the memory for vm_map_entries actually
come from if the slabs are all full. It would appear that the null returned
by uma_zone_slab() just carries all the way up and there's a fixed upper
limit of vm_map_entries... or that I don't understand the way the uma works.
Would someone mind clearing this up for me?

Thanks a ton for the help!
Evan