
Uh oh - Kernel VM space is limited!

So, we changed the world so that only in-use buffers are mapped into kernel memory. 

-Allow buffer cache to grow dynamically. 
-Change the Page Daemon to steal memory from the cache when needed.
-Map buffers only when in use.
-Now need to potentially wait for kva, and for physmem. 
-Challenging as this adds interesting wait states to the kernel - took quite a while to get right. 

