The 4G/4G approach

Kernel and user space need not coexist
Entirely separate spaces can be used

A 4G/4G patch exists
Expands low memory to 4G
Also expands user space
Shipped by Red Hat

Why not adopt it?
Slightly harder user-space access
Must be explicitly mapped
Translation buffer flushed on each context switch
0-30% performance hit
Complicates the VM

4G/4G might go into 2.6
After the 2.7 fork