Some crazy person with PAE thinks that they need more user virtual 
space,  This patch allows PAGE_OFFSET to be in much more arbitrary 
places, with PAE on.  I described the first bit of the patch here:
http://marc.theaimsgroup.com/?l=linux-kernel&m=104197008817507&w=2

This patch also allows us to simulate the conditions that occur when 
we have large amounts of physical RAM (like having pte chains and 
mem_map consuting all of ZONE_NORMAL) with much lower actual amounts 
of RAM.

This part adds the userspace page tables component, so that we can 
_actually_ run userspace programs.  Thanks to Bill Irwin for showing 
me how to do the pgd_alloc() in a relatively sane way.

The second patch I've attached will take you from the normal 3:1 
split, to a 7:1 split.  I've also booted on a machine with PAE on, 4GB 
of physical RAM, and a 15:1 split (PAGE_OFFSET == 0xF0000000).

This is from the 15:1 split:
3615MB HIGHMEM available.
128MB LOWMEM available.

MemTotal:      3790824 kB
MemFree:       3758920 kB
Buffers:          1472 kB
Cached:           7636 kB
SwapCached:          0 kB
Active:           6576 kB
Inactive:         4380 kB
HighTotal:     3702756 kB
HighFree:      3692160 kB
LowTotal:        88068 kB
LowFree:         66760 kB
SwapTotal:           0 kB
SwapFree:            0 kB
Dirty:             116 kB
Writeback:           0 kB
Mapped:           4804 kB
Slab:             5092 kB
Committed_AS:     2776 kB
PageTables:       1868 kB
ReverseMaps:      3091

-- 
Dave Hansen
haveblue@us.ibm.com