Currently the VM decides to start doing background writeback of pages if 
10% of the systems pages are dirty, and starts doing synchronous 
writeback of pages if 40% are dirty.  This is great for smaller memory 
systems, but in larger memory systems (>2GB or so), a process can dirty 
ALL of lowmem (ZONE_NORMAL, 896MB) without hitting the 40% dirty page 
ratio needed to force the process to do writeback.  For this and other 
reasons, it'd be nice to have a zone aware dirty page balancer.  That is 
precisely what these 2 patches try to achieve.

Patch 1/2 - setup_perzone_counters.patch:

This patch does the following things:
	1) balance_dirty_pages uses the first 6 statistics in struct page_state 
(nr_dirty, nr_writeback, nr_unstable, nr_page_table_pages, nr_mapped, 
and nr_slab) to determine when to start writeback.  These stats must 
therefore be maintained on a per-zone basis.  Create a new struct 
perzone_page_state which contains these statistics and embed an array of 
length NR_CPUS of these structures in struct zone.
	2) Add a set of macros mirroring the (mod|inc|dec)_page_state macros 
called (mod|inc|dec)_perzone_page_state to update the per-zone page 
state stats.
	3) Replace all occurences(*) of (mod|inc|dec)_page_state calls that 
modify the per-zone stats with the per-zone versions of the macros.
	4) Modify the get_page_state and get_full_page_state calls to correctly 
gather all the page stats.
	5) Add a get_page_state_zone call to get the page stats for a specific 
zone.

(*) The (mod|inc|dec)_page_state calls in the NFS code have given me 
some trouble.  To determine which zone's statistics to modify, we must 
have a struct page handy, but the NFS code does it's statistics updates 
in bulk, without reference in most cases to the pages.  Thus, the NFS 
code still needs updating, and is noted in the patch.

Comments/criticism requested! :)

Cheers!

-Matt