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 2/2 - numafy_balance_dirty_pages.patch:

This patch does the following things:
	1) Modify balance_dirty_pages_ratelimited() to take a pointer to a 
struct page.  Use this to determine which zone is being dirtied.
	2) Change balance_dirty_pages() to balance_dirty_pages_zone(), which 
now takes a reference to a struct zone to (potentially) balance, rather 
than balancing the whole system.
	3) Finally, create get_dirty_limits_zone(), which takes a reference to 
a struct zone and returns the dirty limits for that specific zone.  The 
old behavior of get_dirty_limits() is still available by passing a NULL 
pointer to get_dirty_limits_zone() (background_writeout() uses this).

Comments/criticism requested! :)

Cheers!

-Matt