Previously the ->protection[] logic was broken.  It was difficult to follow
and basically didn't use the asynch reclaim watermarks (pages_min,
pages_low, pages_high) properly.

Now use ->protection *only* for lower-zone protection.  So the allocator
now explicitly uses the ->pages_low, ->pages_min watermarks and adds
->protection on top of that, instead of trying to use ->protection for
everything.

Pages are allocated down to (->pages_low + ->protection), once this is
reached, kswapd the background reclaim is started; after this, we can
allocate down to (->pages_min + ->protection) without blocking; the memory
below pages_min is reserved for __GFP_HIGH and PF_MEMALLOC allocations. 
kswapd attempts to reclaim memory until ->pages_high is reached.
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>