lockless pagecache 2

This is my second attempt at a lockless pagecache.

Patches are against 2.6.13-rc6, and have had reasonable
stressing (albeit on small SMPs).

Main changes since last seen:
* Code clarity and commenting improvement.

* Fix race where multiple concurrent failed speculative
   reference takers could be confused into thinking a free
   page wasn't free, due to the elevated refcounts.

* Convert radix tree node freeing over to RCU. I completely
   missed this problem in my first attempt. (My first real
   RCU attempt - completely wrong?).

* page_cache_get_speculative previously used only preempt_
   disable to stop the current CPU from entering the page
   allocator. Needs to turn off interrupts too.

   Paul picked this bug up without seeing the code, just a
   vague description of what I was trying to do. All I
   picked up was my jaw from the ground ;)

-- 
SUSE Labs, Novell Inc.

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