memcg: lockless page_cgroup v1

Hi, this is a patch set for lockless page_cgroup.

dropped patches related to mem+swap controller for easy review.
(I'm rewriting it, too.)

Changes from current -mm is.
  - page_cgroup->flags operations is set to be atomic.
  - lock_page_cgroup() is removed.
  - page->page_cgroup is changed from unsigned long to struct page_cgroup*
  - page_cgroup is freed by RCU.
  - For avoiding race, charge/uncharge against mm/memory.c::insert_page() is
    omitted. This is ususally used for mapping device's page. (I think...)

In my quick test, perfomance is improved a little. But the benefit of this
patch is to allow access page_cgroup without lock. I think this is good 
for Yamamoto's Dirty page tracking for memcg.
For I/O tracking people, I added a header file for allowing access to
page_cgroup from out of memcontrol.c

The base kernel is recent mmtom. Any comments are welcome.
This is still under test. I have to do long-run test before removing "RFC".

patch [1-4] is core logic.

[1/7] page_cgroup_atomic_flags.patch
[2/7] delayed_batch_freeing_of_page_cgroup.patch
[3/7] freeing page_cgroup by rcu.patch
[4/7] lockess page_cgroup.patch
[5/7] add prefetch patch
[6/7] make-mapping-null-before-calling-uncharge.patch
[7/7] adding page_cgroup.h header file.patch


Thanks,
-Kame