towards I/O aware memory cgroup v4

This is v4.

Major changes from v3 is 
 - dropped spinlock per page_cgroup (then, 4byte free space.)
 - added more comments
 - clean up.


This set has 2 purposes.
 1. re-desgin struct page_cgroup and makes room for blocckio-cgroup ID.
 2. implement quick updating method for memcg's file stat.

 1. check influence of Mel's new writeback method.
    I think we'll see OOM easier. IIUC, memory cgroup needs a thread like kswapd
    to do background writeback or low-high watermark.
    (By this, we can control priority of background writeout thread priority
     by CFS. This is very good.)

 2. implementing dirty_ratio.
    Now, Greg Thelen is working on. One of biggest problems of previous trial was
    update cost of status. I think this patch set can reduce it.

About reducing size of struct page_cgroup:
We have several choices. So, plz wait.
I don't think packing memcgid, blockio-id to pc->flags is a good choice.
(I'm afraid of races and new limitations added by that.)

One idea:  free spaces in pc->flags can be used...but it's better to store some
           very stable/staic value.
	   One idea is store pfn or section-ID or node-id there. Then, we can remove
	   pc->page pointer. Considering page_cgroup's usage, page->page_cgroup is
	   a usual operation but page_cgroup->page is not. (only used at memory recalim)
	   Then, we can implement a function page_cgroup_to_page() and remove
	   page_cgroup->page pointer. For this, free space in pc->flags can be used.

After MM-Summit, I have another event until Aug16, and will be busy for a while.
(And cannot read e-mail box on my office) If you want to contact me, please
 e-mail kahi at mte biglobe ne jp.

Thanks,
-Kame




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