This patch provides a function to move account information of a page
between mem_cgroups and rewrite force_empty to make use of this.

This moving of page_cgroup is done under
 - lru_lock of source/destination mem_cgroup is held.
 - lock_page_cgroup() is held.

Then, a routine which touches pc->mem_cgroup without lock_page_cgroup()
should confirm pc->mem_cgroup is still valid or not.  Typical code can be
following.

(while page is not under lock_page())
	mem = pc->mem_cgroup;
	mz = page_cgroup_zoneinfo(pc)
	spin_lock_irqsave(&mz->lru_lock);
	if (pc->mem_cgroup == mem)
		...../* some list handling */
	spin_unlock_irqrestore(&mz->lru_lock);

Of course, better way is
	lock_page_cgroup(pc);
	....
	unlock_page_cgroup(pc);

But you should confirm the nest of lock and avoid deadlock.

If you treats page_cgroup from mem_cgroup's LRU under mz->lru_lock,
you don't have to worry about what pc->mem_cgroup points to.
moved pages are added to head of lru, not to tail.

Expected users of this routine is:
  - force_empty (rmdir)
  - moving tasks between cgroup (for moving account information.)
  - hierarchy (maybe useful.)

force_empty(rmdir) uses this move_account and move pages to its parent.
This "move" will not cause OOM (I added "oom" parameter to try_charge().)

If the parent is busy (not enough memory), force_empty calls try_to_free_page()
and reduce usage.

Purpose of this behavior is
  - Fix "forget all" behavior of force_empty and avoid leak of accounting.
  - By "moving first, free if necessary", keep pages on memory as much as
    possible.

Adding a switch to change behavior of force_empty to
  - free first, move if necessary
  - free all, if there is mlocked/busy pages, return -EBUSY.
is under consideration. (I'll add if someone requtests.)

This patch also removes memory.force_empty file, a brutal debug-only interface.

Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Tested-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>