[PATCH][for -mm] Fix and Enhancements for memory cgroup [3/6] add helper function for page_cgroup

[KAMEZAWA Hiroyuki]

On Tue, 9 Oct 2007 20:26:42 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> wrote:

> > 
> > > +		 */
> > > +		if (clear_page_cgroup(page, pc) == pc) {
> > 
> > OK.. so we've come so far and seen that pc has changed underneath us,
> > what do we do with this pc?
> > 
> Hmm... How about this ?
> ==
>  if (clear_page_cgroup(page, pc) == pc) {
> 	/* do usual work */
>  } else {
> 	BUG();
>  }
> == or BUG_ON(clear_page_cgroup(page, pc) != pc)
> 
> I have no clear idea when this race will occur.
After good sleep, I noticed there is a race with force_reclaim (in patch 6).

force_reclaim doesn't check refcnt before clearing page->pc.

My final view will be
==
   if (clear_page_cgroup(page, pc) == pc) {
	/* do usual work */
   } else {
	/* force reclaim clears page->page_cgroup */
   }
==
Anyway, I'll add a meaningful comment here.

Thanks,
-Kame