Re: [PATCH 02/11] mm,migration: Do not try to migrate unmappedanonymous pages

[KAMEZAWA Hiroyuki]

On Thu, 25 Mar 2010 18:59:25 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:

> > > > > Kosaki-san,
> > > > > 
> > > > >  IIUC, the race in memory-hotunplug was fixed by this patch [2/11].
> > > > > 
> > > > >  But, this behavior of unmap_and_move() requires access to _freed_
> > > > >  objects (spinlock). Even if it's safe because of SLAB_DESTROY_BY_RCU,
> > > > >  it't not good habit in general.
> > > > > 
> > > > >  After direct compaction, page-migration will be one of "core" code of
> > > > >  memory management. Then, I agree to patch [1/11] as our direction for
> > > > >  keeping sanity and showing direction to more updates. Maybe adding
> > > > >  refcnt and removing RCU in futuer is good.
> > > > 
> > > > But Christoph seems oppose to remove SLAB_DESTROY_BY_RCU. then refcount
> > > > is meaningless now.
> > > 
> > > Christoph is opposed to removing it because of cache-hotness issues more
> > > so than use-after-free concerns. The refcount is needed with or without
> > > SLAB_DESTROY_BY_RCU.
> > > 
> > 
> > I wonder a code which the easiest to be read will be like following.
> > ==
> > 
> >         if (PageAnon(page)) {
> >                 struct anon_vma anon = page_lock_anon_vma(page);
> > 		/* to take this lock, this page must be mapped. */
> > 		if (!anon_vma)
> > 			goto uncharge;
> > 		increase refcnt
> > 		page_unlock_anon_vma(anon);
> >         }
> > 	....
> > ==
> 
> This seems very good and acceptable to me. This refcnt usage
> obviously reduce rcu-lock holding time.
> 
> I still think no refcount doesn't cause any disaster. but I agree
> this is forward step patch.
> 

BTW, by above change and the change in patch [2/11], 
"A page turnd to be SwapCache and free unmapped but not freed"
page will be never migrated.

Mel, could you change the check as this ??

	if (PageAnon(page)) {
		rcu_read_lock();
		if (!page_mapcount(page)) {
			rcu_read_unlock();
			if (!PageSwapCache(page))
				goto uncharge;
			/* unmapped swap cache can be migrated */
		} else {
			...
		}
	.....
	} else 


Thx,
-Kame


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