Re: [Experimental][PATCH] putback_lru_page rework

[KAMEZAWA Hiroyuki]

On Thu, 19 Jun 2008 17:00:59 +0900
Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:

> > > > -		unlock = putback_lru_page(newpage);
> > > > +		putback_lru_page(newpage);
> > > >  	} else
> > > >  		newpage->mapping = NULL;
> > > 
> > > originally move_to_lru() called in unmap_and_move().
> > > unevictable infrastructure patch move to this point for 
> > > calling putback_lru_page() under page locked.
> > > 
> > > So, your patch remove page locked dependency.
> > > move to unmap_and_move() again is better.
> > > 
> > > it become page lock holding time reducing.
> > > 
> > ok, will look into again.
> > 
> 
> I agree with Kosaki-san.
> 
> And VM_BUG_ON(page_count(newpage) != 1) in unmap_and_move()
> is not correct again, IMHO.
> I got this BUG actually when testing this patch(with
> migratin_entry_wait fix).
> 
> unmap_and_move()
> 	move_to_new_page()
> 		migrate_page()
> 		remove_migration_ptes()
> 		putback_lru_page()			(*1)
> 	  :
>         if (!newpage->mapping)				(*2)
> 		VM_BUG_ON(page_count(newpage) != 1)
> 
> If a anonymous page(without mapping) is migrated successfully,
> this page is moved back to lru by putback_lru_page()(*1),
> and the page count becomes 1(pte only).
> 
yes.

> At the same time(between *1 and *2), if the process
> that owns this page are freeing this page, the page count
> becomes 0 and ->mapping becomes NULL by free_hot_cold_page(),
> so this BUG is caused.
> 
Agree, I see.

> I've not seen this BUG on real HW yet(seen twice on fake-numa
> hvm guest of Xen), but I think it can happen theoretically.
> 
That's (maybe) because page->mapping is not cleared when it's removed
from rmap. (and there is pagevec to dealy freeing....)

But ok, I see your point. KOSAKI-san is now writing patch set to
fix the whole. please see it.

Thanks,
-Kame

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