Re: [patch 1/2] mm: speculative get_page

Dave Kleikamp
2006-08-01T15:55:37Z

On Tue, 2006-08-01 at 23:32 +0400, Oleg Nesterov wrote:
> Nick Piggin wrote:
> >
> > --- linux-2.6.orig/mm/vmscan.c
> > +++ linux-2.6/mm/vmscan.c
> > @@ -380,6 +380,8 @@ int remove_mapping(struct address_space
> > if (!mapping)
> > return 0; /* truncate got there first */
> >
> > + SetPageNoNewRefs(page);
> > + smp_wmb();
> > write_lock_irq(&mapping->tree_lock);
> >
>
> Is it enough?
>
> PG_nonewrefs could be already set by another add_to_page_cache()/remove_mapping(),
> and it will be cleared when we take ->tree_lock.

Isn't the page locked when calling remove_mapping()? It looks like
SetPageNoNewRefs & ClearPageNoNewRefs are called in safe places. Either
the page is locked, or it's newly allocated. I could have missed
something, though.

> For example:
>
> CPU_0 CPU_1 CPU_3
>
> add_to_page_cache:
>
> SetPageNoNewRefs();
> write_lock_irq(->tree_lock);

SetPageLocked(page);

> ...
> write_unlock_irq(->tree_lock);
>
> remove_mapping:
>
> SetPageNoNewRefs();
>
> ClearPageNoNewRefs();
> write_lock_irq(->tree_lock);
>
> check page_count()
>
> page_cache_get_speculative:
>
> increment page_count()
>
> no PG_nonewrefs => return
>
> Oleg.

Shaggy
--
David Kleikamp
IBM Linux Technology Center