2006-08-01 15:08:46

by Oleg Nesterov

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

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. For example:

CPU_0 CPU_1 CPU_3

add_to_page_cache:

SetPageNoNewRefs();
write_lock_irq(->tree_lock);
...
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.

2006-08-01 15:55:37

by Dave Kleikamp

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

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

2006-08-01 16:18:43

by Oleg Nesterov

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

On 08/01, Dave Kleikamp wrote:
>
> 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.

No, I think it is I who missed something, thanks.

Oleg.

2006-08-01 23:54:39

by Nick Piggin

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

Oleg Nesterov wrote:
> On 08/01, Dave Kleikamp wrote:

>>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.
>
>
> No, I think it is I who missed something, thanks.

Yeah, SetPageNoNewRefs is indeed called only under PageLocked or for
newly allocated pages. I should make a note about that, as it isn't
immediately clear.

Thanks

--
SUSE Labs, Novell Inc.
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