Re: [PATCH] mm: re-allow pinning of zero pfns (again)

[Felix Kuehling]

Am 2022-08-09 um 08:31 schrieb Matthew Wilcox:
> On Mon, Aug 08, 2022 at 10:42:24PM -0600, Alex Williamson wrote:
> > The below referenced commit makes the same error as 1c563432588d ("mm: fix
> > is_pinnable_page against a cma page"), re-interpreting the logic to exclude
> > pinning of the zero page, which breaks device assignment with vfio.
> Perhaps we need to admit we're not as good at boolean logic as we think
> we are.
>
> 	if (is_device_coherent_page(page))
> 		return false;
> 	if (is_zone_movable_page(page))
> 		return false;
> 	return is_zero_pfn(page_to_pfn(page));
>
> (or whatever the right logic is ... I just woke up and I'm having
> trouble parsing it).

This implies an assumption that zero-page is never device-coherent or 
moveable, which is probably true, but not part of the original 
condition. A more formally correct rewrite would be:

	if (is_zero_pfn(page_to_pfn(page)))
		return true;
	if (is_device_coherent_page(page))
		return false;
	return !is_zone_moveable_page(page);

Regards,
  Felix


> > Link: https://lore.kernel.org/all/165490039431.944052.12458624139225785964.stgit@omen
> > Fixes: f25cbb7a95a2 ("mm: add zone device coherent type memory support")
> > Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > ---
> >   include/linux/mm.h |    5 ++---
> >   1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 18e01474cf6b..772279ed7010 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1544,9 +1544,8 @@ static inline bool is_longterm_pinnable_page(struct page *page)
> >   	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
> >   		return false;
> >   #endif
> > -	return !(is_device_coherent_page(page) ||
> > -		 is_zone_movable_page(page) ||
> > -		 is_zero_pfn(page_to_pfn(page)));
> > +	return !(is_device_coherent_page(page) || is_zone_movable_page(page)) ||
> > +	       is_zero_pfn(page_to_pfn(page));
> >   }
> >   #else
> >   static inline bool is_longterm_pinnable_page(struct page *page)