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

[Matthew Wilcox <willy@infradead.org>]

On Tue, Aug 09, 2022 at 10:14:12AM -0400, Felix Kuehling wrote:
> 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);

It's definitely true that the zero page is never device-coherent, nor
movable.  Moreover, we want to avoid calling page_to_pfn() if we can.
So it should be the last condition that we check.