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

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Date:   Tue, 9 Aug 2022 15:43:41 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Felix Kuehling <felix.kuehling@....com>
Cc:     Alex Williamson <alex.williamson@...hat.com>, alex.sierra@....com,
        akpm@...ux-foundation.org, hch@....de, apopple@...dia.com,
        david@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: re-allow pinning of zero pfns (again)

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.