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

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Date:   Tue, 9 Aug 2022 19:12:27 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Felix Kuehling <felix.kuehling@....com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        alex.sierra@....com, 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, 9 Aug 2022 10:14:12 -0400 Felix Kuehling <felix.kuehling@....com> 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.

If two people made the same error then surely that's a sign that we
need a comment which explains things to the next visitor.

> > 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);
> 

Yes please, vastly better.

And a nice thing about this layout is that it leaves places where we
can add a nice little comment against each clause of the test, to
explain why we're performing each one.