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

[David Hildenbrand]

On 09.08.22 16:43, Matthew Wilcox wrote:
> 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.

IIRC, with "kernelcore" and/or "movablecore", the zero page could
eventually end up in the movable zone (whereby we can have boottime
allocations being placed into the movable zone). IIRC that's why we have
to special-case on the zero-page here at all.

So taking out the zero page first is correct.

-- 
Thanks,

David / dhildenb