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

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Date:   Tue, 9 Aug 2022 16:51:22 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Felix Kuehling <felix.kuehling@....com>
Cc:     Alex Williamson <alex.williamson@...hat.com>, alex.sierra@....com,
        akpm@...ux-foundation.org, hch@....de, apopple@...dia.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: re-allow pinning of zero pfns (again)

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