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

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Message-ID: <cd25ebd9-52f9-2e66-841e-fc94b3949f7d@amd.com>
Date:   Tue, 9 Aug 2022 10:14:12 -0400
From:   Felix Kuehling <felix.kuehling@....com>
To:     Matthew Wilcox <willy@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     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)

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);

Regards,
   Felix


>
>> Link: https://lore.kernel.org/all/165490039431.944052.12458624139225785964.stgit@omen
>> Fixes: f25cbb7a95a2 ("mm: add zone device coherent type memory support")
>> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
>> ---
>>   include/linux/mm.h |    5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 18e01474cf6b..772279ed7010 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1544,9 +1544,8 @@ static inline bool is_longterm_pinnable_page(struct page *page)
>>   	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
>>   		return false;
>>   #endif
>> -	return !(is_device_coherent_page(page) ||
>> -		 is_zone_movable_page(page) ||
>> -		 is_zero_pfn(page_to_pfn(page)));
>> +	return !(is_device_coherent_page(page) || is_zone_movable_page(page)) ||
>> +	       is_zero_pfn(page_to_pfn(page));
>>   }
>>   #else
>>   static inline bool is_longterm_pinnable_page(struct page *page)
>>
>>
>>