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SubjectRe: [PATCH] mm: re-allow pinning of zero pfns (again)
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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@redhat.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)
>>
>>
>>

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