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.
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 <[email protected]>
---
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)
On 09.08.22 06:42, 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.
>
> 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 <[email protected]>
> ---
> 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)
:/ I guess the code was moved just at the time the old code was still in
place, and when rebasing, the diff in the code was ignored.
Reviewed-by: David Hildenbrand <[email protected]>
I have patches in the works that will properly break COW here to get
anon pages instead of pinning the shared zeropage, which is questionable
in COW mappings.
--
Thanks,
David / dhildenb
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).
> 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 <[email protected]>
> ---
> 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)
>
>
>
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 <[email protected]>
>> ---
>> 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)
>>
>>
>>
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
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.
On Tue, 9 Aug 2022 10:14:12 -0400 Felix Kuehling <[email protected]> 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.