[PATCH v6 01/10] mm: add zone device coherent type memory support

[Sierra Guiza, Alejandro (Alex)]

On 2/11/2022 10:39 AM, David Hildenbrand wrote:
> On 11.02.22 17:15, David Hildenbrand wrote:
>> On 01.02.22 16:48, Alex Sierra wrote:
>>> Device memory that is cache coherent from device and CPU point of view.
>>> This is used on platforms that have an advanced system bus (like CAPI
>>> or CXL). Any page of a process can be migrated to such memory. However,
>>> no one should be allowed to pin such memory so that it can always be
>>> evicted.
>>>
>>> Signed-off-by: Alex Sierra<alex.sierra at amd.com>
>>> Acked-by: Felix Kuehling<Felix.Kuehling at amd.com>
>>> Reviewed-by: Alistair Popple<apopple at nvidia.com>
>> So, I'm currently messing with PageAnon() pages and CoW semantics ...
>> all these PageAnon() ZONE_DEVICE variants don't necessarily make my life
>> easier but I'm not sure yet if they make my life harder. I hope you can
>> help me understand some of that stuff.
>>
>> 1) What are expected CoW semantics for DEVICE_COHERENT?
>>
>> I assume we'll share them just like other PageAnon() pages during fork()
>> readable, and the first sharer writing to them receives an "ordinary"
>> !ZONE_DEVICE copy.
>>
>> So this would be just like DEVICE_EXCLUSIVE CoW handling I assume, just
>> that we don't have to go through the loop of restoring a device
>> exclusive entry?
>>
>> 2) How are these pages freed to clear/invalidate PageAnon() ?
>>
>> I assume for PageAnon() ZONE_DEVICE pages we'll always for via
>> free_devmap_managed_page(), correct?
>>
>>
>> 3) FOLL_PIN
>>
>> While you write "no one should be allowed to pin such memory", patch #2
>> only blocks FOLL_LONGTERM. So I assume we allow ordinary FOLL_PIN and
>> you might want to be a bit more precise?

Device coherent pages can be FOLL_PIN. However, we want to avoid 
FOLL_LONGTERM with
these pages because that would affect our memory manager's ability to 
evict device memory.

Regards,
Alex Sierra

>>
>>
>> ... I'm pretty sure we cannot FOLL_PIN DEVICE_PRIVATE pages, but can we
>> FILL_PIN DEVICE_EXCLUSIVE pages? I strongly assume so?
>>
>>
>> Thanks for any information.
>>
> (digging a bit more, I realized that device exclusive pages are not
> actually/necessarily ZONE_DEVICE pages -- so I assume DEVICE_COHERENT
> will be the actual first PageAnon() ZONE_DEVICE pages that can be
> present in a page table.)

Yes, that's correct. Device coherent pages are pte present.

Regards,
Alex Sierra

>
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