Re: [PATCH v1 0/3] udmabuf: Add support for page migration out of movable zone or CMA

[David Hildenbrand <david@redhat.com>]

On 24.08.23 20:30, Jason Gunthorpe wrote:
> On Thu, Aug 24, 2023 at 08:30:17PM +0200, David Hildenbrand wrote:
>> On 24.08.23 08:31, Kasireddy, Vivek wrote:
>>> Hi David,
>>>
>>>>
>>>>>> - Add a new API to the backing store/allocator to longterm-pin the page.
>>>>>>      For example, something along the lines of
>>>> shmem_pin_mapping_page_longterm()
>>>>>>      for shmem as suggested by Daniel. A similar one needs to be added for
>>>>>>      hugetlbfs as well.
>>>>>
>>>>> This may also be reasonable.
>>>>
>>>> Sounds reasonable to keep the old API (that we unfortunately have) working.
>>> I agree; I'd like to avoid adding new APIs unless absolutely necessary. Given this,
>>> and considering the options I have mentioned earlier, what would be your
>>> recommendation for how page migration needs to be done in udmabuf driver?
>>
>> I guess using proper APIs for shmem and hugetlb. So, turning roughly what
>> you have in patch#1 for now into common code, and only calling into that
>> from udmabug.
> 
> This is a lot of work for an obscure uapi :\

Well, what can we otherwise to to *not* break existing users? I'm not 
happy about this either.

Of course, we can come up with a new uapi, but we have to handle the old 
uapi somehow.

Sure, we can simply always fail when we detect ZONE_MOVABLE or 
MIGRATE_CMA. Maybe that keeps at least some use cases working.

-- 
Cheers,

David / dhildenb