Re: [PATCH v3 1/4] mm: Introduce vm_uffd_ops API

David Hildenbrand
2025-10-01T14:40:02Z

On 01.10.25 16:35, Peter Xu wrote:
> On Wed, Oct 01, 2025 at 03:58:14PM +0200, David Hildenbrand wrote:
>>>>>> I briefly wondered whether we could use actual UFFD_FEATURE_* here, but they
>>>>>> are rather unsuited for this case here (e.g., different feature flags for
>>>>>> hugetlb support/shmem support etc).
>>>>>>
>>>>>> But reading "uffd_ioctls" below, can't we derive the suitable vma flags from
>>>>>> the supported ioctls?
>>>>>>
>>>>>> _UFFDIO_COPY | _UFDIO_ZEROPAGE -> VM_UFFD_MISSING
>>>>>> _UFFDIO_WRITEPROTECT -> VM_UFFD_WP
>>>>>> _UFFDIO_CONTINUE -> VM_UFFD_MINOR
>>>>>
>>>>> Yes we can deduce that, but it'll be unclear then when one stares at a
>>>>> bunch of ioctls and cannot easily digest the modes the memory type
>>>>> supports. Here, the modes should be the most straightforward way to
>>>>> describe the capability of a memory type.
>>>>
>>>> I rather dislike the current split approach between vm-flags and ioctls.
>>>>
>>>> I briefly thought about abstracting it for internal purposes further and
>>>> just have some internal backend ("memory type") flags.
>>>>
>>>> UFFD_BACKEND_FEAT_MISSING -> _UFFDIO_COPY and VM_UFFD_MISSING
>>>> UFFD_BACKEND_FEAT_ZEROPAGE -> _UFDIO_ZEROPAGE
>>>> UFFD_BACKEND_FEAT_WP -> _UFFDIO_WRITEPROTECT and VM_UFFD_WP
>>>> UFFD_BACKEND_FEAT_MINOR -> _UFFDIO_CONTINUE and VM_UFFD_MINOR
>>>> UFFD_BACKEND_FEAT_POISON -> _UFFDIO_POISON
>>>
>>> This layer of mapping can be helpful to some, but maybe confusing to
>>> others.. who is familiar with existing userfaultfd definitions.
>>>
>>
>> Just wondering, is this confusing to you, and if so, which part?
>>
>> To me it makes perfect sense and cleans up this API and not have to sets of
>> flags that are somehow interlinked.
>
> It adds the extra layer of mapping that will only be used in vm_uffd_ops
> and the helper that will consume it.

Agreed, while making the API cleaner. I don't easily see what's
confusing about that, though.

I think it can be done with a handful of LOC and avoid having to use VM_
flags in this API.

--
Cheers

David / dhildenb