linux-kernel - Re: [PATCH v3 1/4] mm: Introduce vm_uffd

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Message-ID: <ad124fb6-a712-4cf5-8a7e-2abacbc2e4be@redhat.com>
Date: Wed, 1 Oct 2025 16:39:50 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Axel Rasmussen <axelrasmussen@...gle.com>, Vlastimil Babka <vbabka@...e.cz>,
 James Houghton <jthoughton@...gle.com>, Nikita Kalyazin
 <kalyazin@...zon.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Ujwal Kundur <ujwal.kundur@...il.com>, Mike Rapoport <rppt@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Andrea Arcangeli <aarcange@...hat.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Michal Hocko
 <mhocko@...e.com>, Muchun Song <muchun.song@...ux.dev>,
 Oscar Salvador <osalvador@...e.de>, Hugh Dickins <hughd@...gle.com>,
 Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v3 1/4] mm: Introduce vm_uffd_ops API

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