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

[David Hildenbrand]

On 20.06.25 21:03, Peter Xu wrote:

Hi Peter,

> Introduce a generic userfaultfd API for vm_operations_struct, so that one
> vma, especially when as a module, can support userfaults without modifying

The sentence is confusing ("vma ... as a module").

Did you mean something like ".. so that a vma that is backed by a 
special-purpose in-memory filesystem like shmem or hugetlb can support 
userfaultfd without modifying the uffd core; this is required when the 
in-memory filesystem is built as a module."

> the core files.  More importantly, when the module can be compiled out of
> the kernel.
>
> So, instead of having core mm referencing modules that may not ever exist,
> we need to have modules opt-in on core mm hooks instead.
>
> After this API applied, if a module wants to support userfaultfd, the
> module should only need to touch its own file and properly define
> vm_uffd_ops, instead of changing anything in core mm.

Talking about modules that much is a bit confusing. I think this is more 
about cleanly supporting in-memory filesystems, without the need to 
special-case each and every one of them; can be viewed a cleanup 
independent of the module requirement from guest_memfd.

> Note that such API will not work for anonymous. Core mm will process
> anonymous memory separately for userfault operations like before.
>
> This patch only introduces the API alone so that we can start to move
> existing users over but without breaking them.
>
> Currently the uffd_copy() API is almost designed to be the simplistic with
> minimum mm changes to move over to the API.

Is there a way to move part of the actual implementation (how this is 
all wired up) from patch #4 into this patch, to then only remove the old 
shmem/hugetlb hooks (that are effectively unused) in patch #4?

--
Cheers,

David / dhildenb