Re: [PATCH v2 3/5] mm: introduce VM_FAULT_UFFD_MINOR fault reason

Peter Xu
2025-11-25T19:21:22Z

Hi, Mike,

On Tue, Nov 25, 2025 at 08:38:38PM +0200, Mike Rapoport wrote:
> From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
>
> When a VMA is registered with userfaulfd in minor mode, its ->fault()
> method should check if a folio exists in the page cache and if yes
> ->fault() should call handle_userfault(VM_UFFD_MISSING).

s/MISSING/MINOR/

>
> Instead of calling handle_userfault() directly from a specific ->fault()
> implementation introduce new fault reason VM_FAULT_UFFD_MINOR that will
> notify the core page fault handler that it should call
> handle_userfaultfd(VM_UFFD_MISSING) to complete a page fault.

Same.

>
> Replace a call to handle_userfault(VM_UFFD_MISSING) in shmem and use the

Same.

> new VM_FAULT_UFFD_MINOR there instead.

Personally I'd keep the fault path as simple as possible, because that's
the more frequently used path (rather than when userfaultfd is armed). I
also see it slightly a pity that even with flags introduced, it only solves
the MINOR problem, not MISSING.

If it's me, I'd simply export handle_userfault().. I confess I still don't
know why exporting it is a problem, but maybe I missed something.

Only my two cents. Feel free to go with whatever way you prefer.

Thanks,

--
Peter Xu