Re: [PATCH v4 0/4] mm/userfaultfd: modulize memory types

[Nikita Kalyazin <kalyazin@amazon.com>]

On 30/10/2025 17:13, Liam R. Howlett wrote:
> * Peter Xu <peterx@redhat.com> [251021 12:28]:
> 
> ...
> 
>> Can you send some patches and show us the code, help everyone to support
>> guest-memfd minor fault, please?
> 
> Patches are here:
> 
> https://git.infradead.org/?p=users/jedix/linux-maple.git;a=shortlog;h=refs/heads/modularized_mem
> 
> This is actually modularized memory types.  That means there is no
> hugetlb.h or shmem.h included in mm/userfaultfd.c code.
> 
> uffd_flag_t has been removed.  This was turning into a middleware and
> it is not necessary.  Neither is supported_ioctls.
> 
> hugetlb now uses the same functions as every other memory type,
> including anon memory.
> 
> Any memory type can change functionality without adding instructions or
> flags or anything to some other code.
> 
> This code passes uffd-unit-test and uffd-wp-mremap (skipped the swap
> tests).
> 
> guest-memfd can implement whatever it needs to (or use others
> implementations), like shmem_uffd_ops here:
> 
> static const struct vm_uffd_ops shmem_uffd_ops = {
>          .copy                   =       shmem_mfill_atomic_pte_copy,
>          .zeropage               =       shmem_mfill_atomic_pte_zeropage,
>          .cont                   =       shmem_mfill_atomic_pte_continue,
>          .poison                 =       mfill_atomic_pte_poison,
>          .writeprotect           =       uffd_writeprotect,
>          .is_dst_valid           =       shmem_is_dst_valid,
>          .increment              =       mfill_size,
>          .failed_do_unlock       =       uffd_failed_do_unlock,
>          .page_shift             =       uffd_page_shift,
>          .complete_register      =       uffd_complete_register,
> };
> 
> Where guest-memfd needs to write the one function:
> guest_memfd_pte_continue(), from what I understand.
> 
> Obviously some of the shmem_ functions would need to be added to a
> header, or such.
> 
> And most of that can come from shmem_mfill_atomic_pte_continue(), from
> what I understand.  This is about 40 lines of code, but may require
> exposing some shmem functions to keep the code that compact.
> 
> So we don't need to expose getting a folio to a module, or decode any
> special flags or whatever.  We just call the function that needs to be
> called on the vma that is found.
> 
> If anyone has tests I can use for guest-memfd and instructions on
> guest-memfd setup, I'll just write it instead of expanding the
> userfaultfd middleware.

I used to use [1] as a test.  The test function can be called from inside

         if (flags & GUEST_MEMFD_FLAG_MMAP) {
                 test_mmap_supported(fd, page_size, total_size);
                 test_fault_overflow(fd, page_size, total_size);
         } else {

in your tree (tools/testing/selftests/kvm/guest_memfd_test.c).

CONFIG_KVM_GUEST_MEMFD=y should be sufficient to enable guest_memfd support.

[1] https://lore.kernel.org/kvm/20250404154352.23078-7-kalyazin@amazon.com/

> 
> Thanks,
> Liam
>