Re: [PATCH v3 4/4] mm: Apply vm_uffd_ops API to core mm

David Hildenbrand
2025-09-30T09:23:26Z

On 26.09.25 23:16, Peter Xu wrote:
> Move userfaultfd core to use new vm_uffd_ops API. After this change file
> systems that implement vm_operations_struct can start using new API for
> userfaultfd operations.
>
> When at it, moving vma_can_userfault() into mm/userfaultfd.c instead,
> because it's getting too big. It's only used in slow paths so it shouldn't
> be an issue. Move the pte marker check before wp_async, which might be
> more intuitive because wp_async depends on pte markers. That shouldn't
> cause any functional change though because only one check would take effect
> depending on whether pte marker was selected in config.
>
> This will also remove quite some hard-coded checks for either shmem or
> hugetlbfs. Now all the old checks should still work but with vm_uffd_ops.
>
> Note that anonymous memory will still need to be processed separately
> because it doesn't have vm_ops at all.
>
> Reviewed-by: James Houghton <jthoughton@google.com>
> Acked-by: Mike Rapoport <rppt@kernel.org>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---

[...]

> +++ b/mm/userfaultfd.c
> @@ -20,6 +20,43 @@
> #include "internal.h"
> #include "swap.h"
>
> +bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
> + bool wp_async)
> +{
> + unsigned long supported;
> +
> + if (vma->vm_flags & VM_DROPPABLE)
> + return false;
> +
> + vm_flags &= __VM_UFFD_FLAGS;
> +
> +#ifndef CONFIG_PTE_MARKER_UFFD_WP

While at it, you can turn that into an
!IS_ENABLED(CONFIG_PTE_MARKER_UFFD_WP) to avoid the ifdef.

> + /*
> + * If user requested uffd-wp but not enabled pte markers for
> + * uffd-wp, then any file system (like shmem or hugetlbfs) are not
> + * supported but only anonymous.
> + */
> + if ((vm_flags & VM_UFFD_WP) && !vma_is_anonymous(vma))
> + return false;
> +#endif
> + /*
> + * If wp async enabled, and WP is the only mode enabled, allow any
> + * memory type.
> + */
> + if (wp_async && (vm_flags == VM_UFFD_WP))
> + return true;

> +
> + if (vma_is_anonymous(vma))
> + /* Anonymous has no page cache, MINOR not supported */
> + supported = VM_UFFD_MISSING | VM_UFFD_WP;
> + else if (vma_get_uffd_ops(vma))
> + supported = vma_get_uffd_ops(vma)->uffd_features;
> + else
> + return false;

To avoid the hidde return here, I think you can just do

supported = 0;

Or even cleaner, just do

unsigned long supported = 0
...
if (vma_is_anonymous(vma))
supported = ...
else if (vma_get_uffd_ops(vma))
supported = ...
return ...

> +
> + return !(vm_flags & (~supported));

I think this can just be:

return !(vm_flags & ~supported);

--
Cheers

David / dhildenb