Re: [PATCH 11/46] mm/huge_memory: properly handle refcounted PUD mappings on fork

[Lance Yang]

On Tue, Oct 27, 2026 at 12:17:31AM +0100, David Hildenbrand (Arm) wrote:
>copy_huge_pud() copies device-DAX entries without taking a folio reference
>or updating mapcounts and the child's RSS.
>
>For example, MADV_GUARD_INSTALL sets VM_MAYBE_GUARD on a device-DAX VMA,
>forcing copy_page_range() to traverse the whole VMA on fork. Any remaining
>huge PUDs are then copied without accounting for the new mappings.
>Unmapping them in the child decrements accounting that was never added.
>
>Use vm_normal_page_pud() to identify refcounted mappings. Take a folio
>reference and add the child's PUD rmap and RSS accounting before installing
>the copied entry. Pass src and dst VMAs consistently instead of src and
>dst MM. Move the TODO.
>
>Fixes: aed877c2b4257 ("device/dax: properly refcount device dax pages when mapping")
>Cc: stable@xxxxxxxxxxxxxxx
>Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>---

Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>