Re: [PATCH v3 43/47] mm/huge_memory: split huge PUDs when adjusting VMA boundaries

[Lance Yang]

On Sat, Oct 31, 2026 at 10:40:12AM +0100, David Hildenbrand (Arm) wrote:
>PFN mappings such as VFIO PCI BAR mappings can use PUD leaves while
>allowing VMA splits at page granularity. However, vma_adjust_trans_huge()
>only handles PMDs.
>
>After faulting a writable PUD-sized VFIO BAR mapping, changing its first
>page to read-only with mprotect() makes the PMD walk treat the PUD leaf
>as a PMD table. Applying MADV_DONTFORK to the first PMD-sized chunk leaves
>the PUD spanning two VMAs: the PMD-aligned boundary skips splitting.
>
>Walk the page tables once at each adjusted boundary and split a huge
>PUD or PMD leaf as needed. Handle PUD leaves even when the boundary is
>PMD-aligned.
>
>Fixes: f9e54c3a2f5b7 ("vfio/pci: implement huge_fault support")
>Cc: stable@xxxxxxxxxxxxxxx
>Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
>Reviewed-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
>Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>---

Nice work!

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