Re: [PATCH v2 43/47] mm/huge_memory: split huge PUDs when adjusting VMA boundaries

[Kiryl Shutsemau]

On Thu, Oct 29, 2026 at 09:33:38PM +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
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Reviewed-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov