Re: [PATCH 07/12] mm: Remove redundant pXd_devmap calls

David Hildenbrand
2025-06-02T09:33:20Z

> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1398,10 +1398,7 @@ static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
> }
>
> entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
> - if (pfn_t_devmap(pfn))
> - entry = pmd_mkdevmap(entry);
> - else
> - entry = pmd_mkspecial(entry);
> + entry = pmd_mkspecial(entry);
> if (write) {

I just stumbled over this, and I think there is something off here in
the PMD/PUD case.

vmf_insert_folio_pmd() does a folio_get() + folio_add_file_rmap_pmd().

But then, we go ahead and turn this into a special mapping by setting it
pmd_mkdevmap()/pmd_mkspecial().

Consequently, vm_normal_page_pmd() would ignore them, not following the
rules documented for vm_normal_page() and behaving differently than
vmf_insert_page_mkwrite()->insert_page().

folio_add_file_rmap_pmd() should never set these things special/devmap
in the first place :/

What am I missing?

Note that fs/dax.c calls vmf_insert_folio_pmd() for PMDs and
vmf_insert_page_mkwrite() for PTEs.

Consequently, PTEs will never be marked special (corner case, shared
zeropage), but PMDs would always.

Hm?

--
Cheers,

David / dhildenb