Re: [PATCH 1/1] mm/huge_memory: fix pgtable withdrawal for huge zero PMDs

[Lance Yang]

On Tue, Sep 15, 2026 at 05:13:17PM +0200, David Hildenbrand (Arm) wrote:
[...]
>
>Ok, it's really only DAX and anonymous VMAs that use the huge zero folio. Other
>(module) code would have a hard time using it, as mm_get_huge_zero_folio() is
>not exported to modules.
>
>DAX uses dax_pmd_load_hole()->vmf_insert_folio_pmd()->insert_pmd() where we
>deposit a page table only if arch_needs_pgtable_deposit().
>
>
>So I think the rule is simply:
>
>arch_needs_pgtable_deposit() -> always deposited
>vma_is_anonymous() -> always deposited
>
>?

YES!

>
>The trick is that we don't have anon THPs in non-anon VMAs.
>
>So could this be simplified further or am I missing something?

Ah, cool! I hadn't thought of that :) You're right, anon THPs cannot live
in non-anon VMAs, so pmdval and folio aren't needed here. Will simplify
it as suggested :) and add a comment explaining the rule.

static bool has_deposited_pgtable(struct vm_area_struct *vma)
{
	return arch_needs_pgtable_deposit() || vma_is_anonymous(vma);
}

Cheers, Lance