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

[David Hildenbrand (Arm)]

On 9/16/26 06:41, Lance Yang wrote:
> 
> 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);
> }

Yes, that's what I had in mind. (while at it, maybe call it "vma_has_*)

-- 
Cheers,

David