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

[Lance Yang <lance.yang@linux.dev>]

On 2026/9/16 14:11, David Hildenbrand (Arm) wrote:
> 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_*)

Good point, will do. Thanks!