Re: [PATCHv7 09/18] mm/hugetlb: Defer vmemmap population for bootmem hugepages

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/17/26 12:28, Kiryl Shutsemau wrote:
> On Mon, Mar 16, 2026 at 05:48:24PM +0100, David Hildenbrand (Arm) wrote:
>> On 2/27/26 20:42, Kiryl Shutsemau (Meta) wrote:
>>> Currently, the vmemmap for bootmem-allocated gigantic pages is populated
>>> early in hugetlb_vmemmap_init_early(). However, the zone information is
>>> only available after zones are initialized. If it is later discovered
>>> that a page spans multiple zones, the HVO mapping must be undone and
>>> replaced with a normal mapping using vmemmap_undo_hvo().
>>>
>>> Defer the actual vmemmap population to hugetlb_vmemmap_init_late(). At
>>> this stage, zones are already initialized, so it can be checked if the
>>> page is valid for HVO before deciding how to populate the vmemmap.
>>>
>>> This allows us to remove vmemmap_undo_hvo() and the complex logic
>>> required to rollback HVO mappings.
>>>
>>> In hugetlb_vmemmap_init_late(), if HVO population fails or if the zones
>>> are invalid, fall back to a normal vmemmap population.
>>>
>>> Postponing population until hugetlb_vmemmap_init_late() also makes zone
>>> information available from within vmemmap_populate_hvo().
>>
>> So we'll keep marking the sections as SECTION_IS_VMEMMAP_PREINIT such
>> that sparse_init_nid() will still properly skip it and leave population
>> to hugetlb_vmemmap_init_late().
>>
>> Should we clear SECTION_IS_VMEMMAP_PREINIT in case we run into the
>> hugetlb_bootmem_page_zones_valid() scenario?
>>
>> I suspect we don't care about SECTION_IS_VMEMMAP_PREINIT after boot and
>> can just leave the flag set. (maybe we wan to add a comment in the code?
>> above the vmemmap_populate() ?)
> 
> I think keeping the flag is right thing to do.
> 
> SECTION_IS_VMEMMAP_PREINIT indicates to core-sparse that the section
> should not be populated and it will be initialized elsewhere. Even in
> !hugetlb_bootmem_page_zones_valid() we take care of it in
> hugetlb_vmemmap_init_late().

Makes sense.

-- 
Cheers,

David