Re: [PATCH 10/12] mm: hugetlb_vmemmap: make vmemmap restoration folio-oriented

[Muchun Song <muchun.song@linux.dev>]

On 2026/10/23 11:46, Lance Yang wrote:
> On Tue, Oct 20, 2026 at 07:58:22PM +0800, Muchun Song wrote:
>> Like optimization, vmemmap restoration operates on one folio at a time,
>> but the current interface exposes a raw address range and requires the
>> HugeTLB caller to prepare part of the operation.  This splits the common
>> mechanism across layers and obscures its natural unit of work.
>>
>> Make the folio the unit of vmemmap restoration and move the common
>> mechanics behind a folio-oriented interface.  This keeps HugeTLB-specific
>> eligibility and state in the caller and gives both directions of HVO
>> consistent folio-level interfaces.
>>
>> No functional change intended.
>>
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>> mm/hugetlb_vmemmap.c | 37 +++++++++++++------------------------
>> 1 file changed, 13 insertions(+), 24 deletions(-)
>>
>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>> index b76203e34881..97748a9cefe2 100644
>> --- a/mm/hugetlb_vmemmap.c
>> +++ b/mm/hugetlb_vmemmap.c
>> @@ -360,28 +360,29 @@ static int vmemmap_optimize_folio(const struct folio *folio,
>> }
>>
>> /**
>> - * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
>> - *			 to the page which is from the @vmemmap_pages
>> - *			 respectively.
>> - * @start:	start address of the vmemmap virtual address range that we want
>> - *		to remap.
>> - * @end:	end address of the vmemmap virtual address range that we want to
>> - *		remap.
>> - * @flags:	modifications to vmemmap_remap_walk flags
>> + * vmemmap_restore_folio - restore the vmemmap backing of a folio
>> + * @folio:	folio whose vmemmap pages should be restored.
>> + * @flags:	vmemmap remap control flags passed to vmemmap_remap_walk.
>> + *
>> + * Remap @folio's vmemmap so optimizable entries are restored from the shared
>> + * tail page back to distinct backing pages allocated for @folio, while
>> + * non-optimizable entries remain unchanged.
>>   *
>>   * Return: %0 on success, negative error code otherwise.
>>   */
>> -static int vmemmap_remap_alloc(unsigned long start, unsigned long end,
>> -			       unsigned long flags)
>> +static int vmemmap_restore_folio(const struct folio *folio, unsigned long flags)
>> {
>> 	LIST_HEAD(vmemmap_pages);
>> -	int nid = page_to_nid((struct page *)start);
>> +	int nid = folio_nid(folio);
>> +	unsigned long start = (unsigned long)folio_page(folio, 0);
>> +	unsigned long end = start + folio_vmemmap_size(folio);
>> 	struct vmemmap_remap_walk walk = {
>> 		.remap_pte	= vmemmap_restore_pte,
>> 		.vmemmap_pages	= &vmemmap_pages,
>> 		.flags		= flags,
>> 	};
>>
>> +	start += VMEMMAP_OPTIMIZATION_SIZE;
>> 	if (alloc_vmemmap_page_list(&vmemmap_pages, (end - start) / PAGE_SIZE, nid))
>> 		return -ENOMEM;
>>
>> @@ -399,7 +400,6 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
>> 					   struct folio *folio, unsigned long flags)
>> {
>> 	int ret;
>> -	unsigned long vmemmap_start, vmemmap_end;
>>
>> 	VM_WARN_ON_ONCE_FOLIO(!folio_test_hugetlb(folio), folio);
>> 	VM_WARN_ON_ONCE_FOLIO(folio_ref_count(folio), folio);
>> @@ -407,18 +407,7 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
>> 	if (!folio_test_hugetlb_vmemmap_optimized(folio))
>> 		return 0;
>>
>> -	vmemmap_start	= (unsigned long)&folio->page;
>> -	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
>> -
>> -	vmemmap_start	+= VMEMMAP_OPTIMIZATION_SIZE;
>> -
>> -	/*
>> -	 * The pages which the vmemmap virtual address range [@vmemmap_start,
>> -	 * @vmemmap_end) are mapped to are freed to the buddy allocator.
>> -	 * When a HugeTLB page is freed to the buddy allocator, previously
>> -	 * discarded vmemmap pages must be allocated and remapping.
>> -	 */
>> -	ret = vmemmap_remap_alloc(vmemmap_start, vmemmap_end, flags);
>> +	ret = vmemmap_restore_folio(folio, flags);
> Nit: we don't use h in __hugetlb_vmemmap_restore_folio() anymore. Could
> we drop it from that helper while at it?

You're really thorough. That's actually something on my follow-up series,
so I didn't include it in the current one.

>
> Otherwise, LGTM!
>
> Reviewed-by: Lance Yang <lance.yang@linux.dev>

Thanks.