Re: [PATCH 2/2] mm/hugetlb: optimize bootmem hugepage tail vmemmap init

[Balbir Singh <balbirs@nvidia.com>]

On Thu, Nov 05, 2026 at 08:06:15PM +0800, Li Zhe wrote:
> Boot-time HugeTLB allocation has to initialize tail struct pages itself
> when HugeTLB vmemmap optimization (HVO) is unavailable or fails.  For
> gigantic pages, this can cover a large number of tail pages.  Most state
> installed by __init_single_page() and prep_compound_tail() is identical
> for every tail page in the same huge page; only PFN-dependent fields
> need refreshing.
> 
> Initialize the first tail page normally, copy it into a local template,
> then initialize the remaining tails from that template with
> memmap_init_page_from_template().  This avoids repeatedly doing the full
> __init_single_page() and prep_compound_tail() sequence for state that is
> identical across tail pages in the same huge page.
> 
> On an arm64 QEMU/KVM guest using 4K pages and 64 boot-time 1G hugepages
> (HVO unavailable on arm64 in v7.4-rc1), the measured fallback tail
> initialization time improved by 36.43%, measured with a temporary
> jiffies-based printk modeled after the existing timing in
> memmap_init_zone_device():
> 
>   base:             318.4 ms
>   with this patch:  202.4 ms
> 
> Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
> ---
>  mm/hugetlb.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index da5d04edf7c1..987f16a5ea27 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3219,19 +3219,32 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
>  	enum zone_type zone = folio_zonenum(folio);
>  	int nid = folio_nid(folio);
>  	struct page *page = folio_page(folio, start_page_number);
> +	struct page template;
>  	unsigned long head_pfn = folio_pfn(folio);
>  	unsigned long pfn, end_pfn = head_pfn + end_page_number;
>  	unsigned int order = huge_page_order(h);
>  
> +	if (start_page_number >= end_page_number)
> +		return;
> +

Can this actually happen?

>  	/*
>  	 * As we marked all tail pages with memblock_reserved_mark_noinit(),
>  	 * we must initialize them ourselves here.
>  	 */
> -	for (pfn = head_pfn + start_page_number; pfn < end_pfn; page++, pfn++) {
> -		__init_single_page(page, pfn, zone, nid);
> -		prep_compound_tail(page, &folio->page, order);
> -		set_page_count(page, 0);
> -	}
> +	pfn = head_pfn + start_page_number;
> +
> +	/*
> +	 * Seed a template from the first real tail page and refresh only
> +	 * PFN-dependent fields before copying it.
> +	 */
> +	__init_single_page(page, pfn, zone, nid);
> +	prep_compound_tail(page, &folio->page, order);
> +	set_page_count(page, 0);
> +	memcpy(&template, page, sizeof(template));

Do we need a separate template and page?

> +
> +	for (pfn++; pfn < end_pfn; pfn++)
> +		memmap_init_page_from_template(pfn_to_page(pfn), pfn, zone,
> +					       &template);
>  }
>  
>  static void __init hugetlb_folio_init_vmemmap(struct folio *folio,

Acked-by: Balbir Singh <balbirs@nvidia.com>