Re: [PATCH v2 08/20] hugetlb: Fully initialize tail struct pages of non-pre-HVOed bootmem folios

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

> On Oct 3, 2026, at 08:21, James Houghton <jthoughton@google.com> wrote:
> 
> alloc_bootmem() marks all but the head struct page of a bootmem gigantic
> folio as noinit, and gather_bootmem_prealloc_node() then only
> initializes the first HUGETLB_VMEMMAP_RESERVE_PAGES struct pages. The
> remaining tail struct pages are initialized only if the folio ends up
> not being HVOed.
> 
> This assumes that a bootmem folio is either pre-HVOed, or will not be
> HVOed at all. However, hugetlb_vmemmap_optimize_bootmem_page() may skip
> pre-HVO because arch_hugetlb_vmemmap_optimization_supported() does not
> yet return true that early in boot (e.g. on arm64, where support depends
> on a system-wide CPU capability), while it does return true by the time
> hugetlb_vmemmap_optimize_bootmem_folios() runs. Such folios are then
> HVOed through the regular remap path with uninitialized tail struct
> pages, which trips the PageTail() WARN in vmemmap_remap_pte().
> 
> Avoid this by initializing all tail struct pages up front in
> gather_bootmem_prealloc_node() for folios that were not pre-HVOed. This
> makes the HVO-failure fallback in prep_and_add_bootmem_folios()
> unnecessary, as pre-HVOed folios are never passed through the regular
> remap path, and all other folios now already have initialized tail
> struct pages. A failed optimization either leaves the original vmemmap
> in place or restores the tail struct pages from the shared tail page.
> Remove the fallback.
> 
> For folios that are not HVOed at all, this does not change the amount
> of initialization work, only where it is done.
> 
> Signed-off-by: James Houghton <jthoughton@google.com>
> ---
> mm/hugetlb.c | 28 +++++++++++++++-------------
> 1 file changed, 15 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4dac7ed1df57..e971e2362412 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3303,17 +3303,6 @@ static void __init prep_and_add_bootmem_folios(struct hstate *h,
> 	hugetlb_vmemmap_optimize_bootmem_folios(h, folio_list);
> 
> 	list_for_each_entry_safe(folio, tmp_f, folio_list, lru) {
> -		if (!folio_test_hugetlb_vmemmap_optimized(folio)) {
> -			/*
> -			 * If HVO fails, initialize all tail struct pages
> -			 * We do not worry about potential long lock hold
> -			 * time as this is early in boot and there should
> -			 * be no contention.
> -			 */
> -			hugetlb_folio_init_tail_vmemmap(folio, h,
> -					HUGETLB_VMEMMAP_RESERVE_PAGES,
> -					pages_per_huge_page(h));
> -		}

Just for your information, I also plan to remove those code, see here [1].
I need to push this patch along quickly. I'm confident it won't take long.

[1] https://lore.kernel.org/all/20260513132044.41690-15-songmuchun@bytedance.com/

Thanks,
Muchun

> 		hugetlb_bootmem_init_migratetype(folio, h);
> 		/* Subdivide locks to achieve better parallel performance */
> 		spin_lock_irqsave(&hugetlb_lock, flags);
> @@ -3337,6 +3326,7 @@ static void __init gather_bootmem_prealloc_node(unsigned long nid)
> 		struct page *page = virt_to_page(m);
> 		struct folio *folio = (void *)page;
> 		const unsigned long pfn = folio_pfn(folio);
> +		bool pre_hvo;
> 
> 		h = m->hstate;
> 		/*
> @@ -3350,11 +3340,23 @@ static void __init gather_bootmem_prealloc_node(unsigned long nid)
> 		VM_BUG_ON(!hstate_is_gigantic(h));
> 		WARN_ON(folio_ref_count(folio) != 1);
> 
> +		pre_hvo = vmemmap_optimizable_order(pfn_to_section_compound_order(pfn));
> +
> +		/*
> +		 * Pre-HVOed folios have their tail struct pages mirrored from
> +		 * the shared tail page, so only the first vmemmap page needs
> +		 * initializing. Otherwise, the tail struct pages (marked noinit
> +		 * in alloc_bootmem()) must all be initialized now: the folio
> +		 * may still be HVOed via the regular remap path (e.g. if the
> +		 * architecture could not determine HVO support at bootmem
> +		 * allocation time), which expects valid tail pages.
> +		 */
> 		hugetlb_folio_init_vmemmap(folio, h,
> -					   HUGETLB_VMEMMAP_RESERVE_PAGES);
> +					   pre_hvo ? HUGETLB_VMEMMAP_RESERVE_PAGES :
> +						     pages_per_huge_page(h));
> 		init_new_hugetlb_folio(folio);
> 
> -		if (vmemmap_optimizable_order(pfn_to_section_compound_order(pfn)))
> +		if (pre_hvo)
> 			folio_set_hugetlb_vmemmap_optimized(folio);
> 		section_set_compound_order_range(pfn, folio_nr_pages(folio), 0);
> 
> -- 
> 2.56.0.rc1.315.gc6ed9934b7-goog
>