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

["Li Zhe" <lizhe.67@bytedance.com>]

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;
+
 	/*
 	 * 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));
+
+	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,
-- 
2.20.1