[v4 4/4] mm: hugetlb: Skip initialization of gigantic tail struct pages if freed by HVO

[Usama Arif <usama.arif@bytedance.com>]

The new boot flow when it comes to initialization of gigantic pages
is as follows:
- At boot time, for a gigantic page during __alloc_bootmem_hugepage,
the region after the first struct page is marked as noinit.
- This results in only the first struct page to be
initialized in reserve_bootmem_region. As the tail struct pages are
not initialized at this point, there can be a significant saving
in boot time if HVO succeeds later on.
- Later on in the boot, the head page is prepped and the first
HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page) - 1 tail struct pages
are initialized.
- HVO is attempted. If it is not successful, then the rest of the
tail struct pages are initialized. If it is successful, no more
tail struct pages need to be initialized saving significant boot time.

Signed-off-by: Usama Arif <usama.arif@bytedance.com>
---
 mm/hugetlb.c         | 61 +++++++++++++++++++++++++++++++++++++-------
 mm/hugetlb_vmemmap.c |  2 +-
 mm/hugetlb_vmemmap.h |  9 ++++---
 mm/internal.h        |  3 +++
 mm/mm_init.c         |  2 +-
 5 files changed, 62 insertions(+), 15 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index c32ca241df4b..540e0386514e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3169,6 +3169,15 @@ int __alloc_bootmem_huge_page(struct hstate *h, int nid)
 	}
 
 found:
+
+	/*
+	 * Only initialize the head struct page in memmap_init_reserved_pages,
+	 * rest of the struct pages will be initialized by the HugeTLB subsystem itself.
+	 * The head struct page is used to get folio information by the HugeTLB
+	 * subsystem like zone id and node id.
+	 */
+	memblock_reserved_mark_noinit(virt_to_phys((void *)m + PAGE_SIZE),
+		huge_page_size(h) - PAGE_SIZE);
 	/* Put them into a private list first because mem_map is not up yet */
 	INIT_LIST_HEAD(&m->list);
 	list_add(&m->list, &huge_boot_pages);
@@ -3176,6 +3185,40 @@ int __alloc_bootmem_huge_page(struct hstate *h, int nid)
 	return 1;
 }
 
+/* Initialize [start_page:end_page_number] tail struct pages of a hugepage */
+static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
+						    unsigned long start_page_number,
+						    unsigned long end_page_number)
+{
+	enum zone_type zone = zone_idx(folio_zone(folio));
+	int nid = folio_nid(folio);
+	unsigned long head_pfn = folio_pfn(folio);
+	unsigned long pfn, end_pfn = head_pfn + end_page_number;
+
+	for (pfn = head_pfn + start_page_number; pfn < end_pfn; pfn++) {
+		struct page *page = pfn_to_page(pfn);
+
+		__init_single_page(page, pfn, zone, nid);
+		prep_compound_tail((struct page *)folio, pfn - head_pfn);
+		set_page_count(page, 0);
+	}
+}
+
+static void __init hugetlb_folio_init_vmemmap(struct folio *folio, struct hstate *h,
+					       unsigned long nr_pages)
+{
+	int ret;
+
+	/* Prepare folio head */
+	__folio_clear_reserved(folio);
+	__folio_set_head(folio);
+	ret = page_ref_freeze(&folio->page, 1);
+	VM_BUG_ON(!ret);
+	/* Initialize the necessary tail struct pages */
+	hugetlb_folio_init_tail_vmemmap(folio, 1, nr_pages);
+	prep_compound_head((struct page *)folio, huge_page_order(h));
+}
+
 /*
  * Put bootmem huge pages into the standard lists after mem_map is up.
  * Note: This only applies to gigantic (order > MAX_ORDER) pages.
@@ -3186,19 +3229,19 @@ static void __init gather_bootmem_prealloc(void)
 
 	list_for_each_entry(m, &huge_boot_pages, list) {
 		struct page *page = virt_to_page(m);
-		struct folio *folio = page_folio(page);
+		struct folio *folio = (void *)page;
 		struct hstate *h = m->hstate;
 
 		VM_BUG_ON(!hstate_is_gigantic(h));
 		WARN_ON(folio_ref_count(folio) != 1);
-		if (prep_compound_gigantic_folio(folio, huge_page_order(h))) {
-			WARN_ON(folio_test_reserved(folio));
-			prep_new_hugetlb_folio(h, folio, folio_nid(folio));
-			free_huge_folio(folio); /* add to the hugepage allocator */
-		} else {
-			/* VERY unlikely inflated ref count on a tail page */
-			free_gigantic_folio(folio, huge_page_order(h));
-		}
+
+		hugetlb_folio_init_vmemmap(folio, h, HUGETLB_VMEMMAP_RESERVE_PAGES);
+		prep_new_hugetlb_folio(h, folio, folio_nid(folio));
+		/* If HVO fails, initialize all tail struct pages */
+		if (!HPageVmemmapOptimized(&folio->page))
+			hugetlb_folio_init_tail_vmemmap(folio, HUGETLB_VMEMMAP_RESERVE_PAGES,
+							pages_per_huge_page(h));
+		free_huge_folio(folio); /* add to the hugepage allocator */
 
 		/*
 		 * We need to restore the 'stolen' pages to totalram_pages
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 3cdb38d87a95..772a877918d7 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -589,7 +589,7 @@ static int __init hugetlb_vmemmap_init(void)
 	const struct hstate *h;
 
 	/* HUGETLB_VMEMMAP_RESERVE_SIZE should cover all used struct pages */
-	BUILD_BUG_ON(__NR_USED_SUBPAGE * sizeof(struct page) > HUGETLB_VMEMMAP_RESERVE_SIZE);
+	BUILD_BUG_ON(__NR_USED_SUBPAGE > HUGETLB_VMEMMAP_RESERVE_PAGES);
 
 	for_each_hstate(h) {
 		if (hugetlb_vmemmap_optimizable(h)) {
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 25bd0e002431..4573899855d7 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -10,15 +10,16 @@
 #define _LINUX_HUGETLB_VMEMMAP_H
 #include <linux/hugetlb.h>
 
-#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
-int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head);
-void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head);
-
 /*
  * Reserve one vmemmap page, all vmemmap addresses are mapped to it. See
  * Documentation/vm/vmemmap_dedup.rst.
  */
 #define HUGETLB_VMEMMAP_RESERVE_SIZE	PAGE_SIZE
+#define HUGETLB_VMEMMAP_RESERVE_PAGES	(HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page))
+
+#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
+int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head);
+void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head);
 
 static inline unsigned int hugetlb_vmemmap_size(const struct hstate *h)
 {
diff --git a/mm/internal.h b/mm/internal.h
index d1d4bf4e63c0..d74061aa6de7 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1154,4 +1154,7 @@ struct vma_prepare {
 	struct vm_area_struct *remove;
 	struct vm_area_struct *remove2;
 };
+
+void __meminit __init_single_page(struct page *page, unsigned long pfn,
+				unsigned long zone, int nid);
 #endif	/* __MM_INTERNAL_H */
diff --git a/mm/mm_init.c b/mm/mm_init.c
index 50f2f34745af..fed4370b02e1 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -555,7 +555,7 @@ static void __init find_zone_movable_pfns_for_nodes(void)
 	node_states[N_MEMORY] = saved_node_state;
 }
 
-static void __meminit __init_single_page(struct page *page, unsigned long pfn,
+void __meminit __init_single_page(struct page *page, unsigned long pfn,
 				unsigned long zone, int nid)
 {
 	mm_zero_struct_page(page);
-- 
2.25.1