[v2 1/6] mm: hugetlb: Skip prep of tail pages when HVO is enabled

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

When vmemmap is optimizable, it will free all the duplicated tail
pages in hugetlb_vmemmap_optimize while preparing the new hugepage.
Hence, there is no need to prepare them.

For 1G x86 hugepages, it avoids preparing
262144 - 64 = 262080 struct pages per hugepage.

The indirection of using __prep_compound_gigantic_folio is also removed,
as it just creates extra functions to indicate demote which can be done
with the argument.

Signed-off-by: Usama Arif <usama.arif@bytedance.com>
---
 mm/hugetlb.c         | 32 ++++++++++++++------------------
 mm/hugetlb_vmemmap.c |  2 +-
 mm/hugetlb_vmemmap.h | 15 +++++++++++----
 3 files changed, 26 insertions(+), 23 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 64a3239b6407..541c07b6d60f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1942,14 +1942,23 @@ static void prep_new_hugetlb_folio(struct hstate *h, struct folio *folio, int ni
 	spin_unlock_irq(&hugetlb_lock);
 }
 
-static bool __prep_compound_gigantic_folio(struct folio *folio,
-					unsigned int order, bool demote)
+static bool prep_compound_gigantic_folio(struct folio *folio, struct hstate *h, bool demote)
 {
 	int i, j;
+	int order = huge_page_order(h);
 	int nr_pages = 1 << order;
 	struct page *p;
 
 	__folio_clear_reserved(folio);
+
+	/*
+	 * No need to prep pages that will be freed later by hugetlb_vmemmap_optimize.
+	 * Hence, reduce nr_pages to the pages that will be kept.
+	 */
+	if (IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP) &&
+			vmemmap_should_optimize(h, &folio->page))
+		nr_pages = HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page);
+
 	for (i = 0; i < nr_pages; i++) {
 		p = folio_page(folio, i);
 
@@ -2019,18 +2028,6 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
 	return false;
 }
 
-static bool prep_compound_gigantic_folio(struct folio *folio,
-							unsigned int order)
-{
-	return __prep_compound_gigantic_folio(folio, order, false);
-}
-
-static bool prep_compound_gigantic_folio_for_demote(struct folio *folio,
-							unsigned int order)
-{
-	return __prep_compound_gigantic_folio(folio, order, true);
-}
-
 /*
  * PageHuge() only returns true for hugetlbfs pages, but not for normal or
  * transparent huge pages.  See the PageTransHuge() documentation for more
@@ -2185,7 +2182,7 @@ static struct folio *alloc_fresh_hugetlb_folio(struct hstate *h,
 	if (!folio)
 		return NULL;
 	if (hstate_is_gigantic(h)) {
-		if (!prep_compound_gigantic_folio(folio, huge_page_order(h))) {
+		if (!prep_compound_gigantic_folio(folio, h, false)) {
 			/*
 			 * Rare failure to convert pages to compound page.
 			 * Free pages and try again - ONCE!
@@ -3201,7 +3198,7 @@ static void __init gather_bootmem_prealloc(void)
 
 		VM_BUG_ON(!hstate_is_gigantic(h));
 		WARN_ON(folio_ref_count(folio) != 1);
-		if (prep_compound_gigantic_folio(folio, huge_page_order(h))) {
+		if (prep_compound_gigantic_folio(folio, h, false)) {
 			WARN_ON(folio_test_reserved(folio));
 			prep_new_hugetlb_folio(h, folio, folio_nid(folio));
 			free_huge_page(page); /* add to the hugepage allocator */
@@ -3624,8 +3621,7 @@ static int demote_free_hugetlb_folio(struct hstate *h, struct folio *folio)
 		subpage = folio_page(folio, i);
 		inner_folio = page_folio(subpage);
 		if (hstate_is_gigantic(target_hstate))
-			prep_compound_gigantic_folio_for_demote(inner_folio,
-							target_hstate->order);
+			prep_compound_gigantic_folio(inner_folio, target_hstate, true);
 		else
 			prep_compound_page(subpage, target_hstate->order);
 		folio_change_private(inner_folio, NULL);
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index c2007ef5e9b0..b721e87de2b3 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -486,7 +486,7 @@ int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head)
 }
 
 /* Return true iff a HugeTLB whose vmemmap should and can be optimized. */
-static bool vmemmap_should_optimize(const struct hstate *h, const struct page *head)
+bool vmemmap_should_optimize(const struct hstate *h, const struct page *head)
 {
 	if (!READ_ONCE(vmemmap_optimize_enabled))
 		return false;
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 25bd0e002431..3e7978a9af73 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -10,16 +10,17 @@
 #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
 
+#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);
+bool vmemmap_should_optimize(const struct hstate *h, const struct page *head);
+
 static inline unsigned int hugetlb_vmemmap_size(const struct hstate *h)
 {
 	return pages_per_huge_page(h) * sizeof(struct page);
@@ -51,6 +52,12 @@ static inline unsigned int hugetlb_vmemmap_optimizable_size(const struct hstate
 {
 	return 0;
 }
+
+static inline bool vmemmap_should_optimize(const struct hstate *h, const struct page *head)
+{
+	return false;
+}
+
 #endif /* CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP */
 
 static inline bool hugetlb_vmemmap_optimizable(const struct hstate *h)
-- 
2.25.1