[RFC 1/4] mm/hugetlb: Skip prep of tail pages when HVO is enabled

[Usama Arif <usama.arif@xxxxxxxxxxxxx>]

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.

Signed-off-by: Usama Arif <usama.arif@xxxxxxxxxxxxx>
---
 mm/hugetlb.c         | 30 +++++++++++++++++++++---------
 mm/hugetlb_vmemmap.c |  2 +-
 mm/hugetlb_vmemmap.h |  1 +
 3 files changed, 23 insertions(+), 10 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 64a3239b6407..24352abbb9e5 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1943,13 +1943,22 @@ static void prep_new_hugetlb_folio(struct hstate *h, struct folio *folio, int ni
 }
 
 static bool __prep_compound_gigantic_folio(struct folio *folio,
-					unsigned int order, bool demote)
+					unsigned int order, bool demote,
+					bool hugetlb_vmemmap_optimizable)
 {
 	int i, j;
 	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
+	 * in prep_new_huge_page. Hence, reduce nr_pages to the pages that will be kept.
+	 */
+	if (hugetlb_vmemmap_optimizable)
+		nr_pages = HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page);
+
 	for (i = 0; i < nr_pages; i++) {
 		p = folio_page(folio, i);
 
@@ -2020,15 +2029,15 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
 }
 
 static bool prep_compound_gigantic_folio(struct folio *folio,
-							unsigned int order)
+							unsigned int order, bool hugetlb_vmemmap_optimizable)
 {
-	return __prep_compound_gigantic_folio(folio, order, false);
+	return __prep_compound_gigantic_folio(folio, order, false, hugetlb_vmemmap_optimizable);
 }
 
 static bool prep_compound_gigantic_folio_for_demote(struct folio *folio,
-							unsigned int order)
+							unsigned int order, bool hugetlb_vmemmap_optimizable)
 {
-	return __prep_compound_gigantic_folio(folio, order, true);
+	return __prep_compound_gigantic_folio(folio, order, true, hugetlb_vmemmap_optimizable);
 }
 
 /*
@@ -2185,7 +2194,8 @@ 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, huge_page_order(h),
+							 vmemmap_should_optimize(h, &folio->page))) {
 			/*
 			 * Rare failure to convert pages to compound page.
 			 * Free pages and try again - ONCE!
@@ -3201,7 +3211,8 @@ 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, huge_page_order(h),
+						vmemmap_should_optimize(h, page))) {
 			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 +3635,9 @@ 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_for_demote(folio,
+							target_hstate->order,
+							vmemmap_should_optimize(target_hstate, subpage));
 		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..3525c514c061 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -57,4 +57,5 @@ static inline bool hugetlb_vmemmap_optimizable(const struct hstate *h)
 {
 	return hugetlb_vmemmap_optimizable_size(h) != 0;
 }
+bool vmemmap_should_optimize(const struct hstate *h, const struct page *head);
 #endif /* _LINUX_HUGETLB_VMEMMAP_H */
-- 
2.25.1