[PATCH v15 8/8] mm: hugetlb: optimize the code with the help of the compiler

[Muchun Song]

When the "struct page size" crosses page boundaries we cannot
make use of this feature. Let free_vmemmap_pages_per_hpage()
return zero if that is the case, most of the functions can be
optimized away.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
---
 include/linux/hugetlb.h |  3 ++-
 mm/hugetlb_vmemmap.c    | 13 +++++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 822ab2f5542a..7bfb06e16298 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -878,7 +878,8 @@ extern bool hugetlb_free_vmemmap_enabled;
 
 static inline bool is_hugetlb_free_vmemmap_enabled(void)
 {
-	return hugetlb_free_vmemmap_enabled;
+	return hugetlb_free_vmemmap_enabled &&
+	       is_power_of_2(sizeof(struct page));
 }
 #else
 static inline bool is_hugetlb_free_vmemmap_enabled(void)
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index a67301a9d19a..2e7e1d6ee458 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -211,6 +211,12 @@ early_param("hugetlb_free_vmemmap", early_hugetlb_free_vmemmap_param);
  */
 static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
 {
+	/*
+	 * This check aims to let the compiler help us optimize the code as
+	 * much as possible.
+	 */
+	if (!is_power_of_2(sizeof(struct page)))
+		return 0;
 	return h->nr_free_vmemmap_pages;
 }
 
@@ -280,6 +286,13 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 	BUILD_BUG_ON(NR_USED_SUBPAGE >=
 		     RESERVE_VMEMMAP_SIZE / sizeof(struct page));
 
+	/*
+	 * The compiler can help us to optimize this function to null
+	 * when the size of the struct page is not power of 2.
+	 */
+	if (!is_power_of_2(sizeof(struct page)))
+		return;
+
 	if (!hugetlb_free_vmemmap_enabled)
 		return;
 
-- 
2.11.0