[PATCH v3 2/4] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key

[Muchun Song <songmuchun@bytedance.com>]

The page_head_if_fake() is used throughout memory management and the
conditional check requires checking a global variable, although the
overhead of this check may be small, it increases when the memory
cache comes under pressure. Also, the global variable will not be
modified after system boot, so it is very appropriate to use static
key machanism.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 include/linux/hugetlb.h    | 10 ++++++++--
 include/linux/page-flags.h |  6 ++++--
 mm/hugetlb_vmemmap.c       | 12 ++++++------
 mm/memory_hotplug.c        |  2 +-
 4 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 1faebe1cd0ed..4cc647a5dbf8 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -1066,9 +1066,15 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
 #endif	/* CONFIG_HUGETLB_PAGE */
 
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
-extern bool hugetlb_free_vmemmap_enabled;
+static inline bool hugetlb_free_vmemmap_enabled(void)
+{
+	return static_key_enabled(&hugetlb_free_vmemmap_enabled_key);
+}
 #else
-#define hugetlb_free_vmemmap_enabled	false
+static inline bool hugetlb_free_vmemmap_enabled(void)
+{
+	return false;
+}
 #endif
 
 static inline spinlock_t *huge_pte_lock(struct hstate *h,
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index b47a7f51d2c3..54e119e44496 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -185,7 +185,8 @@ enum pageflags {
 #ifndef __GENERATING_BOUNDS_H
 
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
-extern bool hugetlb_free_vmemmap_enabled;
+DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+			 hugetlb_free_vmemmap_enabled_key);
 
 /*
  * If the feature of freeing some vmemmap pages associated with each HugeTLB
@@ -205,7 +206,8 @@ extern bool hugetlb_free_vmemmap_enabled;
  */
 static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
 {
-	if (!hugetlb_free_vmemmap_enabled)
+	if (!static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+				 &hugetlb_free_vmemmap_enabled_key))
 		return page;
 	/*
 	 * Only addresses aligned with PAGE_SIZE of struct page may be fake head
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 527bcaa44a48..47517e878ed5 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -188,9 +188,9 @@
 #define RESERVE_VMEMMAP_NR		1U
 #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 
-bool hugetlb_free_vmemmap_enabled __read_mostly =
-	IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
-EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled);
+DEFINE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+			hugetlb_free_vmemmap_enabled_key);
+EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled_key);
 
 static int __init early_hugetlb_free_vmemmap_param(char *buf)
 {
@@ -204,9 +204,9 @@ static int __init early_hugetlb_free_vmemmap_param(char *buf)
 		return -EINVAL;
 
 	if (!strcmp(buf, "on"))
-		hugetlb_free_vmemmap_enabled = true;
+		static_branch_enable(&hugetlb_free_vmemmap_enabled_key);
 	else if (!strcmp(buf, "off"))
-		hugetlb_free_vmemmap_enabled = false;
+		static_branch_disable(&hugetlb_free_vmemmap_enabled_key);
 	else
 		return -EINVAL;
 
@@ -284,7 +284,7 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 	BUILD_BUG_ON(__NR_USED_SUBPAGE >=
 		     RESERVE_VMEMMAP_SIZE / sizeof(struct page));
 
-	if (!hugetlb_free_vmemmap_enabled)
+	if (!hugetlb_free_vmemmap_enabled())
 		return;
 
 	vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 0488eed3327c..89c1fde02162 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1341,7 +1341,7 @@ bool mhp_supports_memmap_on_memory(unsigned long size)
 	 *       populate a single PMD.
 	 */
 	return memmap_on_memory &&
-	       !hugetlb_free_vmemmap_enabled &&
+	       !hugetlb_free_vmemmap_enabled() &&
 	       IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) &&
 	       size == memory_block_size_bytes() &&
 	       IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
-- 
2.11.0