[PATCH 1/3] mm, debug_pagelloc: use static keys to enable debugging

[Vlastimil Babka <vbabka@xxxxxxx>]

CONFIG_DEBUG_PAGEALLOC has been redesigned by 031bc5743f15
("mm/debug-pagealloc: make debug-pagealloc boottime configurable") to allow
being always enabled in a distro kernel, but only perform its expensive
functionality when booted with debug_pagelloc=on. We can further reduce
the overhead when not boot-enabled (including page allocator fast paths) using
static keys. This patch introduces one for debug_pagealloc core functionality,
and another for the optional guard page functionality (enabled by booting with
debug_guardpage_minorder=X).

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
---
 include/linux/mm.h | 15 +++++++++++----
 mm/page_alloc.c    | 23 +++++++++++++++++------
 2 files changed, 28 insertions(+), 10 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0e8834ac32b7..c71ed22769f3 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2685,11 +2685,18 @@ static inline void kernel_poison_pages(struct page *page, int numpages,
 					int enable) { }
 #endif
 
-extern bool _debug_pagealloc_enabled;
+#ifdef CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT
+DECLARE_STATIC_KEY_TRUE(_debug_pagealloc_enabled);
+#else
+DECLARE_STATIC_KEY_FALSE(_debug_pagealloc_enabled);
+#endif
 
 static inline bool debug_pagealloc_enabled(void)
 {
-	return IS_ENABLED(CONFIG_DEBUG_PAGEALLOC) && _debug_pagealloc_enabled;
+	if (!IS_ENABLED(CONFIG_DEBUG_PAGEALLOC))
+		return false;
+
+	return static_branch_unlikely(&_debug_pagealloc_enabled);
 }
 
 #if defined(CONFIG_DEBUG_PAGEALLOC) || defined(CONFIG_ARCH_HAS_SET_DIRECT_MAP)
@@ -2843,7 +2850,7 @@ extern struct page_ext_operations debug_guardpage_ops;
 
 #ifdef CONFIG_DEBUG_PAGEALLOC
 extern unsigned int _debug_guardpage_minorder;
-extern bool _debug_guardpage_enabled;
+DECLARE_STATIC_KEY_FALSE(_debug_guardpage_enabled);
 
 static inline unsigned int debug_guardpage_minorder(void)
 {
@@ -2852,7 +2859,7 @@ static inline unsigned int debug_guardpage_minorder(void)
 
 static inline bool debug_guardpage_enabled(void)
 {
-	return _debug_guardpage_enabled;
+	return static_branch_unlikely(&_debug_guardpage_enabled);
 }
 
 static inline bool page_is_guard(struct page *page)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d66bc8abe0af..639f1f9e74c5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -646,16 +646,27 @@ void prep_compound_page(struct page *page, unsigned int order)
 
 #ifdef CONFIG_DEBUG_PAGEALLOC
 unsigned int _debug_guardpage_minorder;
-bool _debug_pagealloc_enabled __read_mostly
-			= IS_ENABLED(CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT);
+
+#ifdef CONFIG_DEBUG_PAGEALLOC_ENABLE_DEFAULT
+DEFINE_STATIC_KEY_TRUE(_debug_pagealloc_enabled);
+#else
+DEFINE_STATIC_KEY_FALSE(_debug_pagealloc_enabled);
+#endif
 EXPORT_SYMBOL(_debug_pagealloc_enabled);
-bool _debug_guardpage_enabled __read_mostly;
+
+DEFINE_STATIC_KEY_FALSE(_debug_guardpage_enabled);
 
 static int __init early_debug_pagealloc(char *buf)
 {
-	if (!buf)
+	bool enable = false;
+
+	if (kstrtobool(buf, &enable))
 		return -EINVAL;
-	return kstrtobool(buf, &_debug_pagealloc_enabled);
+
+	if (enable)
+		static_branch_enable(&_debug_pagealloc_enabled);
+
+	return 0;
 }
 early_param("debug_pagealloc", early_debug_pagealloc);
 
@@ -679,7 +690,7 @@ static void init_debug_guardpage(void)
 	if (!debug_guardpage_minorder())
 		return;
 
-	_debug_guardpage_enabled = true;
+	static_branch_enable(&_debug_guardpage_enabled);
 }
 
 struct page_ext_operations debug_guardpage_ops = {
-- 
2.21.0