[PATCH v2 2/5] mm, page_poison: use static key more efficiently

[Vlastimil Babka]

Commit 11c9c7edae06 ("mm/page_poison.c: replace bool variable with static key")
changed page_poisoning_enabled() to a static key check. However, the function
is not inlined, so each check still involves a function call with overhead not
eliminated when page poisoning is disabled.

Analogically to how debug_pagealloc is handled, this patch converts
page_poisoning_enabled() back to boolean check, and introduces
page_poisoning_enabled_static() for fast paths. Both functions are inlined.

The function kernel_poison_pages() is also called unconditionally and does
the static key check inside. Remove it from there and put it to callers. Also
split it to two functions kernel_poison_pages() and kernel_unpoison_pages()
instead of the confusing bool parameter.

Also optimize the check that enables page poisoning instead of debug_pagealloc
for architectures without proper debug_pagealloc support. Move the check to
init_mem_debugging_and_hardening() to enable a single static key instead of
having two static branches in page_poisoning_enabled_static().

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 drivers/virtio/virtio_balloon.c |  2 +-
 include/linux/mm.h              | 23 +++++++++++---
 mm/page_alloc.c                 | 19 ++++++++++--
 mm/page_poison.c                | 53 +++++----------------------------
 4 files changed, 43 insertions(+), 54 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 481611c09dae..e53faed6ba93 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -1116,7 +1116,7 @@ static int virtballoon_validate(struct virtio_device *vdev)
 	 */
 	if (!want_init_on_free() &&
 	    (IS_ENABLED(CONFIG_PAGE_POISONING_NO_SANITY) ||
-	     !page_poisoning_enabled()))
+	     !page_poisoning_enabled_static()))
 		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_POISON);
 	else if (!virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON))
 		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_REPORTING);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index e0bc1c8aa2a6..4d6dd9f44571 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2867,12 +2867,27 @@ extern int apply_to_existing_page_range(struct mm_struct *mm,
 
 extern void init_mem_debugging_and_hardening(void);
 #ifdef CONFIG_PAGE_POISONING
-extern bool page_poisoning_enabled(void);
-extern void kernel_poison_pages(struct page *page, int numpages, int enable);
+extern void kernel_poison_pages(struct page *page, int numpages);
+extern void kernel_unpoison_pages(struct page *page, int numpages);
+extern bool _page_poisoning_enabled_early;
+DECLARE_STATIC_KEY_FALSE(_page_poisoning_enabled);
+static inline bool page_poisoning_enabled(void)
+{
+	return _page_poisoning_enabled_early;
+}
+/*
+ * For use in fast paths after init_mem_debugging() has run, or when a
+ * false negative result is not harmful when called too early.
+ */
+static inline bool page_poisoning_enabled_static(void)
+{
+	return (static_branch_unlikely(&_page_poisoning_enabled));
+}
 #else
 static inline bool page_poisoning_enabled(void) { return false; }
-static inline void kernel_poison_pages(struct page *page, int numpages,
-					int enable) { }
+static inline bool page_poisoning_enabled_static(void) { return false; }
+static inline void kernel_poison_pages(struct page *page, int numpages) { }
+static inline void kernel_unpoison_pages(struct page *page, int numpages) { }
 #endif
 
 DECLARE_STATIC_KEY_FALSE(init_on_alloc);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 44d596c9c764..fd7f9345adc0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -775,6 +775,17 @@ void init_mem_debugging_and_hardening(void)
 			static_branch_enable(&init_on_free);
 	}
 
+#ifdef CONFIG_PAGE_POISONING
+	/*
+	 * Page poisoning is debug page alloc for some arches. If
+	 * either of those options are enabled, enable poisoning.
+	 */
+	if (page_poisoning_enabled() ||
+	     (!IS_ENABLED(CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC) &&
+	      debug_pagealloc_enabled()))
+		static_branch_enable(&_page_poisoning_enabled);
+#endif
+
 #ifdef CONFIG_DEBUG_PAGEALLOC
 	if (!debug_pagealloc_enabled())
 		return;
@@ -1260,7 +1271,8 @@ static __always_inline bool free_pages_prepare(struct page *page,
 	if (want_init_on_free())
 		kernel_init_free_pages(page, 1 << order);
 
-	kernel_poison_pages(page, 1 << order, 0);
+	if (page_poisoning_enabled_static())
+		kernel_poison_pages(page, 1 << order);
 	/*
 	 * arch_free_page() can make the page's contents inaccessible.  s390
 	 * does this.  So nothing which can access the page's contents should
@@ -2206,7 +2218,7 @@ static inline int check_new_page(struct page *page)
 static inline bool free_pages_prezeroed(void)
 {
 	return (IS_ENABLED(CONFIG_PAGE_POISONING_ZERO) &&
-		page_poisoning_enabled()) || want_init_on_free();
+		page_poisoning_enabled_static()) || want_init_on_free();
 }
 
 #ifdef CONFIG_DEBUG_VM
@@ -2269,7 +2281,8 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
 	if (debug_pagealloc_enabled_static())
 		kernel_map_pages(page, 1 << order, 1);
 	kasan_alloc_pages(page, order);
-	kernel_poison_pages(page, 1 << order, 1);
+	if (page_poisoning_enabled_static())
+		kernel_unpoison_pages(page, 1 << order);
 	set_page_owner(page, order, gfp_flags);
 }
 
diff --git a/mm/page_poison.c b/mm/page_poison.c
index ae0482cded87..dd7aeada036f 100644
--- a/mm/page_poison.c
+++ b/mm/page_poison.c
@@ -8,45 +8,17 @@
 #include <linux/ratelimit.h>
 #include <linux/kasan.h>
 
-static DEFINE_STATIC_KEY_FALSE_RO(want_page_poisoning);
+bool _page_poisoning_enabled_early;
+EXPORT_SYMBOL(_page_poisoning_enabled_early);
+DEFINE_STATIC_KEY_FALSE(_page_poisoning_enabled);
+EXPORT_SYMBOL(_page_poisoning_enabled);
 
 static int __init early_page_poison_param(char *buf)
 {
-	int ret;
-	bool tmp;
-
-	ret = strtobool(buf, &tmp);
-	if (ret)
-		return ret;
-
-	if (tmp)
-		static_branch_enable(&want_page_poisoning);
-	else
-		static_branch_disable(&want_page_poisoning);
-
-	return 0;
+	return kstrtobool(buf, &_page_poisoning_enabled_early);
 }
 early_param("page_poison", early_page_poison_param);
 
-/**
- * page_poisoning_enabled - check if page poisoning is enabled
- *
- * Return true if page poisoning is enabled, or false if not.
- */
-bool page_poisoning_enabled(void)
-{
-	/*
-	 * Assumes that debug_pagealloc_enabled is set before
-	 * memblock_free_all.
-	 * Page poisoning is debug page alloc for some arches. If
-	 * either of those options are enabled, enable poisoning.
-	 */
-	return (static_branch_unlikely(&want_page_poisoning) ||
-		(!IS_ENABLED(CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC) &&
-		debug_pagealloc_enabled()));
-}
-EXPORT_SYMBOL_GPL(page_poisoning_enabled);
-
 static void poison_page(struct page *page)
 {
 	void *addr = kmap_atomic(page);
@@ -58,7 +30,7 @@ static void poison_page(struct page *page)
 	kunmap_atomic(addr);
 }
 
-static void poison_pages(struct page *page, int n)
+void kernel_poison_pages(struct page *page, int n)
 {
 	int i;
 
@@ -117,7 +89,7 @@ static void unpoison_page(struct page *page)
 	kunmap_atomic(addr);
 }
 
-static void unpoison_pages(struct page *page, int n)
+void kernel_unpoison_pages(struct page *page, int n)
 {
 	int i;
 
@@ -125,17 +97,6 @@ static void unpoison_pages(struct page *page, int n)
 		unpoison_page(page + i);
 }
 
-void kernel_poison_pages(struct page *page, int numpages, int enable)
-{
-	if (!page_poisoning_enabled())
-		return;
-
-	if (enable)
-		unpoison_pages(page, numpages);
-	else
-		poison_pages(page, numpages);
-}
-
 #ifndef CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC
 void __kernel_map_pages(struct page *page, int numpages, int enable)
 {
-- 
2.29.1