[patch] mm: non-atomic test-clear of PG_mlocked on free

[Johannes Weiner]

By the time PG_mlocked is cleared in the page freeing path, nobody
else is looking at our page->flags anymore.

It is thus safe to make the test-and-clear non-atomic and thereby
removing an unnecessary and expensive operation from a hotpath.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 include/linux/page-flags.h |   12 +++++++++---
 mm/page_alloc.c            |    4 ++--
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index e2e5ce5..10e6011 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -158,6 +158,9 @@ static inline int TestSetPage##uname(struct page *page)			\
 static inline int TestClearPage##uname(struct page *page)		\
 		{ return test_and_clear_bit(PG_##lname, &page->flags); }
 
+#define __TESTCLEARFLAG(uname, lname)					\
+static inline int __TestClearPage##uname(struct page *page)		\
+		{ return __test_and_clear_bit(PG_##lname, &page->flags); }
 
 #define PAGEFLAG(uname, lname) TESTPAGEFLAG(uname, lname)		\
 	SETPAGEFLAG(uname, lname) CLEARPAGEFLAG(uname, lname)
@@ -184,6 +187,9 @@ static inline void __ClearPage##uname(struct page *page) {  }
 #define TESTCLEARFLAG_FALSE(uname)					\
 static inline int TestClearPage##uname(struct page *page) { return 0; }
 
+#define __TESTCLEARFLAG_FALSE(uname)					\
+static inline int __TestClearPage##uname(struct page *page) { return 0; }
+
 struct page;	/* forward declaration */
 
 TESTPAGEFLAG(Locked, locked) TESTSETFLAG(Locked, locked)
@@ -250,11 +256,11 @@ PAGEFLAG(Unevictable, unevictable) __CLEARPAGEFLAG(Unevictable, unevictable)
 #ifdef CONFIG_HAVE_MLOCKED_PAGE_BIT
 #define MLOCK_PAGES 1
 PAGEFLAG(Mlocked, mlocked) __CLEARPAGEFLAG(Mlocked, mlocked)
-	TESTSCFLAG(Mlocked, mlocked)
+	TESTSCFLAG(Mlocked, mlocked) __TESTCLEARFLAG(Mlocked, mlocked)
 #else
 #define MLOCK_PAGES 0
-PAGEFLAG_FALSE(Mlocked)
-	SETPAGEFLAG_NOOP(Mlocked) TESTCLEARFLAG_FALSE(Mlocked)
+PAGEFLAG_FALSE(Mlocked) SETPAGEFLAG_NOOP(Mlocked)
+	TESTCLEARFLAG_FALSE(Mlocked) __TESTCLEARFLAG_FALSE(Mlocked)
 #endif
 
 #ifdef CONFIG_IA64_UNCACHED_ALLOCATOR
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index caa9268..b0c8758 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -557,7 +557,7 @@ static void __free_pages_ok(struct page *page, unsigned int order)
 	unsigned long flags;
 	int i;
 	int bad = 0;
-	int wasMlocked = TestClearPageMlocked(page);
+	int wasMlocked = __TestClearPageMlocked(page);
 
 	kmemcheck_free_shadow(page, order);
 
@@ -1021,7 +1021,7 @@ static void free_hot_cold_page(struct page *page, int cold)
 	struct zone *zone = page_zone(page);
 	struct per_cpu_pages *pcp;
 	unsigned long flags;
-	int wasMlocked = TestClearPageMlocked(page);
+	int wasMlocked = __TestClearPageMlocked(page);
 
 	kmemcheck_free_shadow(page, 0);
 
-- 
1.6.3

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