[RFC 2/2] mm: additional checks to page flag set/clear

[Sasha Levin <sasha.levin@oracle.com>]

Check if the flag is already set before setting it, and vice versa
for clearing.

Obviously setting or clearing a flag twice isn't a problem on it's
own, but it implies that there's an issue where some piece of code
assumed an opposite state of the flag.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
---
 include/linux/page-flags.h | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index d1fe1a7..36b0bef 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -130,6 +130,12 @@ enum pageflags {
 
 #ifndef __GENERATING_BOUNDS_H
 
+#ifdef CONFIG_DEBUG_VM_PAGE_FLAGS
+#define VM_ASSERT_FLAG(assert, page) VM_BUG_ON_PAGE(assert, page)
+#else
+#define VM_ASSERT_FLAG(assert, page) do { } while (0)
+#endif
+
 /*
  * Macros to create function definitions for page flags
  */
@@ -139,11 +145,13 @@ static inline int Page##uname(const struct page *page)			\
 
 #define SETPAGEFLAG(uname, lname)					\
 static inline void SetPage##uname(struct page *page)			\
-			{ set_bit(PG_##lname, &page->flags); }
+			{ VM_ASSERT_FLAG(Page##uname(page), page);	\
+			set_bit(PG_##lname, &page->flags); }
 
 #define CLEARPAGEFLAG(uname, lname)					\
 static inline void ClearPage##uname(struct page *page)			\
-			{ clear_bit(PG_##lname, &page->flags); }
+			{ VM_ASSERT_FLAG(!Page##uname(page), page);	\
+			clear_bit(PG_##lname, &page->flags); }
 
 #define __SETPAGEFLAG(uname, lname)					\
 static inline void __SetPage##uname(struct page *page)			\
-- 
1.8.3.2

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