mm: do not allocate page->ptl dynamically, if spinlock_t fits to long

In struct page we have enough space to fit long-size page->ptl there,
but we use dynamically-allocated page->ptl if size(spinlock_t) is larger
than sizeof(int).

It hurts 64-bit architectures with CONFIG_GENERIC_LOCKBREAK, where
sizeof(spinlock_t) == 8, but it easily fits into struct page.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/lockref.h

@@ -19,7 +19,7 @@
 
 #define USE_CMPXCHG_LOCKREF \
 	(IS_ENABLED(CONFIG_ARCH_USE_CMPXCHG_LOCKREF) && \
-	 IS_ENABLED(CONFIG_SMP) && !BLOATED_SPINLOCKS)
+	 IS_ENABLED(CONFIG_SMP) && SPINLOCK_SIZE <= 4)
 
 struct lockref {
 	union {

include/linux/mm.h

@@ -1317,7 +1317,7 @@ static inline pmd_t *pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long a
 #endif /* CONFIG_MMU && !__ARCH_HAS_4LEVEL_HACK */
 
 #if USE_SPLIT_PTE_PTLOCKS
-#if BLOATED_SPINLOCKS
+#if ALLOC_SPLIT_PTLOCKS
 extern bool ptlock_alloc(struct page *page);
 extern void ptlock_free(struct page *page);
 
@@ -1325,7 +1325,7 @@ static inline spinlock_t *ptlock_ptr(struct page *page)
 {
 	return page->ptl;
 }
-#else /* BLOATED_SPINLOCKS */
+#else /* ALLOC_SPLIT_PTLOCKS */
 static inline bool ptlock_alloc(struct page *page)
 {
 	return true;
@@ -1339,7 +1339,7 @@ static inline spinlock_t *ptlock_ptr(struct page *page)
 {
 	return &page->ptl;
 }
-#endif /* BLOATED_SPINLOCKS */
+#endif /* ALLOC_SPLIT_PTLOCKS */
 
 static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
 {

include/linux/mm_types.h

@@ -26,6 +26,7 @@ struct address_space;
 #define USE_SPLIT_PTE_PTLOCKS	(NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS)
 #define USE_SPLIT_PMD_PTLOCKS	(USE_SPLIT_PTE_PTLOCKS && \
 		IS_ENABLED(CONFIG_ARCH_ENABLE_SPLIT_PMD_PTLOCK))
+#define ALLOC_SPLIT_PTLOCKS	(SPINLOCK_SIZE > BITS_PER_LONG/8)
 
 /*
  * Each physical page in the system has a struct page associated with
@@ -155,7 +156,7 @@ struct page {
 						 * system if PG_buddy is set.
 						 */
 #if USE_SPLIT_PTE_PTLOCKS
-#if BLOATED_SPINLOCKS
+#if ALLOC_SPLIT_PTLOCKS
 		spinlock_t *ptl;
 #else
 		spinlock_t ptl;

kernel/bounds.c

@@ -22,6 +22,6 @@ void foo(void)
 #ifdef CONFIG_SMP
 	DEFINE(NR_CPUS_BITS, ilog2(CONFIG_NR_CPUS));
 #endif
-	DEFINE(BLOATED_SPINLOCKS, sizeof(spinlock_t) > sizeof(int));
+	DEFINE(SPINLOCK_SIZE, sizeof(spinlock_t));
 	/* End of constants */
 }

mm/memory.c

@@ -4271,7 +4271,7 @@ void copy_user_huge_page(struct page *dst, struct page *src,
 }
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
 
-#if USE_SPLIT_PTE_PTLOCKS && BLOATED_SPINLOCKS
+#if ALLOC_SPLIT_PTLOCKS
 bool ptlock_alloc(struct page *page)
 {
 	spinlock_t *ptl;