[PATCH v4 05/16] mm: Move 'private' union within struct page

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>

By moving page->private to the fourth word of struct page, we can put
the SLUB counters in the same word as SLAB's s_mem and still do the
cmpxchg_double trick.  Now the SLUB counters no longer overlap with the
mapcount or refcount so we can drop the call to page_mapcount_reset().

Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 include/linux/mm_types.h | 56 ++++++++++++++++++----------------------
 mm/slub.c                |  1 -
 2 files changed, 25 insertions(+), 32 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index e97a310a6abe..23378a789af4 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -65,15 +65,9 @@ struct hmm;
  */
 #ifdef CONFIG_HAVE_ALIGNED_STRUCT_PAGE
 #define _struct_page_alignment	__aligned(2 * sizeof(unsigned long))
-#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE)
-#define _slub_counter_t		unsigned long
 #else
-#define _slub_counter_t		unsigned int
-#endif
-#else /* !CONFIG_HAVE_ALIGNED_STRUCT_PAGE */
 #define _struct_page_alignment
-#define _slub_counter_t		unsigned int
-#endif /* !CONFIG_HAVE_ALIGNED_STRUCT_PAGE */
+#endif
 
 struct page {
 	/* First double word block */
@@ -95,6 +89,30 @@ struct page {
 		/* page_deferred_list().prev	-- second tail page */
 	};
 
+	union {
+		/*
+		 * Mapping-private opaque data:
+		 * Usually used for buffer_heads if PagePrivate
+		 * Used for swp_entry_t if PageSwapCache
+		 * Indicates order in the buddy system if PageBuddy
+		 */
+		unsigned long private;
+#if USE_SPLIT_PTE_PTLOCKS
+#if ALLOC_SPLIT_PTLOCKS
+		spinlock_t *ptl;
+#else
+		spinlock_t ptl;
+#endif
+#endif
+		void *s_mem;			/* slab first object */
+		unsigned long counters;		/* SLUB */
+		struct {			/* SLUB */
+			unsigned inuse:16;
+			unsigned objects:15;
+			unsigned frozen:1;
+		};
+	};
+
 	union {
 		/*
 		 * If the page is neither PageSlab nor mappable to userspace,
@@ -104,13 +122,7 @@ struct page {
 		 */
 		unsigned int page_type;
 
-		_slub_counter_t counters;
 		unsigned int active;		/* SLAB */
-		struct {			/* SLUB */
-			unsigned inuse:16;
-			unsigned objects:15;
-			unsigned frozen:1;
-		};
 		int units;			/* SLOB */
 
 		struct {			/* Page cache */
@@ -179,24 +191,6 @@ struct page {
 #endif
 	};
 
-	union {
-		/*
-		 * Mapping-private opaque data:
-		 * Usually used for buffer_heads if PagePrivate
-		 * Used for swp_entry_t if PageSwapCache
-		 * Indicates order in the buddy system if PageBuddy
-		 */
-		unsigned long private;
-#if USE_SPLIT_PTE_PTLOCKS
-#if ALLOC_SPLIT_PTLOCKS
-		spinlock_t *ptl;
-#else
-		spinlock_t ptl;
-#endif
-#endif
-		void *s_mem;			/* slab first object */
-	};
-
 #ifdef CONFIG_MEMCG
 	struct mem_cgroup *mem_cgroup;
 #endif
diff --git a/mm/slub.c b/mm/slub.c
index 7fc13c46e975..0b4b58740ed8 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1689,7 +1689,6 @@ static void __free_slab(struct kmem_cache *s, struct page *page)
 	__ClearPageSlabPfmemalloc(page);
 	__ClearPageSlab(page);
 
-	page_mapcount_reset(page);
 	page->mapping = NULL;
 	if (current->reclaim_state)
 		current->reclaim_state->reclaimed_slab += pages;
-- 
2.17.0