Make struct slab independent of struct page.  It still uses the
underlying memory in struct page for storing slab-specific data,
but slab and slub can now be weaned off using struct page directly.
Some of the wrapper functions (slab_address() and slab_order())
still need to cast to struct page, but this is a significant
disentanglement.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
  include/linux/mm_types.h   | 56 +++++++++++++++++++++++++++++
  include/linux/page-flags.h | 29 +++++++++++++++
  mm/slab.h                  | 73 ++++++++++++++++++++++++++++++++++++++
  mm/slub.c                  |  8 ++---
  4 files changed, 162 insertions(+), 4 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 7f8ee09c711f..c2ea71aba84e 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h

@@ -239,6 +239,62 @@ struct page {
  #endif
  } _struct_page_alignment;
  
+/* Reuses the bits in struct page */
+struct slab {
+	unsigned long flags;
+	union {
+		struct list_head slab_list;
+		struct {	/* Partial pages */
+			struct slab *next;
+#ifdef CONFIG_64BIT
+			int slabs;	/* Nr of slabs left */
+			int pobjects;	/* Approximate count */
+#else
+			short int slabs;
+			short int pobjects;
+#endif
+		};
+		struct rcu_head rcu_head;
+	};
+	struct kmem_cache *slab_cache; /* not slob */
+	/* Double-word boundary */
+	void *freelist;		/* first free object */
+	union {
+		void *s_mem;	/* slab: first object */
+		unsigned long counters;		/* SLUB */
+		struct {			/* SLUB */
+			unsigned inuse:16;
+			unsigned objects:15;
+			unsigned frozen:1;
+		};
+	};
+
+	union {
+		unsigned int active;		/* SLAB */
+		int units;			/* SLOB */
+	};
+	atomic_t _refcount;
+#ifdef CONFIG_MEMCG
+	unsigned long memcg_data;
+#endif
+};