mm: Split slab into its own type

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 folio, but this is a significant disentanglement.

[ [email protected]: Rebase on folios, use folio instead of page where
  possible.

  Do not duplicate flags field in struct slab, instead make the related
  accessors go through slab_folio(). For testing pfmemalloc use the
  folio_*_active flag accessors directly so the PageSlabPfmemalloc
  wrappers can be removed later.

  Make folio_slab() expect only folio_test_slab() == true folios and
  virt_to_slab() return NULL when folio_test_slab() == false.

  Move struct slab to mm/slab.h.

  Don't represent with struct slab pages that are not true slab pages,
  but just a compound page obtained directly rom page allocator (with
  large kmalloc() for SLUB and SLOB). ]

Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Reviewed-by: Roman Gushchin <[email protected]>

Author: Matthew Wilcox (Oracle)

include/linux/mm_types.h

@@ -56,11 +56,11 @@ struct mem_cgroup;
  * in each subpage, but you may need to restore some of their values
  * afterwards.
  *
- * SLUB uses cmpxchg_double() to atomically update its freelist and
- * counters.  That requires that freelist & counters be adjacent and
- * double-word aligned.  We align all struct pages to double-word
- * boundaries, and ensure that 'freelist' is aligned within the
- * struct.
+ * SLUB uses cmpxchg_double() to atomically update its freelist and counters.
+ * That requires that freelist & counters in struct slab be adjacent and
+ * double-word aligned. Because struct slab currently just reinterprets the
+ * bits of struct page, we align all struct pages to double-word boundaries,
+ * and ensure that 'freelist' is aligned within struct slab.
  */
 #ifdef CONFIG_HAVE_ALIGNED_STRUCT_PAGE
 #define _struct_page_alignment	__aligned(2 * sizeof(unsigned long))

mm/slab.h

@@ -5,6 +5,173 @@
  * Internal slab definitions
  */
 
+/* Reuses the bits in struct page */
+struct slab {
+	unsigned long __page_flags;
+	union {
+		struct list_head slab_list;
+		struct {	/* Partial pages */
+			struct slab *next;
+#ifdef CONFIG_64BIT
+			int slabs;	/* Nr of slabs left */
+#else
+			short int slabs;
+#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 __page_refcount;
+#ifdef CONFIG_MEMCG
+	unsigned long memcg_data;
+#endif
+};
+
+#define SLAB_MATCH(pg, sl)						\
+	static_assert(offsetof(struct page, pg) == offsetof(struct slab, sl))
+SLAB_MATCH(flags, __page_flags);
+SLAB_MATCH(compound_head, slab_list);	/* Ensure bit 0 is clear */
+SLAB_MATCH(slab_list, slab_list);
+SLAB_MATCH(rcu_head, rcu_head);
+SLAB_MATCH(slab_cache, slab_cache);
+SLAB_MATCH(s_mem, s_mem);
+SLAB_MATCH(active, active);
+SLAB_MATCH(_refcount, __page_refcount);
+#ifdef CONFIG_MEMCG
+SLAB_MATCH(memcg_data, memcg_data);
+#endif
+#undef SLAB_MATCH
+static_assert(sizeof(struct slab) <= sizeof(struct page));
+
+/**
+ * folio_slab - Converts from folio to slab.
+ * @folio: The folio.
+ *
+ * Currently struct slab is a different representation of a folio where
+ * folio_test_slab() is true.
+ *
+ * Return: The slab which contains this folio.
+ */
+#define folio_slab(folio)	(_Generic((folio),			\
+	const struct folio *:	(const struct slab *)(folio),		\
+	struct folio *:		(struct slab *)(folio)))
+
+/**
+ * slab_folio - The folio allocated for a slab
+ * @slab: The slab.
+ *
+ * Slabs are allocated as folios that contain the individual objects and are
+ * using some fields in the first struct page of the folio - those fields are
+ * now accessed by struct slab. It is occasionally necessary to convert back to
+ * a folio in order to communicate with the rest of the mm.  Please use this
+ * helper function instead of casting yourself, as the implementation may change
+ * in the future.
+ */
+#define slab_folio(s)		(_Generic((s),				\
+	const struct slab *:	(const struct folio *)s,		\
+	struct slab *:		(struct folio *)s))
+
+/**
+ * page_slab - Converts from first struct page to slab.
+ * @p: The first (either head of compound or single) page of slab.
+ *
+ * A temporary wrapper to convert struct page to struct slab in situations where
+ * we know the page is the compound head, or single order-0 page.
+ *
+ * Long-term ideally everything would work with struct slab directly or go
+ * through folio to struct slab.
+ *
+ * Return: The slab which contains this page
+ */
+#define page_slab(p)		(_Generic((p),				\
+	const struct page *:	(const struct slab *)(p),		\
+	struct page *:		(struct slab *)(p)))
+
+/**
+ * slab_page - The first struct page allocated for a slab
+ * @slab: The slab.
+ *
+ * A convenience wrapper for converting slab to the first struct page of the
+ * underlying folio, to communicate with code not yet converted to folio or
+ * struct slab.
+ */
+#define slab_page(s) folio_page(slab_folio(s), 0)
+
+/*
+ * If network-based swap is enabled, sl*b must keep track of whether pages
+ * were allocated from pfmemalloc reserves.
+ */
+static inline bool slab_test_pfmemalloc(const struct slab *slab)
+{
+	return folio_test_active((struct folio *)slab_folio(slab));
+}
+
+static inline void slab_set_pfmemalloc(struct slab *slab)
+{
+	folio_set_active(slab_folio(slab));
+}
+
+static inline void slab_clear_pfmemalloc(struct slab *slab)
+{
+	folio_clear_active(slab_folio(slab));
+}
+
+static inline void __slab_clear_pfmemalloc(struct slab *slab)
+{
+	__folio_clear_active(slab_folio(slab));
+}
+
+static inline void *slab_address(const struct slab *slab)
+{
+	return folio_address(slab_folio(slab));
+}
+
+static inline int slab_nid(const struct slab *slab)
+{
+	return folio_nid(slab_folio(slab));
+}
+
+static inline pg_data_t *slab_pgdat(const struct slab *slab)
+{
+	return folio_pgdat(slab_folio(slab));
+}
+
+static inline struct slab *virt_to_slab(const void *addr)
+{
+	struct folio *folio = virt_to_folio(addr);
+
+	if (!folio_test_slab(folio))
+		return NULL;
+
+	return folio_slab(folio);
+}
+
+static inline int slab_order(const struct slab *slab)
+{
+	return folio_order((struct folio *)slab_folio(slab));
+}
+
+static inline size_t slab_size(const struct slab *slab)
+{
+	return PAGE_SIZE << slab_order(slab);
+}
+
 #ifdef CONFIG_SLOB
 /*
  * Common fields provided in kmem_cache by all slab allocators

mm/slub.c

@@ -3787,7 +3787,7 @@ static unsigned int slub_min_objects;
  * requested a higher minimum order then we start with that one instead of
  * the smallest order which will fit the object.
  */
-static inline unsigned int slab_order(unsigned int size,
+static inline unsigned int calc_slab_order(unsigned int size,
 		unsigned int min_objects, unsigned int max_order,
 		unsigned int fract_leftover)
 {
@@ -3851,7 +3851,7 @@ static inline int calculate_order(unsigned int size)
 
 		fraction = 16;
 		while (fraction >= 4) {
-			order = slab_order(size, min_objects,
+			order = calc_slab_order(size, min_objects,
 					slub_max_order, fraction);
 			if (order <= slub_max_order)
 				return order;
@@ -3864,14 +3864,14 @@ static inline int calculate_order(unsigned int size)
 	 * We were unable to place multiple objects in a slab. Now
 	 * lets see if we can place a single object there.
 	 */
-	order = slab_order(size, 1, slub_max_order, 1);
+	order = calc_slab_order(size, 1, slub_max_order, 1);
 	if (order <= slub_max_order)
 		return order;
 
 	/*
 	 * Doh this slab cannot be placed using slub_max_order.
 	 */
-	order = slab_order(size, 1, MAX_ORDER, 1);
+	order = calc_slab_order(size, 1, MAX_ORDER, 1);
 	if (order < MAX_ORDER)
 		return order;
 	return -ENOSYS;