mm/sl*b: Differentiate struct slab fields by sl*b implementations

With a struct slab definition separate from struct page, we can go
further and define only fields that the chosen sl*b implementation uses.
This means everything between __page_flags and __page_refcount
placeholders now depends on the chosen CONFIG_SL*B. Some fields exist in
all implementations (slab_list) but can be part of a union in some, so
it's simpler to repeat them than complicate the definition with ifdefs
even more.

The patch doesn't change physical offsets of the fields, although it
could be done later - for example it's now clear that tighter packing in
SLOB could be possible.

This should also prevent accidental use of fields that don't exist in
given implementation. Before this patch virt_to_cache() and
cache_from_obj() were visible for SLOB (albeit not used), although they
rely on the slab_cache field that isn't set by SLOB. With this patch
it's now a compile error, so these functions are now hidden behind
an #ifndef CONFIG_SLOB.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Roman Gushchin <guro@fb.com>
Tested-by: Marco Elver <elver@google.com> # kfence
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: <kasan-dev@googlegroups.com>

mm/kfence/core.c

@@ -427,10 +427,11 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
 	/* Set required slab fields. */
 	slab = virt_to_slab((void *)meta->addr);
 	slab->slab_cache = cache;
-	if (IS_ENABLED(CONFIG_SLUB))
-		slab->objects = 1;
-	if (IS_ENABLED(CONFIG_SLAB))
-		slab->s_mem = addr;
+#if defined(CONFIG_SLUB)
+	slab->objects = 1;
+#elif defined(CONFIG_SLAB)
+	slab->s_mem = addr;
+#endif
 
 	/* Memory initialization. */
 	for_each_canary(meta, set_canary_byte);

mm/slab.h

@@ -8,35 +8,57 @@
 /* Reuses the bits in struct page */
 struct slab {
 	unsigned long __page_flags;
+
+#if defined(CONFIG_SLAB)
+
 	union {
 		struct list_head slab_list;
-		struct {	/* Partial pages */
+		struct rcu_head rcu_head;
+	};
+	struct kmem_cache *slab_cache;
+	void *freelist;	/* array of free object indexes */
+	void *s_mem;	/* first object */
+	unsigned int active;
+
+#elif defined(CONFIG_SLUB)
+
+	union {
+		struct list_head slab_list;
+		struct rcu_head rcu_head;
+		struct {
 			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 */
+	struct kmem_cache *slab_cache;
 	/* Double-word boundary */
 	void *freelist;		/* first free object */
 	union {
-		void *s_mem;	/* slab: first object */
-		unsigned long counters;		/* SLUB */
-		struct {			/* SLUB */
+		unsigned long counters;
+		struct {
 			unsigned inuse:16;
 			unsigned objects:15;
 			unsigned frozen:1;
 		};
 	};
+	unsigned int __unused;
+
+#elif defined(CONFIG_SLOB)
+
+	struct list_head slab_list;
+	void *__unused_1;
+	void *freelist;		/* first free block */
+	void *__unused_2;
+	int units;
+
+#else
+#error "Unexpected slab allocator configured"
+#endif
 
-	union {
-		unsigned int active;		/* SLAB */
-		int units;			/* SLOB */
-	};
 	atomic_t __page_refcount;
 #ifdef CONFIG_MEMCG
 	unsigned long memcg_data;
@@ -48,10 +70,14 @@ struct slab {
 SLAB_MATCH(flags, __page_flags);
 SLAB_MATCH(compound_head, slab_list);	/* Ensure bit 0 is clear */
 SLAB_MATCH(slab_list, slab_list);
+#ifndef CONFIG_SLOB
 SLAB_MATCH(rcu_head, rcu_head);
 SLAB_MATCH(slab_cache, slab_cache);
+#endif
+#ifdef CONFIG_SLAB
 SLAB_MATCH(s_mem, s_mem);
 SLAB_MATCH(active, active);
+#endif
 SLAB_MATCH(_refcount, __page_refcount);
 #ifdef CONFIG_MEMCG
 SLAB_MATCH(memcg_data, memcg_data);
@@ -599,6 +625,7 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s,
 }
 #endif /* CONFIG_MEMCG_KMEM */
 
+#ifndef CONFIG_SLOB
 static inline struct kmem_cache *virt_to_cache(const void *obj)
 {
 	struct slab *slab;
@@ -645,6 +672,7 @@ static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
 		print_tracking(cachep, x);
 	return cachep;
 }
+#endif /* CONFIG_SLOB */
 
 static inline size_t slab_ksize(const struct kmem_cache *s)
 {