mm/slub: Define struct slab fields for CONFIG_SLUB_CPU_PARTIAL only when enabled

The fields 'next' and 'slabs' are only used when CONFIG_SLUB_CPU_PARTIAL
is enabled. We can put their definition to #ifdef to prevent accidental
use when disabled.

Currenlty show_slab_objects() and slabs_cpu_partial_show() contain code
accessing the slabs field that's effectively dead with
CONFIG_SLUB_CPU_PARTIAL=n through the wrappers slub_percpu_partial() and
slub_percpu_partial_read_once(), but to prevent a compile error, we need
to hide all this code behind #ifdef.
Signed-off-by: NVlastimil Babka <vbabka@suse.cz>
Reviewed-by: NHyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: NRoman Gushchin <guro@fb.com>
Tested-by: NHyeonggon Yoo <42.hyeyoo@gmail.com>

mm/slab.h

@@ -25,10 +25,12 @@ struct slab {
 	union {
 		struct list_head slab_list;
 		struct rcu_head rcu_head;
+#ifdef CONFIG_SLUB_CPU_PARTIAL
 		struct {
 			struct slab *next;
 			int slabs;	/* Nr of slabs left */
 		};
+#endif
 	};
 	struct kmem_cache *slab_cache;
 	/* Double-word boundary */

mm/slub.c

@@ -5258,6 +5258,7 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
 			total += x;
 			nodes[node] += x;
 
+#ifdef CONFIG_SLUB_CPU_PARTIAL
 			slab = slub_percpu_partial_read_once(c);
 			if (slab) {
 				node = slab_nid(slab);
@@ -5270,6 +5271,7 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
 				total += x;
 				nodes[node] += x;
 			}
+#endif
 		}
 	}
 
@@ -5469,9 +5471,10 @@ static ssize_t slabs_cpu_partial_show(struct kmem_cache *s, char *buf)
 {
 	int objects = 0;
 	int slabs = 0;
-	int cpu;
+	int cpu __maybe_unused;
 	int len = 0;
 
+#ifdef CONFIG_SLUB_CPU_PARTIAL
 	for_each_online_cpu(cpu) {
 		struct slab *slab;
 
@@ -5480,12 +5483,13 @@ static ssize_t slabs_cpu_partial_show(struct kmem_cache *s, char *buf)
 		if (slab)
 			slabs += slab->slabs;
 	}
+#endif
 
 	/* Approximate half-full slabs, see slub_set_cpu_partial() */
 	objects = (slabs * oo_objects(s->oo)) / 2;
 	len += sysfs_emit_at(buf, len, "%d(%d)", objects, slabs);
 
-#ifdef CONFIG_SMP
+#if defined(CONFIG_SLUB_CPU_PARTIAL) && defined(CONFIG_SMP)
 	for_each_online_cpu(cpu) {
 		struct slab *slab;