mm/slob: Use object_size field in kmem_cache_size()

Fields object_size and size are not the same: the latter might include
slab metadata. Return object_size field in kmem_cache_size().
Also, improve trace accuracy by correctly tracing reported size.

Cc: Pekka Enberg <penberg@kernel.org>
Cc: Matt Mackall <mpm@selenic.com>
Acked-by: NChristoph Lameter <cl@linux.com>
Acked-by: NDavid Rientjes <rientjes@google.com>
Signed-off-by: NEzequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: NPekka Enberg <penberg@kernel.org>

mm/slob.c

@@ -554,12 +554,12 @@ void *kmem_cache_alloc_node(struct kmem_cache *c, gfp_t flags, int node)
 
 	if (c->size < PAGE_SIZE) {
 		b = slob_alloc(c->size, flags, c->align, node);
-		trace_kmem_cache_alloc_node(_RET_IP_, b, c->size,
+		trace_kmem_cache_alloc_node(_RET_IP_, b, c->object_size,
 					    SLOB_UNITS(c->size) * SLOB_UNIT,
 					    flags, node);
 	} else {
 		b = slob_new_pages(flags, get_order(c->size), node);
-		trace_kmem_cache_alloc_node(_RET_IP_, b, c->size,
+		trace_kmem_cache_alloc_node(_RET_IP_, b, c->object_size,
 					    PAGE_SIZE << get_order(c->size),
 					    flags, node);
 	}
@@ -606,7 +606,7 @@ EXPORT_SYMBOL(kmem_cache_free);
 
 unsigned int kmem_cache_size(struct kmem_cache *c)
 {
-	return c->size;
+	return c->object_size;
 }
 EXPORT_SYMBOL(kmem_cache_size);