user_ns: improve the user_ns on-the-slab packaging

Currently on 64-bit arch the user_namespace is 2096 and when being
kmalloc-ed it resides on a 4k slab wasting 2003 bytes.

If we allocate a separate cache for it and reduce the hash size from 128
to 64 chains the packaging becomes *much* better - the struct is 1072
bytes and the hole between is 98 bytes.

[[e-mail]: s/__initcall/module_init/]
Signed-off-by: Pavel Emelyanov <[e-mail]>
Acked-by: Serge E. Hallyn <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/user_namespace.h

···
 #include <linux/sched.h>
 #include <linux/err.h>
 
-#define UIDHASH_BITS	(CONFIG_BASE_SMALL ? 3 : 8)
+#define UIDHASH_BITS	(CONFIG_BASE_SMALL ? 3 : 7)
 #define UIDHASH_SZ	(1 << UIDHASH_BITS)
 
 struct user_namespace {

kernel/user_namespace.c

···
 #include <linux/highuid.h>
 #include <linux/cred.h>
 
+static struct kmem_cache *user_ns_cachep __read_mostly;
+
 /*
  * Create a new user namespace, deriving the creator from the user in the
  * passed credentials, and replacing that user with the new root user for the
···
 	struct user_struct *root_user;
 	int n;
 
-	ns = kmalloc(sizeof(struct user_namespace), GFP_KERNEL);
+	ns = kmem_cache_alloc(user_ns_cachep, GFP_KERNEL);
 	if (!ns)
 		return -ENOMEM;
 
···
 	/* Alloc new root user.  */
 	root_user = alloc_uid(ns, 0);
 	if (!root_user) {
-		kfree(ns);
+		kmem_cache_free(user_ns_cachep, ns);
 		return -ENOMEM;
 	}
 
···
 	struct user_namespace *ns =
 		container_of(work, struct user_namespace, destroyer);
 	free_uid(ns->creator);
-	kfree(ns);
+	kmem_cache_free(user_ns_cachep, ns);
 }
 
 void free_user_ns(struct kref *kref)
···
 	/* No useful relationship so no mapping */
 	return overflowgid;
 }
+
+static __init int user_namespaces_init(void)
+{
+	user_ns_cachep = KMEM_CACHE(user_namespace, SLAB_PANIC);
+	return 0;
+}
+module_init(user_namespaces_init);