pid namespaces: create a slab-cache for 'struct pid_namespace'

This will help fixing memory leaks due to bad reference counting.
Signed-off-by: NSukadev Bhattiprolu <sukadev@us.ibm.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/pid.c

@@ -40,6 +40,7 @@
 static struct hlist_head *pid_hash;
 static int pidhash_shift;
 struct pid init_struct_pid = INIT_STRUCT_PID;
+static struct kmem_cache *pid_ns_cachep;
 
 int pid_max = PID_MAX_DEFAULT;
 
@@ -486,7 +487,7 @@ static struct pid_namespace *create_pid_namespace(int level)
 	struct pid_namespace *ns;
 	int i;
 
-	ns = kmalloc(sizeof(struct pid_namespace), GFP_KERNEL);
+	ns = kmem_cache_alloc(pid_ns_cachep, GFP_KERNEL);
 	if (ns == NULL)
 		goto out;
 
@@ -516,7 +517,7 @@ static struct pid_namespace *create_pid_namespace(int level)
 out_free_map:
 	kfree(ns->pidmap[0].page);
 out_free:
-	kfree(ns);
+	kmem_cache_free(pid_ns_cachep, ns);
 out:
 	return ERR_PTR(-ENOMEM);
 }
@@ -527,7 +528,7 @@ static void destroy_pid_namespace(struct pid_namespace *ns)
 
 	for (i = 0; i < PIDMAP_ENTRIES; i++)
 		kfree(ns->pidmap[i].page);
-	kfree(ns);
+	kmem_cache_free(pid_ns_cachep, ns);
 }
 
 struct pid_namespace *copy_pid_ns(unsigned long flags, struct pid_namespace *old_ns)
@@ -601,4 +602,6 @@ void __init pidmap_init(void)
 	init_pid_ns.pid_cachep = create_pid_cachep(1);
 	if (init_pid_ns.pid_cachep == NULL)
 		panic("Can't create pid_1 cachep\n");
+
+	pid_ns_cachep = KMEM_CACHE(pid_namespace, SLAB_PANIC);
 }