net: make default_rps_mask a per netns attribute

That really was meant to be a per netns attribute from the beginning.

The idea is that once proper isolation is in place in the main
namespace, additional demux in the child namespaces will be redundant.
Let's make child netns default rps mask empty by default.

To avoid bloating the netns with a possibly large cpumask, allocate
it on-demand during the first write operation.

Signed-off-by: Paolo Abeni <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/netdevice.h

···
 #include <linux/static_key.h>
 extern struct static_key_false rps_needed;
 extern struct static_key_false rfs_needed;
-extern struct cpumask rps_default_mask;
 #endif
 
 struct neighbour;

include/net/netns/core.h

···
 
 struct ctl_table_header;
 struct prot_inuse;
+struct cpumask;
 
 struct netns_core {
 	/* core sysctls */
···
 
 #ifdef CONFIG_PROC_FS
 	struct prot_inuse __percpu *prot_inuse;
+#endif
+
+#if IS_ENABLED(CONFIG_RPS) && IS_ENABLED(CONFIG_SYSCTL)
+	struct cpumask *rps_default_mask;
 #endif
 };
 

net/core/net-sysfs.c

···
 	.get_ownership = rx_queue_get_ownership,
 };
 
+static int rx_queue_default_mask(struct net_device *dev,
+				 struct netdev_rx_queue *queue)
+{
+#if IS_ENABLED(CONFIG_RPS) && IS_ENABLED(CONFIG_SYSCTL)
+	struct cpumask *rps_default_mask = READ_ONCE(dev_net(dev)->core.rps_default_mask);
+
+	if (rps_default_mask && !cpumask_empty(rps_default_mask))
+		return netdev_rx_queue_set_rps_mask(queue, rps_default_mask);
+#endif
+	return 0;
+}
+
 static int rx_queue_add_kobject(struct net_device *dev, int index)
 {
 	struct netdev_rx_queue *queue = dev->_rx + index;
···
 			goto err;
 	}
 
-#if IS_ENABLED(CONFIG_RPS) && IS_ENABLED(CONFIG_SYSCTL)
-	if (!cpumask_empty(&rps_default_mask)) {
-		error = netdev_rx_queue_set_rps_mask(queue, &rps_default_mask);
-		if (error)
-			goto err;
-	}
-#endif
+	error = rx_queue_default_mask(dev, queue);
+	if (error)
+		goto err;
+
 	kobject_uevent(kobj, KOBJ_ADD);
 
 	return error;

net/core/sysctl_net_core.c

···
 #endif
 
 #ifdef CONFIG_RPS
-struct cpumask rps_default_mask;
+
+static struct cpumask *rps_default_mask_cow_alloc(struct net *net)
+{
+	struct cpumask *rps_default_mask;
+
+	if (net->core.rps_default_mask)
+		return net->core.rps_default_mask;
+
+	rps_default_mask = kzalloc(cpumask_size(), GFP_KERNEL);
+	if (!rps_default_mask)
+		return NULL;
+
+	/* pairs with READ_ONCE in rx_queue_default_mask() */
+	WRITE_ONCE(net->core.rps_default_mask, rps_default_mask);
+	return rps_default_mask;
+}
 
 static int rps_default_mask_sysctl(struct ctl_table *table, int write,
 				   void *buffer, size_t *lenp, loff_t *ppos)
 {
+	struct net *net = (struct net *)table->data;
 	int err = 0;
 
 	rtnl_lock();
 	if (write) {
-		err = cpumask_parse(buffer, &rps_default_mask);
+		struct cpumask *rps_default_mask = rps_default_mask_cow_alloc(net);
+
+		err = -ENOMEM;
+		if (!rps_default_mask)
+			goto done;
+
+		err = cpumask_parse(buffer, rps_default_mask);
 		if (err)
 			goto done;
 
-		err = rps_cpumask_housekeeping(&rps_default_mask);
+		err = rps_cpumask_housekeeping(rps_default_mask);
 		if (err)
 			goto done;
 	} else {
-		dump_cpumask(buffer, lenp, ppos, &rps_default_mask);
+		dump_cpumask(buffer, lenp, ppos,
+			     net->core.rps_default_mask ? : cpu_none_mask);
 	}
 
 done:
···
 		.mode		= 0644,
 		.proc_handler	= rps_sock_flow_sysctl
 	},
-	{
-		.procname	= "rps_default_mask",
-		.mode		= 0644,
-		.proc_handler	= rps_default_mask_sysctl
-	},
 #endif
 #ifdef CONFIG_NET_FLOW_LIMIT
 	{
···
 };
 
 static struct ctl_table netns_core_table[] = {
+#if IS_ENABLED(CONFIG_RPS)
+	{
+		.procname	= "rps_default_mask",
+		.data		= &init_net,
+		.mode		= 0644,
+		.proc_handler	= rps_default_mask_sysctl
+	},
+#endif
 	{
 		.procname	= "somaxconn",
 		.data		= &init_net.core.sysctl_somaxconn,
···
 	tbl = net->core.sysctl_hdr->ctl_table_arg;
 	unregister_net_sysctl_table(net->core.sysctl_hdr);
 	BUG_ON(tbl == netns_core_table);
+#if IS_ENABLED(CONFIG_RPS)
+	kfree(net->core.rps_default_mask);
+#endif
 	kfree(tbl);
 }
 
···
 
 static __init int sysctl_core_init(void)
 {
-#if IS_ENABLED(CONFIG_RPS)
-	cpumask_copy(&rps_default_mask, cpu_none_mask);
-#endif
-
 	register_net_sysctl(&init_net, "net/core", net_core_table);
 	return register_pernet_subsys(&sysctl_core_ops);
 }