net: move net_cookie into net_aligned_data

Using per-cpu data for net->net_cookie generation is overkill,
because even busy hosts do not create hundreds of netns per second.

Make sure to put net_cookie in a private cache line to avoid
potential false sharing.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250630093540.3052835-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/aligned_data.h

@@ -2,6 +2,7 @@
 #ifndef _NET_ALIGNED_DATA_H
 #define _NET_ALIGNED_DATA_H
 
+#include <linux/atomic.h>
 #include <linux/types.h>
 
 /* Structure holding cacheline aligned fields on SMP builds.
@@ -9,6 +10,7 @@
  * attribute to ensure no accidental false sharing can happen.
  */
 struct net_aligned_data {
+	atomic64_t	net_cookie ____cacheline_aligned_in_smp;
 };
 
 extern struct net_aligned_data net_aligned_data;

net/core/net_namespace.c

@@ -19,9 +19,9 @@
 #include <linux/net_namespace.h>
 #include <linux/sched/task.h>
 #include <linux/uidgid.h>
-#include <linux/cookie.h>
 #include <linux/proc_fs.h>
 
+#include <net/aligned_data.h>
 #include <net/sock.h>
 #include <net/netlink.h>
 #include <net/net_namespace.h>
@@ -64,8 +64,6 @@ DECLARE_RWSEM(pernet_ops_rwsem);
 
 static unsigned int max_gen_ptrs = INITIAL_NET_GEN_PTRS;
 
-DEFINE_COOKIE(net_cookie);
-
 static struct net_generic *net_alloc_generic(void)
 {
 	unsigned int gen_ptrs = READ_ONCE(max_gen_ptrs);
@@ -434,9 +432,7 @@ static __net_init int setup_net(struct net *net)
 	LIST_HEAD(net_exit_list);
 	int error = 0;
 
-	preempt_disable();
-	net->net_cookie = gen_cookie_next(&net_cookie);
-	preempt_enable();
+	net->net_cookie = atomic64_inc_return(&net_aligned_data.net_cookie);
 
 	list_for_each_entry(ops, &pernet_list, list) {
 		error = ops_init(ops, net);