Currently, unhash_nsid() scans the entire net_namespace_list for each
netns in a destruction batch during cleanup_net(). This leads to
an O(M * N) complexity, where M is the batch size and N is the total
number of namespaces in the system.

Reduce the complexity to O(N) by introducing an 'is_dying' flag to mark
the entire batch of namespaces being destroyed. This allows unhash_nsid()
to perform a single-pass traversal over the system's namespaces. In
this pass, for each survivor namespace, iterate through its netns_ids
and remove any mappings that point to a marked namespace.

Signed-off-by: Qiliang Yuan <realwujing@gmail.com>
Signed-off-by: Qiliang Yuan <yuanql9@chinatelecom.cn>
---
 include/net/net_namespace.h |  1 +
 net/core/net_namespace.c    | 50 +++++++++++++++++++++++++++----------
 2 files changed, 38 insertions(+), 13 deletions(-)

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index cb664f6e3558..bd1acc6056ac 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -69,6 +69,7 @@ struct net {
 
 	unsigned int		dev_base_seq;	/* protected by rtnl_mutex */
 	u32			ifindex;
+	bool			is_dying;
 
 	spinlock_t		nsid_lock;
 	atomic_t		fnhe_genid;
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index a6e6a964a287..d24e46c034f2 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -413,6 +413,8 @@ static __net_init int preinit_net(struct net *net, struct user_namespace *user_n
 
 	get_random_bytes(&net->hash_mix, sizeof(u32));
 	net->dev_base_seq = 1;
+	net->ifindex = 0;
+	net->is_dying = false;
 	net->user_ns = user_ns;
 
 	idr_init(&net->netns_ids);
@@ -624,9 +626,10 @@ void net_ns_get_ownership(const struct net *net, kuid_t *uid, kgid_t *gid)
 }
 EXPORT_SYMBOL_GPL(net_ns_get_ownership);
 
-static void unhash_nsid(struct net *net, struct net *last)
+static void unhash_nsid(struct net *last)
 {
 	struct net *tmp;
+
 	/* This function is only called from cleanup_net() work,
 	 * and this work is the only process, that may delete
 	 * a net from net_namespace_list. So, when the below
@@ -636,20 +639,34 @@ static void unhash_nsid(struct net *net, struct net *last)
 	for_each_net(tmp) {
 		int id;
 
-		spin_lock(&tmp->nsid_lock);
-		id = __peernet2id(tmp, net);
-		if (id >= 0)
-			idr_remove(&tmp->netns_ids, id);
-		spin_unlock(&tmp->nsid_lock);
-		if (id >= 0)
-			rtnl_net_notifyid(tmp, RTM_DELNSID, id, 0, NULL,
-					  GFP_KERNEL);
+		for (id = 0; ; id++) {
+			struct net *peer;
+			bool dying;
+
+			rcu_read_lock();
+			peer = idr_get_next(&tmp->netns_ids, &id);
+			dying = peer && peer->is_dying;
+			rcu_read_unlock();
+
+			if (!peer)
+				break;
+			if (!dying)
+				continue;
+
+			spin_lock(&tmp->nsid_lock);
+			if (idr_find(&tmp->netns_ids, id) == peer)
+				idr_remove(&tmp->netns_ids, id);
+			else
+				peer = NULL;
+			spin_unlock(&tmp->nsid_lock);
+
+			if (peer)
+				rtnl_net_notifyid(tmp, RTM_DELNSID, id, 0,
+						  NULL, GFP_KERNEL);
+		}
 		if (tmp == last)
 			break;
 	}
-	spin_lock(&net->nsid_lock);
-	idr_destroy(&net->netns_ids);
-	spin_unlock(&net->nsid_lock);
 }
 
 static LLIST_HEAD(cleanup_list);
@@ -688,8 +705,15 @@ static void cleanup_net(struct work_struct *work)
 	last = list_last_entry(&net_namespace_list, struct net, list);
 	up_write(&net_rwsem);
 
+	llist_for_each_entry(net, net_kill_list, cleanup_list)
+		net->is_dying = true;
+
+	unhash_nsid(last);
+
 	llist_for_each_entry(net, net_kill_list, cleanup_list) {
-		unhash_nsid(net, last);
+		spin_lock(&net->nsid_lock);
+		idr_destroy(&net->netns_ids);
+		spin_unlock(&net->nsid_lock);
 		list_add_tail(&net->exit_list, &net_exit_list);
 	}
 
-- 
2.51.0