netns: optimize netns cleaning by batching unhash_nsid calls

Currently, unhash_nsid() scans the entire system for each netns being
killed, leading to O(L_dying_net * M_alive_net * N_id) complexity, as
__peernet2id() also performs a linear search in the IDR.

Optimize this to O(M_alive_net * N_id) by batching unhash operations. Move
unhash_nsid() out of the per-netns loop in cleanup_net() to perform a
single-pass traversal over survivor namespaces.

Identify dying peers by an 'is_dying' flag, which is set under net_rwsem
write lock after the netns is removed from the global list. This batches
the unhashing work and eliminates the O(L_dying_net) multiplier.

To minimize the impact on struct net size, 'is_dying' is placed in an
existing hole after 'hash_mix' in struct net.

Use a restartable idr_get_next() loop for iteration. This avoids the
unsafe modification issue inherent to idr_for_each() callbacks and allows
dropping the nsid_lock to safely call sleepy rtnl_net_notifyid().

Clean up redundant nsid_lock and simplify the destruction loop now that
unhashing is centralized.

Signed-off-by: Qiliang Yuan <yuanql9@chinatelecom.cn>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260204074854.3506916-1-realwujing@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 22 insertions, 13 deletions

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index 66d3de1d935f..d7bec49ee9ea 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -121,6 +121,7 @@ struct net {
 	 * it is critical that it is on a read_mostly cache line.
 	 */
 	u32			hash_mix;
+	bool			is_dying;
 
 	struct net_device       *loopback_dev;          /* The loopback */
 
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index a6e6a964a287..aef44e617361 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -624,9 +624,10 @@ 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;
+	struct net *tmp, *peer;
+
 	/* 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
@@ -634,22 +635,26 @@ static void unhash_nsid(struct net *net, struct net *last)
 	 * use for_each_net_rcu() or net_rwsem.
 	 */
 	for_each_net(tmp) {
-		int id;
+		int id = 0;
 
 		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,
+		while ((peer = idr_get_next(&tmp->netns_ids, &id))) {
+			int curr_id = id;
+
+			id++;
+			if (!peer->is_dying)
+				continue;
+
+			idr_remove(&tmp->netns_ids, curr_id);
+			spin_unlock(&tmp->nsid_lock);
+			rtnl_net_notifyid(tmp, RTM_DELNSID, curr_id, 0, NULL,
 					  GFP_KERNEL);
+			spin_lock(&tmp->nsid_lock);
+		}
+		spin_unlock(&tmp->nsid_lock);
 		if (tmp == last)
 			break;
 	}
-	spin_lock(&net->nsid_lock);
-	idr_destroy(&net->netns_ids);
-	spin_unlock(&net->nsid_lock);
 }
 
 static LLIST_HEAD(cleanup_list);
@@ -674,6 +679,7 @@ static void cleanup_net(struct work_struct *work)
 	llist_for_each_entry(net, net_kill_list, cleanup_list) {
 		ns_tree_remove(net);
 		list_del_rcu(&net->list);
+		net->is_dying = true;
 	}
 	/* Cache last net. After we unlock rtnl, no one new net
 	 * added to net_namespace_list can assign nsid pointer
@@ -688,8 +694,10 @@ static void cleanup_net(struct work_struct *work)
 	last = list_last_entry(&net_namespace_list, struct net, list);
 	up_write(&net_rwsem);
 
+	unhash_nsid(last);
+
 	llist_for_each_entry(net, net_kill_list, cleanup_list) {
-		unhash_nsid(net, last);
+		idr_destroy(&net->netns_ids);
 		list_add_tail(&net->exit_list, &net_exit_list);
 	}