[PATCH v5] netns: optimize netns cleaning by batching unhash_nsid calls

[Qiliang Yuan <realwujing@gmail.com>]

Currently, unhash_nsid() scans the entire system for each netns being
killed, leading to O(M_batch * N_system * log(N_ids)) complexity.

Optimize this to O(N_system * N_ids) 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(M_batch) multiplier.

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>
---
v5:
 - Use idr_get_next() for restartable iteration safely handling removals.
 - Drop unhash_nsid_callback() to avoid context safety issues.
v4:
 - Move unhash_nsid() out of the batch loop to reduce complexity from O(M*N) to O(N).
 - Use idr_for_each() for efficient, single-pass IDR traversal.
 - Mark 'is_dying' under net_rwsem to safely identify and batch unhashing.
 - Simplify destruction loop by removing redundant locking and per-netns unhash logic.
v3:
 - Update target tree to net-next.
 - Post as a new thread instead of a reply.
v2:
 - Move 'is_dying' setting to __put_net() to eliminate the O(M_batch) loop.
 - Remove redundant initializations in preinit_net().
v1:
 - Initial implementation of batch unhash_nsid().

 include/net/net_namespace.h |  1 +
 net/core/net_namespace.c    | 34 ++++++++++++++++++++--------------
 2 files changed, 21 insertions(+), 14 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..3061c83786ac 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -624,9 +624,11 @@ 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;
+	int id;
+
 	/* 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 +636,23 @@ static void unhash_nsid(struct net *net, struct net *last)
 	 * use for_each_net_rcu() or net_rwsem.
 	 */
 	for_each_net(tmp) {
-		int id;
-
+		id = 0;
 		spin_lock(&tmp->nsid_lock);
-		id = __peernet2id(tmp, net);
-		if (id >= 0)
-			idr_remove(&tmp->netns_ids, id);
+		while ((peer = idr_get_next(&tmp->netns_ids, &id))) {
+			if (peer->is_dying) {
+				idr_remove(&tmp->netns_ids, id);
+				spin_unlock(&tmp->nsid_lock);
+				rtnl_net_notifyid(tmp, RTM_DELNSID, id, 0, NULL,
+						  GFP_KERNEL);
+				spin_lock(&tmp->nsid_lock);
+			} else {
+				id++;
+			}
+		}
 		spin_unlock(&tmp->nsid_lock);
-		if (id >= 0)
-			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);
@@ -674,6 +677,7 @@ 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 +692,10 @@ 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);
 	}
 
-- 
2.51.0