[PATCH v5] netns: optimize netns cleaning by batching unhash_nsid calls
From: Qiliang Yuan
Date: Sun Feb 01 2026 - 21:40:15 EST
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@xxxxxxxxxxxxxxx>
---
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