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Message-ID: <CANn89i+pyHq-jfqoaXpk_07LVCn+REmW1wPTbKmxV7ykZCEHfg@mail.gmail.com>
Date: Mon, 26 Jan 2026 11:46:07 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Qiliang Yuan <realwujing@...il.com>
Cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
brauner@...nel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
Qiliang Yuan <yuanql9@...natelecom.cn>
Subject: Re: [PATCH] netns: optimize netns cleaning by batching unhash_nsid calls
On Mon, Jan 26, 2026 at 10:47 AM Qiliang Yuan <realwujing@...il.com> wrote:
>
> 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@...il.com>
> Signed-off-by: Qiliang Yuan <yuanql9@...natelecom.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;
Unrelated change (and not needed)
> + net->is_dying = false;
No need to clear fields, whole net is cleared at alloc time (zalloc)
> 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();
Hopefully the number of entries in netns_ids is small, otherwise this
could have O(N*M) complexity.
> +
> + 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;
Move this to __put_net(), no need for yet another loop.
> +
> + 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
>
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