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Message-ID: <CAAVpQUD=_UX0ZqwQQ6hdACC43AKX0xt__4fR_+XQO=fWgymWyw@mail.gmail.com>
Date: Wed, 4 Feb 2026 23:56:41 -0800
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Qiliang Yuan <realwujing@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, Jeff Layton <jlayton@...nel.org>,
Qiliang Yuan <yuanql9@...natelecom.cn>, Simon Horman <horms@...nel.org>, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6] netns: optimize netns cleaning by batching unhash_nsid calls
On Tue, Feb 3, 2026 at 11:49 PM Qiliang Yuan <realwujing@...il.com> wrote:
>
> Currently, unhash_nsid() scans the entire system for each netns being
> killed, leading to O(M_dying_net * N_alive_net * N_id) complexity, as
Sorry I should've used L_dying_net, M_alive_net .. as N is not
the same here.
> __peernet2id() also performs a linear search in the IDR.
>
> Optimize this to O(N_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(M_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@...natelecom.cn>
Code-wise, looks good to me:
Reviewed-by: Kuniyuki Iwashima <kuniyu@...gle.com>
> ---
> v6:
> - Use M_dying_net and N_alive_net terminology for clarity.
> - Correct complexity analysis: __peernet2id() performs a linear search.
> - Move 'is_dying' to a structural hole after 'hash_mix' to save memory.
> - Scope 'id' variable locally within the traversal loop.
> - Simplify IDR traversal logic with unconditional increment.
> 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, 22 insertions(+), 13 deletions(-)
>
> diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
> index cb664f6e3558..db291cc7afe3 100644
> --- a/include/net/net_namespace.h
> +++ b/include/net/net_namespace.h
> @@ -120,6 +120,7 @@ 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);
> }
>
> --
> 2.51.0
>
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