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>

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index 66d3de1d935fd37eadd3b26f2b26a60985054798..d7bec49ee9ea91b495f4d1cc297327ede093ae57 100644 (file)
--- 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 a6e6a964a28768dbf4760972452c8fac2f4b33ae..aef44e61736195a9ca0dc213744bcfd567eb1085 100644 (file)
--- 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);
        }