From 3e52417318473782012b236d0325bf7d2266a597 Mon Sep 17 00:00:00 2001 From: Usama Arif Date: Thu, 21 May 2026 03:29:26 -0700 Subject: [PATCH] xfrm: move policy_bydst RCU sync from per-netns .exit to .pre_exit The struct pernet_operations docstring in include/net/net_namespace.h explicitly warns against blocking RCU primitives in .exit handlers: Exit methods using blocking RCU primitives, such as synchronize_rcu(), should be implemented via exit_batch. [...] Please, avoid synchronize_rcu() at all, where it's possible. Note that a combination of pre_exit() and exit() can be used, since a synchronize_rcu() is guaranteed between the calls. xfrm_policy_fini() violates this: it calls synchronize_rcu() before freeing the policy_bydst hash tables (so no RCU reader is mid- traversal at free time), but runs from xfrm_net_ops.exit -- once per namespace -- so a cleanup_net() of N namespaces pays N full RCU grace periods serially. Use the documented pre_exit/exit split. Move the policy flush (and the workqueue drains it depends on) into a new .pre_exit handler; xfrm_policy_fini() then runs in .exit and frees the hash tables after the synchronize_rcu_expedited() that cleanup_net() guarantees between the two phases. Providing O(1) RCU grace periods per batch instead of O(N). Observed on Linux 6.18 with a workload doing unshare(CLONE_NEWNET) at ~13/sec sustained: cleanup_net() and the netns_wq rescuer kthread both stuck in xfrm_policy_fini()'s synchronize_rcu(), >300k struct net accumulated in the cleanup queue, Percpu in /proc/meminfo climbed to 130+ GB on 256-CPU hosts, and memcg OOMs followed. setup_net and __put_net counts were balanced, ruling out a refcount leak. Fixes: 069daad4f2ae ("xfrm: Wait for RCU readers during policy netns exit") Signed-off-by: Usama Arif Signed-off-by: Steffen Klassert --- net/xfrm/xfrm_policy.c | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-) diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c index 59968dcbafe1..dd09d2063da2 100644 --- a/net/xfrm/xfrm_policy.c +++ b/net/xfrm/xfrm_policy.c @@ -4276,21 +4276,21 @@ static int __net_init xfrm_policy_init(struct net *net) return -ENOMEM; } -static void xfrm_policy_fini(struct net *net) +static void __net_exit xfrm_net_pre_exit(struct net *net) { - struct xfrm_pol_inexact_bin *b, *t; - unsigned int sz; - int dir; - disable_work_sync(&net->xfrm.policy_hthresh.work); - flush_work(&net->xfrm.policy_hash_work); #ifdef CONFIG_XFRM_SUB_POLICY xfrm_policy_flush(net, XFRM_POLICY_TYPE_SUB, false); #endif xfrm_policy_flush(net, XFRM_POLICY_TYPE_MAIN, false); +} - synchronize_rcu(); +static void xfrm_policy_fini(struct net *net) +{ + struct xfrm_pol_inexact_bin *b, *t; + unsigned int sz; + int dir; WARN_ON(!list_empty(&net->xfrm.policy_all)); @@ -4368,6 +4368,7 @@ static void __net_exit xfrm_net_exit(struct net *net) static struct pernet_operations __net_initdata xfrm_net_ops = { .init = xfrm_net_init, + .pre_exit = xfrm_net_pre_exit, .exit = xfrm_net_exit, };