Re: [PATCH] xfrm: move policy_bydst RCU sync from per-netns .exit to .pre_exit

[Steffen Klassert <steffen.klassert@secunet.com>]

On Thu, May 21, 2026 at 03:29:26AM -0700, Usama Arif wrote:
> 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 <usama.arif@linux.dev>

Applied, thanks Usama!