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

[Usama Arif <usama.arif@linux.dev>]

On 26/05/2026 11:25, Steffen Klassert wrote:
> 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!


Thanks! Forgot to cc stable@vger.kernel.org

Adding it here