[PATCH v2 6/7] mm, slab: call kvfree_rcu_barrier() from kmem_cache_destroy()

[Vlastimil Babka]

On 2/25/25 7:21 PM, Uladzislau Rezki wrote:
>>
> WQ_MEM_RECLAIM-patch fixes this for me:

Sounds good, can you send a formal patch then?
Some nits below:

> <snip>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 4030907b6b7d..1b5ed5512782 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1304,6 +1304,8 @@ module_param(rcu_min_cached_objs, int, 0444);
>  static int rcu_delay_page_cache_fill_msec = 5000;
>  module_param(rcu_delay_page_cache_fill_msec, int, 0444);
> 
> +static struct workqueue_struct *rcu_reclaim_wq;
> +
>  /* Maximum number of jiffies to wait before draining a batch. */
>  #define KFREE_DRAIN_JIFFIES (5 * HZ)
>  #define KFREE_N_BATCHES 2
> @@ -1632,10 +1634,10 @@ __schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp)
>         if (delayed_work_pending(&krcp->monitor_work)) {
>                 delay_left = krcp->monitor_work.timer.expires - jiffies;
>                 if (delay < delay_left)
> -                       mod_delayed_work(system_unbound_wq, &krcp->monitor_work, delay);
> +                       mod_delayed_work(rcu_reclaim_wq, &krcp->monitor_work, delay);
>                 return;
>         }
> -       queue_delayed_work(system_unbound_wq, &krcp->monitor_work, delay);
> +       queue_delayed_work(rcu_reclaim_wq, &krcp->monitor_work, delay);
>  }
> 
>  static void
> @@ -1733,7 +1735,7 @@ kvfree_rcu_queue_batch(struct kfree_rcu_cpu *krcp)
>                         // "free channels", the batch can handle. Break
>                         // the loop since it is done with this CPU thus
>                         // queuing an RCU work is _always_ success here.
> -                       queued = queue_rcu_work(system_unbound_wq, &krwp->rcu_work);
> +                       queued = queue_rcu_work(rcu_reclaim_wq, &krwp->rcu_work);
>                         WARN_ON_ONCE(!queued);
>                         break;
>                 }
> @@ -1883,7 +1885,7 @@ run_page_cache_worker(struct kfree_rcu_cpu *krcp)
>         if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
>                         !atomic_xchg(&krcp->work_in_progress, 1)) {
>                 if (atomic_read(&krcp->backoff_page_cache_fill)) {
> -                       queue_delayed_work(system_unbound_wq,
> +                       queue_delayed_work(rcu_reclaim_wq,
>                                 &krcp->page_cache_work,
>                                         msecs_to_jiffies(rcu_delay_page_cache_fill_msec));
>                 } else {
> @@ -2120,6 +2122,10 @@ void __init kvfree_rcu_init(void)
>         int i, j;
>         struct shrinker *kfree_rcu_shrinker;
> 
> +       rcu_reclaim_wq = alloc_workqueue("rcu_reclaim",

Should we name it "kvfree_rcu_reclaim"? rcu_reclaim sounds too generic
as if it's part of rcu itself?

> +               WQ_UNBOUND | WQ_MEM_RECLAIM, 0);

Do we want WQ_SYSFS? Or maybe only when someone asks, with a use case?

Thanks,
Vlastimil

> +       WARN_ON(!rcu_reclaim_wq);
> +
>         /* Clamp it to [0:100] seconds interval. */
>         if (rcu_delay_page_cache_fill_msec < 0 ||
>                 rcu_delay_page_cache_fill_msec > 100 * MSEC_PER_SEC) {
> <snip>
> 
> it passes:
> 
> <snip>
> [   15.972416] KTAP version 1
> [   15.972421] 1..1
> [   15.973467]     KTAP version 1
> [   15.973470]     # Subtest: slub_test
> [   15.973472]     # module: slub_kunit
> [   15.973474]     1..10
> [   15.974483]     ok 1 test_clobber_zone
> [   15.974927]     ok 2 test_next_pointer
> [   15.975308]     ok 3 test_first_word
> [   15.975672]     ok 4 test_clobber_50th_byte
> [   15.976035]     ok 5 test_clobber_redzone_free
> [   15.976128] stackdepot: allocating hash table of 1048576 entries via kvcalloc
> [   15.979505]     ok 6 test_kmalloc_redzone_access
> [   16.014408]     ok 7 test_kfree_rcu
> [   17.726602]     ok 8 test_kfree_rcu_wq_destroy
> [   17.750323]     ok 9 test_leak_destroy
> [   17.750883]     ok 10 test_krealloc_redzone_zeroing
> [   17.750887] # slub_test: pass:10 fail:0 skip:0 total:10
> [   17.750890] # Totals: pass:10 fail:0 skip:0 total:10
> [   17.750891] ok 1 slub_test
> <snip>
> 
> --
> Uladzislau Rezki