[PATCH V2] mm: slub: fix flush_cpu_slab()/__free_slab() invocations in task context.

[PATCH V2] mm: slub: fix flush_cpu_slab()/__free_slab() invocations in task context.

[Maurizio Lombardi]

Commit 5a836bf6b09f ("mm: slub: move flush_cpu_slab() invocations
__free_slab() invocations out of IRQ context") moved all flush_cpu_slab()
invocations to the global workqueue to avoid a problem related
with deactivate_slab()/__free_slab() being called from an IRQ context
on PREEMPT_RT kernels.

When the flush_all_cpu_locked() function is called from a task context
it may happen that a workqueue with WQ_MEM_RECLAIM bit set ends up
flushing the global workqueue, this will cause a dependency issue.

 workqueue: WQ_MEM_RECLAIM nvme-delete-wq:nvme_delete_ctrl_work [nvme_core]
   is flushing !WQ_MEM_RECLAIM events:flush_cpu_slab
 WARNING: CPU: 37 PID: 410 at kernel/workqueue.c:2637
   check_flush_dependency+0x10a/0x120
 Workqueue: nvme-delete-wq nvme_delete_ctrl_work [nvme_core]
 RIP: 0010:check_flush_dependency+0x10a/0x120[  453.262125] Call Trace:
 __flush_work.isra.0+0xbf/0x220
 ? __queue_work+0x1dc/0x420
 flush_all_cpus_locked+0xfb/0x120
 __kmem_cache_shutdown+0x2b/0x320
 kmem_cache_destroy+0x49/0x100
 bioset_exit+0x143/0x190
 blk_release_queue+0xb9/0x100
 kobject_cleanup+0x37/0x130
 nvme_fc_ctrl_free+0xc6/0x150 [nvme_fc]
 nvme_free_ctrl+0x1ac/0x2b0 [nvme_core]

Fix this bug by creating a workqueue for the flush operation with
the WQ_MEM_RECLAIM bit set.

v2: Create a workqueue with WQ_MEM_RECLAIM
    instead of trying to revert the changes.

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
---
 mm/slub.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 862dbd9af4f5..cdb8011884d0 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -310,6 +310,11 @@ static inline void stat(const struct kmem_cache *s, enum stat_item si)
  */
 static nodemask_t slab_nodes;
 
+/*
+ * Workqueue used for flush_cpu_slab().
+ */
+static struct workqueue_struct *flushwq;
+
 /********************************************************************
  * 			Core slab cache functions
  *******************************************************************/
@@ -2730,7 +2735,7 @@ static void flush_all_cpus_locked(struct kmem_cache *s)
 		INIT_WORK(&sfw->work, flush_cpu_slab);
 		sfw->skip = false;
 		sfw->s = s;
-		schedule_work_on(cpu, &sfw->work);
+		queue_work_on(cpu, flushwq, &sfw->work);
 	}
 
 	for_each_online_cpu(cpu) {
@@ -4858,6 +4863,8 @@ void __init kmem_cache_init(void)
 
 void __init kmem_cache_init_late(void)
 {
+	flushwq = create_workqueue("slub_flushwq");
+	BUG_ON(!flushwq);
 }
 
 struct kmem_cache *
-- 
2.31.1

Re: [PATCH V2] mm: slub: fix flush_cpu_slab()/__free_slab() invocations in task context.

[Hyeonggon Yoo]

On Mon, Sep 12, 2022 at 06:11:40PM +0200, Maurizio Lombardi wrote:
> Commit 5a836bf6b09f ("mm: slub: move flush_cpu_slab() invocations
> __free_slab() invocations out of IRQ context") moved all flush_cpu_slab()
> invocations to the global workqueue to avoid a problem related
> with deactivate_slab()/__free_slab() being called from an IRQ context
> on PREEMPT_RT kernels.
> 
> When the flush_all_cpu_locked() function is called from a task context
> it may happen that a workqueue with WQ_MEM_RECLAIM bit set ends up
> flushing the global workqueue, this will cause a dependency issue.
> 
>  workqueue: WQ_MEM_RECLAIM nvme-delete-wq:nvme_delete_ctrl_work [nvme_core]
>    is flushing !WQ_MEM_RECLAIM events:flush_cpu_slab
>  WARNING: CPU: 37 PID: 410 at kernel/workqueue.c:2637
>    check_flush_dependency+0x10a/0x120
>  Workqueue: nvme-delete-wq nvme_delete_ctrl_work [nvme_core]
>  RIP: 0010:check_flush_dependency+0x10a/0x120[  453.262125] Call Trace:
>  __flush_work.isra.0+0xbf/0x220
>  ? __queue_work+0x1dc/0x420
>  flush_all_cpus_locked+0xfb/0x120
>  __kmem_cache_shutdown+0x2b/0x320
>  kmem_cache_destroy+0x49/0x100
>  bioset_exit+0x143/0x190
>  blk_release_queue+0xb9/0x100
>  kobject_cleanup+0x37/0x130
>  nvme_fc_ctrl_free+0xc6/0x150 [nvme_fc]
>  nvme_free_ctrl+0x1ac/0x2b0 [nvme_core]
> 
> Fix this bug by creating a workqueue for the flush operation with
> the WQ_MEM_RECLAIM bit set.
> 
> v2: Create a workqueue with WQ_MEM_RECLAIM
>     instead of trying to revert the changes.
> 
> Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
> ---
>  mm/slub.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 862dbd9af4f5..cdb8011884d0 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -310,6 +310,11 @@ static inline void stat(const struct kmem_cache *s, enum stat_item si)
>   */
>  static nodemask_t slab_nodes;
>  
> +/*
> + * Workqueue used for flush_cpu_slab().
> + */
> +static struct workqueue_struct *flushwq;
> +
>  /********************************************************************
>   * 			Core slab cache functions
>   *******************************************************************/
> @@ -2730,7 +2735,7 @@ static void flush_all_cpus_locked(struct kmem_cache *s)
>  		INIT_WORK(&sfw->work, flush_cpu_slab);
>  		sfw->skip = false;
>  		sfw->s = s;
> -		schedule_work_on(cpu, &sfw->work);
> +		queue_work_on(cpu, flushwq, &sfw->work);
>  	}
>  
>  	for_each_online_cpu(cpu) {
> @@ -4858,6 +4863,8 @@ void __init kmem_cache_init(void)
>  
>  void __init kmem_cache_init_late(void)
>  {
> +	flushwq = create_workqueue("slub_flushwq");
> +	BUG_ON(!flushwq);
>  }
>  
>  struct kmem_cache *
> -- 
> 2.31.1
>

No objection about this patch,
but it seems create_workqueue is deprecated?


-- 
Thanks,
Hyeonggon

Re: [PATCH V2] mm: slub: fix flush_cpu_slab()/__free_slab() invocations in task context.

[Maurizio Lombardi]

čt 15. 9. 2022 v 14:48 odesílatel Hyeonggon Yoo <42.hyeyoo@gmail.com> napsal:
> No objection about this patch,
> but it seems create_workqueue is deprecated?

You're right, the documentation says that create_workqueue() is deprecated,
too bad there is not a comment in the source code.
Thanks for reporting this.

I will wait for the maintainers to review this one and eventually I
will submit a V3.

Maurizio



>
>
> --
> Thanks,
> Hyeonggon
>

Re: [PATCH V2] mm: slub: fix flush_cpu_slab()/__free_slab() invocations in task context.

[Vlastimil Babka]

On 9/12/22 18:11, Maurizio Lombardi wrote:
> @@ -2730,7 +2735,7 @@ static void flush_all_cpus_locked(struct kmem_cache *s)
>  		INIT_WORK(&sfw->work, flush_cpu_slab);
>  		sfw->skip = false;
>  		sfw->s = s;
> -		schedule_work_on(cpu, &sfw->work);
> +		queue_work_on(cpu, flushwq, &sfw->work);
>  	}
>  
>  	for_each_online_cpu(cpu) {
> @@ -4858,6 +4863,8 @@ void __init kmem_cache_init(void)
>  
>  void __init kmem_cache_init_late(void)
>  {
> +	flushwq = create_workqueue("slub_flushwq");
> +	BUG_ON(!flushwq);

Adding new BUG_ON is essentially prohibited these days, so please use WARN_ON().
Other than that deprecation issue, looks fine to me.

>  }
>  
>  struct kmem_cache *