Re: [PATCH V3] mm: slub: fix flush_cpu_slab()/__free_slab() invocations in task context.

[Hyeonggon Yoo]

On Mon, Sep 19, 2022 at 06:39:29PM +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.
> 
> v3: replace create_workqueue() with alloc_workqueue() and BUG_ON() with
>     WARN_ON()
> 
> Signed-off-by: Maurizio Lombardi <mlombard@xxxxxxxxxx>
> ---
>  mm/slub.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 862dbd9af4f5..016da09608fb 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);

Hi. what happens here if flushwq failed?

I think avoiding BUG_ON() makes sense,
but shouldn't we have fallback method?

>  	}
>  
>  	for_each_online_cpu(cpu) {
> @@ -4858,6 +4863,8 @@ void __init kmem_cache_init(void)
>  
>  void __init kmem_cache_init_late(void)
>  {
> +	flushwq = alloc_workqueue("slub_flushwq", WQ_MEM_RECLAIM, 0);
> +	WARN_ON(!flushwq);
>  }

>  
>  struct kmem_cache *
> -- 
> 2.31.1
> 
> 

-- 
Thanks,
Hyeonggon