Subject: Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache_free callback

From: Uladzislau Rezki <[email-masked]>
Date: Mon, 17 Jun 2024 18:30:53 +0200

On Mon, Jun 17, 2024 at 04:56:17PM +0200, Jason A. Donenfeld wrote:
> On Mon, Jun 17, 2024 at 03:50:56PM +0200, Uladzislau Rezki wrote:
> > On Fri, Jun 14, 2024 at 09:33:45PM +0200, Jason A. Donenfeld wrote:
> > > On Fri, Jun 14, 2024 at 02:35:33PM +0200, Uladzislau Rezki wrote:
> > > > +	/* Should a destroy process be deferred? */
> > > > +	if (s->flags & SLAB_DEFER_DESTROY) {
> > > > +		list_move_tail(&s->list, &slab_caches_defer_destroy);
> > > > +		schedule_delayed_work(&slab_caches_defer_destroy_work, HZ);
> > > > +		goto out_unlock;
> > > > +	}
> > > 
> > > Wouldn't it be smoother to have the actual kmem_cache_free() function
> > > check to see if it's been marked for destruction and the refcount is
> > > zero, rather than polling every one second? I mentioned this approach
> > > in: https://lore.kernel.org/all/[email-masked]/ -
> > > 
> > >     I wonder if the right fix to this would be adding a `should_destroy`
> > >     boolean to kmem_cache, which kmem_cache_destroy() sets to true. And
> > >     then right after it checks `if (number_of_allocations == 0)
> > >     actually_destroy()`, and likewise on each kmem_cache_free(), it
> > >     could check `if (should_destroy && number_of_allocations == 0)
> > >     actually_destroy()`. 
> > > 
> > I do not find pooling as bad way we can go with. But your proposal
> > sounds reasonable to me also. We can combine both "prototypes" to
> > one and offer.
> > 
> > Can you post a prototype here?
> 
> This is untested, but the simplest, shortest possible version would be:
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index 5f8f47c5bee0..907c0ea56c01 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -275,6 +275,7 @@ struct kmem_cache {
>  	unsigned int inuse;		/* Offset to metadata */
>  	unsigned int align;		/* Alignment */
>  	unsigned int red_left_pad;	/* Left redzone padding size */
> +	bool is_destroyed;		/* Destruction happens when no objects */
>  	const char *name;		/* Name (only for display!) */
>  	struct list_head list;		/* List of slab caches */
>  #ifdef CONFIG_SYSFS
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 1560a1546bb1..f700bed066d9 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -494,8 +494,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  		goto out_unlock;
> 
>  	err = shutdown_cache(s);
> -	WARN(err, "%s %s: Slab cache still has objects when called from %pS",
> -	     __func__, s->name, (void *)_RET_IP_);
> +	if (err)
> +		s->is_destroyed = true;
>
Here if an "err" is less then "0" means there are still objects
whereas "is_destroyed" is set to "true" which is not correlated
with a comment:

"Destruction happens when no objects"

>  out_unlock:
>  	mutex_unlock(&slab_mutex);
>  	cpus_read_unlock();
> diff --git a/mm/slub.c b/mm/slub.c
> index 1373ac365a46..7db8fe90a323 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4510,6 +4510,8 @@ void kmem_cache_free(struct kmem_cache *s, void *x)
>  		return;
>  	trace_kmem_cache_free(_RET_IP_, x, s);
>  	slab_free(s, virt_to_slab(x), x, _RET_IP_);
> +	if (s->is_destroyed)
> +		kmem_cache_destroy(s);
>  }
>  EXPORT_SYMBOL(kmem_cache_free);
> 
> @@ -5342,9 +5344,6 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
>  		if (!slab->inuse) {
>  			remove_partial(n, slab);
>  			list_add(&slab->slab_list, &discard);
> -		} else {
> -			list_slab_objects(s, slab,
> -			  "Objects remaining in %s on __kmem_cache_shutdown()");
>  		}
>  	}
>  	spin_unlock_irq(&n->list_lock);
> 
Anyway it looks like it was not welcome to do it in the kmem_cache_free()
function due to performance reason.

--
Uladzislau Rezki