Re: [PATCH RFC v2 12/20] slab: remove defer_deactivate_slab()

[Hao Li <hao.li@linux.dev>]

On Mon, Jan 12, 2026 at 04:17:06PM +0100, Vlastimil Babka wrote:
> There are no more cpu slabs so we don't need their deferred
> deactivation. The function is now only used from places where we
> allocate a new slab but then can't spin on node list_lock to put it on
> the partial list. Instead of the deferred action we can free it directly
> via __free_slab(), we just need to tell it to use _nolock() freeing of
> the underlying pages and take care of the accounting.
> 
> Since free_frozen_pages_nolock() variant does not yet exist for code
> outside of the page allocator, create it as a trivial wrapper for
> __free_frozen_pages(..., FPI_TRYLOCK).
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/internal.h   |  1 +
>  mm/page_alloc.c |  5 +++++
>  mm/slab.h       |  8 +-------
>  mm/slub.c       | 51 ++++++++++++++++-----------------------------------
>  4 files changed, 23 insertions(+), 42 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index e430da900430..1f44ccb4badf 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -846,6 +846,7 @@ static inline struct page *alloc_frozen_pages_noprof(gfp_t gfp, unsigned int ord
>  struct page *alloc_frozen_pages_nolock_noprof(gfp_t gfp_flags, int nid, unsigned int order);
>  #define alloc_frozen_pages_nolock(...) \
>  	alloc_hooks(alloc_frozen_pages_nolock_noprof(__VA_ARGS__))
> +void free_frozen_pages_nolock(struct page *page, unsigned int order);
>  
>  extern void zone_pcp_reset(struct zone *zone);
>  extern void zone_pcp_disable(struct zone *zone);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 822e05f1a964..8a288ecfdd93 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2981,6 +2981,11 @@ void free_frozen_pages(struct page *page, unsigned int order)
>  	__free_frozen_pages(page, order, FPI_NONE);
>  }
>  
> +void free_frozen_pages_nolock(struct page *page, unsigned int order)
> +{
> +	__free_frozen_pages(page, order, FPI_TRYLOCK);
> +}
> +
>  /*
>   * Free a batch of folios
>   */
> diff --git a/mm/slab.h b/mm/slab.h
> index e77260720994..4efec41b6445 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -71,13 +71,7 @@ struct slab {
>  	struct kmem_cache *slab_cache;
>  	union {
>  		struct {
> -			union {
> -				struct list_head slab_list;
> -				struct { /* For deferred deactivate_slab() */
> -					struct llist_node llnode;
> -					void *flush_freelist;
> -				};
> -			};
> +			struct list_head slab_list;
>  			/* Double-word boundary */
>  			struct freelist_counters;
>  		};
> diff --git a/mm/slub.c b/mm/slub.c
> index 522a7e671a26..0effeb3b9552 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3248,7 +3248,7 @@ static struct slab *new_slab(struct kmem_cache *s, gfp_t flags, int node)
>  		flags & (GFP_RECLAIM_MASK | GFP_CONSTRAINT_MASK), node);
>  }
>  
> -static void __free_slab(struct kmem_cache *s, struct slab *slab)
> +static void __free_slab(struct kmem_cache *s, struct slab *slab, bool allow_spin)
>  {
>  	struct page *page = slab_page(slab);
>  	int order = compound_order(page);
> @@ -3262,11 +3262,20 @@ static void __free_slab(struct kmem_cache *s, struct slab *slab)
>  	free_frozen_pages(page, order);

Here we missed using the newly added allow_spin.
It should call free_frozen_pages_nolock() when !allow_spin.

-- 
Thanks,
Hao

>  }
>  
> +static void free_new_slab_nolock(struct kmem_cache *s, struct slab *slab)
> +{
> +	/*
> +	 * Since it was just allocated, we can skip the actions in
> +	 * discard_slab() and free_slab().
> +	 */
> +	__free_slab(s, slab, false);
> +}
> +
>  static void rcu_free_slab(struct rcu_head *h)
>  {
>  	struct slab *slab = container_of(h, struct slab, rcu_head);
>  
> -	__free_slab(slab->slab_cache, slab);
> +	__free_slab(slab->slab_cache, slab, true);
>  }
>  
>  static void free_slab(struct kmem_cache *s, struct slab *slab)
> @@ -3282,7 +3291,7 @@ static void free_slab(struct kmem_cache *s, struct slab *slab)
>  	if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU))
>  		call_rcu(&slab->rcu_head, rcu_free_slab);
>  	else
> -		__free_slab(s, slab);
> +		__free_slab(s, slab, true);
>  }
>  
>  static void discard_slab(struct kmem_cache *s, struct slab *slab)
> @@ -3375,8 +3384,6 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
>  	return object;
>  }
>  
> -static void defer_deactivate_slab(struct slab *slab, void *flush_freelist);
> -
>  /*
>   * Called only for kmem_cache_debug() caches to allocate from a freshly
>   * allocated slab. Allocate a single object instead of whole freelist
> @@ -3392,8 +3399,8 @@ static void *alloc_single_from_new_slab(struct kmem_cache *s, struct slab *slab,
>  	void *object;
>  
>  	if (!allow_spin && !spin_trylock_irqsave(&n->list_lock, flags)) {
> -		/* Unlucky, discard newly allocated slab */
> -		defer_deactivate_slab(slab, NULL);
> +		/* Unlucky, discard newly allocated slab. */
> +		free_new_slab_nolock(s, slab);
>  		return NULL;
>  	}
>  
> @@ -4262,7 +4269,7 @@ static unsigned int alloc_from_new_slab(struct kmem_cache *s, struct slab *slab,
>  
>  		if (!spin_trylock_irqsave(&n->list_lock, flags)) {
>  			/* Unlucky, discard newly allocated slab */
> -			defer_deactivate_slab(slab, NULL);
> +			free_new_slab_nolock(s, slab);
>  			return 0;
>  		}
>  	}
> @@ -6031,7 +6038,6 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
>  
>  struct defer_free {
>  	struct llist_head objects;
> -	struct llist_head slabs;
>  	struct irq_work work;
>  };
>  
> @@ -6039,7 +6045,6 @@ static void free_deferred_objects(struct irq_work *work);
>  
>  static DEFINE_PER_CPU(struct defer_free, defer_free_objects) = {
>  	.objects = LLIST_HEAD_INIT(objects),
> -	.slabs = LLIST_HEAD_INIT(slabs),
>  	.work = IRQ_WORK_INIT(free_deferred_objects),
>  };
>  
> @@ -6052,10 +6057,9 @@ static void free_deferred_objects(struct irq_work *work)
>  {
>  	struct defer_free *df = container_of(work, struct defer_free, work);
>  	struct llist_head *objs = &df->objects;
> -	struct llist_head *slabs = &df->slabs;
>  	struct llist_node *llnode, *pos, *t;
>  
> -	if (llist_empty(objs) && llist_empty(slabs))
> +	if (llist_empty(objs))
>  		return;
>  
>  	llnode = llist_del_all(objs);
> @@ -6079,16 +6083,6 @@ static void free_deferred_objects(struct irq_work *work)
>  
>  		__slab_free(s, slab, x, x, 1, _THIS_IP_);
>  	}
> -
> -	llnode = llist_del_all(slabs);
> -	llist_for_each_safe(pos, t, llnode) {
> -		struct slab *slab = container_of(pos, struct slab, llnode);
> -
> -		if (slab->frozen)
> -			deactivate_slab(slab->slab_cache, slab, slab->flush_freelist);
> -		else
> -			free_slab(slab->slab_cache, slab);
> -	}
>  }
>  
>  static void defer_free(struct kmem_cache *s, void *head)
> @@ -6102,19 +6096,6 @@ static void defer_free(struct kmem_cache *s, void *head)
>  		irq_work_queue(&df->work);
>  }
>  
> -static void defer_deactivate_slab(struct slab *slab, void *flush_freelist)
> -{
> -	struct defer_free *df;
> -
> -	slab->flush_freelist = flush_freelist;
> -
> -	guard(preempt)();
> -
> -	df = this_cpu_ptr(&defer_free_objects);
> -	if (llist_add(&slab->llnode, &df->slabs))
> -		irq_work_queue(&df->work);
> -}
> -
>  void defer_free_barrier(void)
>  {
>  	int cpu;
> 
> -- 
> 2.52.0
>