Message-ID: <dk5mggvowqulzlbv7o2n7ha5p4adq5uzh5i4d473jjwsyivi7l@lvlqxz4si37f>
Date: Tue, 27 Jan 2026 12:31:27 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Harry Yoo <harry.yoo@...cle.com>, Petr Tesarik <ptesarik@...e.com>,
        Christoph Lameter <cl@...two.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>, Hao Li <hao.li@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Alexei Starovoitov <ast@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
        bpf@...r.kernel.org, kasan-dev@...glegroups.com
Subject: Re: [PATCH v4 07/22] slab: introduce percpu sheaves bootstrap

* Vlastimil Babka <vbabka@...e.cz> [260123 01:53]:
> Until now, kmem_cache->cpu_sheaves was !NULL only for caches with
> sheaves enabled. Since we want to enable them for almost all caches,
> it's suboptimal to test the pointer in the fast paths, so instead
> allocate it for all caches in do_kmem_cache_create(). Instead of testing
> the cpu_sheaves pointer to recognize caches (yet) without sheaves, test
> kmem_cache->sheaf_capacity for being 0, where needed, using a new
> cache_has_sheaves() helper.
> 
> However, for the fast paths sake we also assume that the main sheaf
> always exists (pcs->main is !NULL), and during bootstrap we cannot
> allocate sheaves yet.
> 
> Solve this by introducing a single static bootstrap_sheaf that's
> assigned as pcs->main during bootstrap. It has a size of 0, so during
> allocations, the fast path will find it's empty. Since the size of 0
> matches sheaf_capacity of 0, the freeing fast paths will find it's
> "full". In the slow path handlers, we use cache_has_sheaves() to
> recognize that the cache doesn't (yet) have real sheaves, and fall back.
> Thus sharing the single bootstrap sheaf like this for multiple caches
> and cpus is safe.
> 
> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>

> ---
>  mm/slab.h        |  12 ++++++
>  mm/slab_common.c |   2 +-
>  mm/slub.c        | 123 ++++++++++++++++++++++++++++++++++++-------------------
>  3 files changed, 95 insertions(+), 42 deletions(-)
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index cb48ce5014ba..a20a6af6e0ef 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -277,6 +277,18 @@ struct kmem_cache {
>  	struct kmem_cache_node *node[MAX_NUMNODES];
>  };
>  
> +/*
> + * Every cache has !NULL s->cpu_sheaves but they may point to the
> + * bootstrap_sheaf temporarily during init, or permanently for the boot caches
> + * and caches with debugging enabled, or all caches with CONFIG_SLUB_TINY. This
> + * helper distinguishes whether cache has real non-bootstrap sheaves.
> + */
> +static inline bool cache_has_sheaves(struct kmem_cache *s)
> +{
> +	/* Test CONFIG_SLUB_TINY for code elimination purposes */
> +	return !IS_ENABLED(CONFIG_SLUB_TINY) && s->sheaf_capacity;
> +}
> +
>  #if defined(CONFIG_SYSFS) && !defined(CONFIG_SLUB_TINY)
>  #define SLAB_SUPPORTS_SYSFS 1
>  void sysfs_slab_unlink(struct kmem_cache *s);
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 5c15a4ce5743..8d0d6b0cb896 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -2163,7 +2163,7 @@ EXPORT_SYMBOL_GPL(kvfree_rcu_barrier);
>   */
>  void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
>  {
> -	if (s->cpu_sheaves) {
> +	if (cache_has_sheaves(s)) {
>  		flush_rcu_sheaves_on_cache(s);
>  		rcu_barrier();
>  	}
> diff --git a/mm/slub.c b/mm/slub.c
> index 594f5fac39b3..41e1bf35707c 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2846,12 +2846,23 @@ static void pcs_destroy(struct kmem_cache *s)
>  {
>  	int cpu;
>  
> +	/*
> +	 * We may be unwinding cache creation that failed before or during the
> +	 * allocation of this.
> +	 */
> +	if (!s->cpu_sheaves)
> +		return;
> +
> +	/* pcs->main can only point to the bootstrap sheaf, nothing to free */
> +	if (!cache_has_sheaves(s))
> +		goto free_pcs;
> +
>  	for_each_possible_cpu(cpu) {
>  		struct slub_percpu_sheaves *pcs;
>  
>  		pcs = per_cpu_ptr(s->cpu_sheaves, cpu);
>  
> -		/* can happen when unwinding failed create */
> +		/* This can happen when unwinding failed cache creation. */
>  		if (!pcs->main)
>  			continue;
>  
> @@ -2873,6 +2884,7 @@ static void pcs_destroy(struct kmem_cache *s)
>  		}
>  	}
>  
> +free_pcs:
>  	free_percpu(s->cpu_sheaves);
>  	s->cpu_sheaves = NULL;
>  }
> @@ -4030,7 +4042,7 @@ static bool has_pcs_used(int cpu, struct kmem_cache *s)
>  {
>  	struct slub_percpu_sheaves *pcs;
>  
> -	if (!s->cpu_sheaves)
> +	if (!cache_has_sheaves(s))
>  		return false;
>  
>  	pcs = per_cpu_ptr(s->cpu_sheaves, cpu);
> @@ -4052,7 +4064,7 @@ static void flush_cpu_slab(struct work_struct *w)
>  
>  	s = sfw->s;
>  
> -	if (s->cpu_sheaves)
> +	if (cache_has_sheaves(s))
>  		pcs_flush_all(s);
>  
>  	flush_this_cpu_slab(s);
> @@ -4157,7 +4169,7 @@ void flush_all_rcu_sheaves(void)
>  	mutex_lock(&slab_mutex);
>  
>  	list_for_each_entry(s, &slab_caches, list) {
> -		if (!s->cpu_sheaves)
> +		if (!cache_has_sheaves(s))
>  			continue;
>  		flush_rcu_sheaves_on_cache(s);
>  	}
> @@ -4179,7 +4191,7 @@ static int slub_cpu_dead(unsigned int cpu)
>  	mutex_lock(&slab_mutex);
>  	list_for_each_entry(s, &slab_caches, list) {
>  		__flush_cpu_slab(s, cpu);
> -		if (s->cpu_sheaves)
> +		if (cache_has_sheaves(s))
>  			__pcs_flush_all_cpu(s, cpu);
>  	}
>  	mutex_unlock(&slab_mutex);
> @@ -4979,6 +4991,12 @@ __pcs_replace_empty_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs,
>  
>  	lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
>  
> +	/* Bootstrap or debug cache, back off */
> +	if (unlikely(!cache_has_sheaves(s))) {
> +		local_unlock(&s->cpu_sheaves->lock);
> +		return NULL;
> +	}
> +
>  	if (pcs->spare && pcs->spare->size > 0) {
>  		swap(pcs->main, pcs->spare);
>  		return pcs;
> @@ -5165,6 +5183,11 @@ unsigned int alloc_from_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
>  		struct slab_sheaf *full;
>  		struct node_barn *barn;
>  
> +		if (unlikely(!cache_has_sheaves(s))) {
> +			local_unlock(&s->cpu_sheaves->lock);
> +			return allocated;
> +		}
> +
>  		if (pcs->spare && pcs->spare->size > 0) {
>  			swap(pcs->main, pcs->spare);
>  			goto do_alloc;
> @@ -5244,8 +5267,7 @@ static __fastpath_inline void *slab_alloc_node(struct kmem_cache *s, struct list
>  	if (unlikely(object))
>  		goto out;
>  
> -	if (s->cpu_sheaves)
> -		object = alloc_from_pcs(s, gfpflags, node);
> +	object = alloc_from_pcs(s, gfpflags, node);
>  
>  	if (!object)
>  		object = __slab_alloc_node(s, gfpflags, node, addr, orig_size);
> @@ -5353,18 +5375,10 @@ kmem_cache_prefill_sheaf(struct kmem_cache *s, gfp_t gfp, unsigned int size)
>  	struct slab_sheaf *sheaf = NULL;
>  	struct node_barn *barn;
>  
> -	if (unlikely(size > s->sheaf_capacity)) {
> +	if (unlikely(!size))
> +		return NULL;
>  
> -		/*
> -		 * slab_debug disables cpu sheaves intentionally so all
> -		 * prefilled sheaves become "oversize" and we give up on
> -		 * performance for the debugging. Same with SLUB_TINY.
> -		 * Creating a cache without sheaves and then requesting a
> -		 * prefilled sheaf is however not expected, so warn.
> -		 */
> -		WARN_ON_ONCE(s->sheaf_capacity == 0 &&
> -			     !IS_ENABLED(CONFIG_SLUB_TINY) &&
> -			     !(s->flags & SLAB_DEBUG_FLAGS));
> +	if (unlikely(size > s->sheaf_capacity)) {
>  
>  		sheaf = kzalloc(struct_size(sheaf, objects, size), gfp);
>  		if (!sheaf)
> @@ -6082,6 +6096,12 @@ __pcs_replace_full_main(struct kmem_cache *s, struct slub_percpu_sheaves *pcs)
>  restart:
>  	lockdep_assert_held(this_cpu_ptr(&s->cpu_sheaves->lock));
>  
> +	/* Bootstrap or debug cache, back off */
> +	if (unlikely(!cache_has_sheaves(s))) {
> +		local_unlock(&s->cpu_sheaves->lock);
> +		return NULL;
> +	}
> +
>  	barn = get_barn(s);
>  	if (!barn) {
>  		local_unlock(&s->cpu_sheaves->lock);
> @@ -6295,6 +6315,12 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
>  		struct slab_sheaf *empty;
>  		struct node_barn *barn;
>  
> +		/* Bootstrap or debug cache, fall back */
> +		if (unlikely(!cache_has_sheaves(s))) {
> +			local_unlock(&s->cpu_sheaves->lock);
> +			goto fail;
> +		}
> +
>  		if (pcs->spare && pcs->spare->size == 0) {
>  			pcs->rcu_free = pcs->spare;
>  			pcs->spare = NULL;
> @@ -6691,9 +6717,8 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
>  	if (unlikely(!slab_free_hook(s, object, slab_want_init_on_free(s), false)))
>  		return;
>  
> -	if (s->cpu_sheaves && likely(!IS_ENABLED(CONFIG_NUMA) ||
> -				     slab_nid(slab) == numa_mem_id())
> -			   && likely(!slab_test_pfmemalloc(slab))) {
> +	if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id())
> +	    && likely(!slab_test_pfmemalloc(slab))) {
>  		if (likely(free_to_pcs(s, object)))
>  			return;
>  	}
> @@ -7396,7 +7421,7 @@ void kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p)
>  	 * freeing to sheaves is so incompatible with the detached freelist so
>  	 * once we go that way, we have to do everything differently
>  	 */
> -	if (s && s->cpu_sheaves) {
> +	if (s && cache_has_sheaves(s)) {
>  		free_to_pcs_bulk(s, size, p);
>  		return;
>  	}
> @@ -7507,8 +7532,7 @@ int kmem_cache_alloc_bulk_noprof(struct kmem_cache *s, gfp_t flags, size_t size,
>  		size--;
>  	}
>  
> -	if (s->cpu_sheaves)
> -		i = alloc_from_pcs_bulk(s, size, p);
> +	i = alloc_from_pcs_bulk(s, size, p);
>  
>  	if (i < size) {
>  		/*
> @@ -7719,6 +7743,7 @@ static inline int alloc_kmem_cache_cpus(struct kmem_cache *s)
>  
>  static int init_percpu_sheaves(struct kmem_cache *s)
>  {
> +	static struct slab_sheaf bootstrap_sheaf = {};
>  	int cpu;
>  
>  	for_each_possible_cpu(cpu) {
> @@ -7728,7 +7753,28 @@ static int init_percpu_sheaves(struct kmem_cache *s)
>  
>  		local_trylock_init(&pcs->lock);
>  
> -		pcs->main = alloc_empty_sheaf(s, GFP_KERNEL);
> +		/*
> +		 * Bootstrap sheaf has zero size so fast-path allocation fails.
> +		 * It has also size == s->sheaf_capacity, so fast-path free
> +		 * fails. In the slow paths we recognize the situation by
> +		 * checking s->sheaf_capacity. This allows fast paths to assume
> +		 * s->cpu_sheaves and pcs->main always exists and are valid.
> +		 * It's also safe to share the single static bootstrap_sheaf
> +		 * with zero-sized objects array as it's never modified.
> +		 *
> +		 * Bootstrap_sheaf also has NULL pointer to kmem_cache so we
> +		 * recognize it and not attempt to free it when destroying the
> +		 * cache.
> +		 *
> +		 * We keep bootstrap_sheaf for kmem_cache and kmem_cache_node,
> +		 * caches with debug enabled, and all caches with SLUB_TINY.
> +		 * For kmalloc caches it's used temporarily during the initial
> +		 * bootstrap.
> +		 */
> +		if (!s->sheaf_capacity)
> +			pcs->main = &bootstrap_sheaf;
> +		else
> +			pcs->main = alloc_empty_sheaf(s, GFP_KERNEL);
>  
>  		if (!pcs->main)
>  			return -ENOMEM;
> @@ -7803,8 +7849,7 @@ static void free_kmem_cache_nodes(struct kmem_cache *s)
>  void __kmem_cache_release(struct kmem_cache *s)
>  {
>  	cache_random_seq_destroy(s);
> -	if (s->cpu_sheaves)
> -		pcs_destroy(s);
> +	pcs_destroy(s);
>  #ifdef CONFIG_PREEMPT_RT
>  	if (s->cpu_slab)
>  		lockdep_unregister_key(&s->lock_key);
> @@ -7826,7 +7871,7 @@ static int init_kmem_cache_nodes(struct kmem_cache *s)
>  			continue;
>  		}
>  
> -		if (s->cpu_sheaves) {
> +		if (cache_has_sheaves(s)) {
>  			barn = kmalloc_node(sizeof(*barn), GFP_KERNEL, node);
>  
>  			if (!barn)
> @@ -8149,7 +8194,7 @@ int __kmem_cache_shutdown(struct kmem_cache *s)
>  	flush_all_cpus_locked(s);
>  
>  	/* we might have rcu sheaves in flight */
> -	if (s->cpu_sheaves)
> +	if (cache_has_sheaves(s))
>  		rcu_barrier();
>  
>  	/* Attempt to free all objects */
> @@ -8461,7 +8506,7 @@ static int slab_mem_going_online_callback(int nid)
>  		if (get_node(s, nid))
>  			continue;
>  
> -		if (s->cpu_sheaves) {
> +		if (cache_has_sheaves(s)) {
>  			barn = kmalloc_node(sizeof(*barn), GFP_KERNEL, nid);
>  
>  			if (!barn) {
> @@ -8669,12 +8714,10 @@ int do_kmem_cache_create(struct kmem_cache *s, const char *name,
>  
>  	set_cpu_partial(s);
>  
> -	if (s->sheaf_capacity) {
> -		s->cpu_sheaves = alloc_percpu(struct slub_percpu_sheaves);
> -		if (!s->cpu_sheaves) {
> -			err = -ENOMEM;
> -			goto out;
> -		}
> +	s->cpu_sheaves = alloc_percpu(struct slub_percpu_sheaves);
> +	if (!s->cpu_sheaves) {
> +		err = -ENOMEM;
> +		goto out;
>  	}
>  
>  #ifdef CONFIG_NUMA
> @@ -8693,11 +8736,9 @@ int do_kmem_cache_create(struct kmem_cache *s, const char *name,
>  	if (!alloc_kmem_cache_cpus(s))
>  		goto out;
>  
> -	if (s->cpu_sheaves) {
> -		err = init_percpu_sheaves(s);
> -		if (err)
> -			goto out;
> -	}
> +	err = init_percpu_sheaves(s);
> +	if (err)
> +		goto out;
>  
>  	err = 0;
>  
> 
> -- 
> 2.52.0
> 
> 

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