Extend the sheaf infrastructure for more efficient kfree_rcu() handling.
For caches with sheaves, on each cpu maintain a rcu_free sheaf in
addition to main and spare sheaves.

kfree_rcu() operations will try to put objects on this sheaf. Once full,
the sheaf is detached and submitted to call_rcu() with a handler that
will try to put it in the barn, or flush to slab pages using bulk free,
when the barn is full. Then a new empty sheaf must be obtained to put
more objects there.

It's possible that no free sheaves are available to use for a new
rcu_free sheaf, and the allocation in kfree_rcu() context can only use
GFP_NOWAIT and thus may fail. In that case, fall back to the existing
kfree_rcu() implementation.

Expected advantages:
- batching the kfree_rcu() operations, that could eventually replace the
  existing batching
- sheaves can be reused for allocations via barn instead of being
  flushed to slabs, which is more efficient
  - this includes cases where only some cpus are allowed to process rcu
    callbacks (Android)

Possible disadvantage:
- objects might be waiting for more than their grace period (it is
  determined by the last object freed into the sheaf), increasing memory
  usage - but the existing batching does that too.

Only implement this for CONFIG_KVFREE_RCU_BATCHED as the tiny
implementation favors smaller memory footprint over performance.

Add CONFIG_SLUB_STATS counters free_rcu_sheaf and free_rcu_sheaf_fail to
count how many kfree_rcu() used the rcu_free sheaf successfully and how
many had to fall back to the existing implementation.

Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slab.h        |   2 +
 mm/slab_common.c |  24 +++++++
 mm/slub.c        | 193 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 214 insertions(+), 5 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index 1980330c2fcb4a4613a7e4f7efc78b349993fd89..44c9b70eaabbd87c06fb39b79dfb791d515acbde 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -459,6 +459,8 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s)
 	return !(s->flags & (SLAB_CACHE_DMA|SLAB_ACCOUNT|SLAB_RECLAIM_ACCOUNT));
 }
 
+bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj);
+
 #define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | \
 			 SLAB_CACHE_DMA32 | SLAB_PANIC | \
 			 SLAB_TYPESAFE_BY_RCU | SLAB_DEBUG_OBJECTS | \
diff --git a/mm/slab_common.c b/mm/slab_common.c
index e2b197e47866c30acdbd1fee4159f262a751c5a7..2d806e02568532a1000fd3912db6978e945dcfa8 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1608,6 +1608,27 @@ static void kfree_rcu_work(struct work_struct *work)
 		kvfree_rcu_list(head);
 }
 
+static bool kfree_rcu_sheaf(void *obj)
+{
+	struct kmem_cache *s;
+	struct folio *folio;
+	struct slab *slab;
+
+	if (is_vmalloc_addr(obj))
+		return false;
+
+	folio = virt_to_folio(obj);
+	if (unlikely(!folio_test_slab(folio)))
+		return false;
+
+	slab = folio_slab(folio);
+	s = slab->slab_cache;
+	if (s->cpu_sheaves)
+		return __kfree_rcu_sheaf(s, obj);
+
+	return false;
+}
+
 static bool
 need_offload_krc(struct kfree_rcu_cpu *krcp)
 {
@@ -1952,6 +1973,9 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
 	if (!head)
 		might_sleep();
 
+	if (kfree_rcu_sheaf(ptr))
+		return;
+
 	// Queue the object but don't yet schedule the batch.
 	if (debug_rcu_head_queue(ptr)) {
 		// Probable double kfree_rcu(), just leak.
diff --git a/mm/slub.c b/mm/slub.c
index 6543aaade60b0adaab232b2256d65c1042c62e1c..f6d86cd3983533784583f1df6add186c4a74cd97 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -350,6 +350,8 @@ enum stat_item {
 	ALLOC_FASTPATH,		/* Allocation from cpu slab */
 	ALLOC_SLOWPATH,		/* Allocation by getting a new cpu slab */
 	FREE_PCS,		/* Free to percpu sheaf */
+	FREE_RCU_SHEAF,		/* Free to rcu_free sheaf */
+	FREE_RCU_SHEAF_FAIL,	/* Failed to free to a rcu_free sheaf */
 	FREE_FASTPATH,		/* Free to cpu slab */
 	FREE_SLOWPATH,		/* Freeing not to cpu slab */
 	FREE_FROZEN,		/* Freeing to frozen slab */
@@ -444,6 +446,7 @@ struct slab_sheaf {
 		struct rcu_head rcu_head;
 		struct list_head barn_list;
 	};
+	struct kmem_cache *cache;
 	unsigned int size;
 	void *objects[];
 };
@@ -452,6 +455,7 @@ struct slub_percpu_sheaves {
 	local_trylock_t lock;
 	struct slab_sheaf *main; /* never NULL when unlocked */
 	struct slab_sheaf *spare; /* empty or full, may be NULL */
+	struct slab_sheaf *rcu_free; /* for batching kfree_rcu() */
 	struct node_barn *barn;
 };
 
@@ -2490,6 +2494,8 @@ static struct slab_sheaf *alloc_empty_sheaf(struct kmem_cache *s, gfp_t gfp)
 	if (unlikely(!sheaf))
 		return NULL;
 
+	sheaf->cache = s;
+
 	stat(s, SHEAF_ALLOC);
 
 	return sheaf;
@@ -2614,6 +2620,43 @@ static void sheaf_flush_unused(struct kmem_cache *s, struct slab_sheaf *sheaf)
 	sheaf->size = 0;
 }
 
+static void __rcu_free_sheaf_prepare(struct kmem_cache *s,
+				     struct slab_sheaf *sheaf)
+{
+	bool init = slab_want_init_on_free(s);
+	void **p = &sheaf->objects[0];
+	unsigned int i = 0;
+
+	while (i < sheaf->size) {
+		struct slab *slab = virt_to_slab(p[i]);
+
+		memcg_slab_free_hook(s, slab, p + i, 1);
+		alloc_tagging_slab_free_hook(s, slab, p + i, 1);
+
+		if (unlikely(!slab_free_hook(s, p[i], init, true))) {
+			p[i] = p[--sheaf->size];
+			continue;
+		}
+
+		i++;
+	}
+}
+
+static void rcu_free_sheaf_nobarn(struct rcu_head *head)
+{
+	struct slab_sheaf *sheaf;
+	struct kmem_cache *s;
+
+	sheaf = container_of(head, struct slab_sheaf, rcu_head);
+	s = sheaf->cache;
+
+	__rcu_free_sheaf_prepare(s, sheaf);
+
+	sheaf_flush_unused(s, sheaf);
+
+	free_empty_sheaf(s, sheaf);
+}
+
 /*
  * Caller needs to make sure migration is disabled in order to fully flush
  * single cpu's sheaves
@@ -2626,7 +2669,7 @@ static void sheaf_flush_unused(struct kmem_cache *s, struct slab_sheaf *sheaf)
 static void pcs_flush_all(struct kmem_cache *s)
 {
 	struct slub_percpu_sheaves *pcs;
-	struct slab_sheaf *spare;
+	struct slab_sheaf *spare, *rcu_free;
 
 	local_lock(&s->cpu_sheaves->lock);
 	pcs = this_cpu_ptr(s->cpu_sheaves);
@@ -2634,6 +2677,9 @@ static void pcs_flush_all(struct kmem_cache *s)
 	spare = pcs->spare;
 	pcs->spare = NULL;
 
+	rcu_free = pcs->rcu_free;
+	pcs->rcu_free = NULL;
+
 	local_unlock(&s->cpu_sheaves->lock);
 
 	if (spare) {
@@ -2641,6 +2687,9 @@ static void pcs_flush_all(struct kmem_cache *s)
 		free_empty_sheaf(s, spare);
 	}
 
+	if (rcu_free)
+		call_rcu(&rcu_free->rcu_head, rcu_free_sheaf_nobarn);
+
 	sheaf_flush_main(s);
 }
 
@@ -2657,6 +2706,11 @@ static void __pcs_flush_all_cpu(struct kmem_cache *s, unsigned int cpu)
 		free_empty_sheaf(s, pcs->spare);
 		pcs->spare = NULL;
 	}
+
+	if (pcs->rcu_free) {
+		call_rcu(&pcs->rcu_free->rcu_head, rcu_free_sheaf_nobarn);
+		pcs->rcu_free = NULL;
+	}
 }
 
 static void pcs_destroy(struct kmem_cache *s)
@@ -2682,6 +2736,7 @@ static void pcs_destroy(struct kmem_cache *s)
 		 */
 
 		WARN_ON(pcs->spare);
+		WARN_ON(pcs->rcu_free);
 
 		if (!WARN_ON(pcs->main->size)) {
 			free_empty_sheaf(s, pcs->main);
@@ -3742,7 +3797,7 @@ static bool has_pcs_used(int cpu, struct kmem_cache *s)
 
 	pcs = per_cpu_ptr(s->cpu_sheaves, cpu);
 
-	return (pcs->spare || pcs->main->size);
+	return (pcs->spare || pcs->rcu_free || pcs->main->size);
 }
 
 static void pcs_flush_all(struct kmem_cache *s);
@@ -5339,6 +5394,127 @@ bool free_to_pcs(struct kmem_cache *s, void *object)
 	return true;
 }
 
+static void rcu_free_sheaf(struct rcu_head *head)
+{
+	struct slab_sheaf *sheaf;
+	struct node_barn *barn;
+	struct kmem_cache *s;
+
+	sheaf = container_of(head, struct slab_sheaf, rcu_head);
+
+	s = sheaf->cache;
+
+	/*
+	 * This may remove some objects due to slab_free_hook() returning false,
+	 * so that the sheaf might no longer be completely full. But it's easier
+	 * to handle it as full (unless it became completely empty), as the code
+	 * handles it fine. The only downside is that sheaf will serve fewer
+	 * allocations when reused. It only happens due to debugging, which is a
+	 * performance hit anyway.
+	 */
+	__rcu_free_sheaf_prepare(s, sheaf);
+
+	barn = get_node(s, numa_mem_id())->barn;
+
+	/* due to slab_free_hook() */
+	if (unlikely(sheaf->size == 0))
+		goto empty;
+
+	/*
+	 * Checking nr_full/nr_empty outside lock avoids contention in case the
+	 * barn is at the respective limit. Due to the race we might go over the
+	 * limit but that should be rare and harmless.
+	 */
+
+	if (data_race(barn->nr_full) < MAX_FULL_SHEAVES) {
+		stat(s, BARN_PUT);
+		barn_put_full_sheaf(barn, sheaf);
+		return;
+	}
+
+	stat(s, BARN_PUT_FAIL);
+	sheaf_flush_unused(s, sheaf);
+
+empty:
+	if (data_race(barn->nr_empty) < MAX_EMPTY_SHEAVES) {
+		barn_put_empty_sheaf(barn, sheaf);
+		return;
+	}
+
+	free_empty_sheaf(s, sheaf);
+}
+
+bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
+{
+	struct slub_percpu_sheaves *pcs;
+	struct slab_sheaf *rcu_sheaf;
+
+	if (!local_trylock(&s->cpu_sheaves->lock))
+		goto fail;
+
+	pcs = this_cpu_ptr(s->cpu_sheaves);
+
+	if (unlikely(!pcs->rcu_free)) {
+
+		struct slab_sheaf *empty;
+
+		if (pcs->spare && pcs->spare->size == 0) {
+			pcs->rcu_free = pcs->spare;
+			pcs->spare = NULL;
+			goto do_free;
+		}
+
+		empty = barn_get_empty_sheaf(pcs->barn);
+
+		if (empty) {
+			pcs->rcu_free = empty;
+			goto do_free;
+		}
+
+		local_unlock(&s->cpu_sheaves->lock);
+
+		empty = alloc_empty_sheaf(s, GFP_NOWAIT);
+
+		if (!empty)
+			goto fail;
+
+		if (!local_trylock(&s->cpu_sheaves->lock)) {
+			barn_put_empty_sheaf(pcs->barn, empty);
+			goto fail;
+		}
+
+		pcs = this_cpu_ptr(s->cpu_sheaves);
+
+		if (unlikely(pcs->rcu_free))
+			barn_put_empty_sheaf(pcs->barn, empty);
+		else
+			pcs->rcu_free = empty;
+	}
+
+do_free:
+
+	rcu_sheaf = pcs->rcu_free;
+
+	rcu_sheaf->objects[rcu_sheaf->size++] = obj;
+
+	if (likely(rcu_sheaf->size < s->sheaf_capacity))
+		rcu_sheaf = NULL;
+	else
+		pcs->rcu_free = NULL;
+
+	local_unlock(&s->cpu_sheaves->lock);
+
+	if (rcu_sheaf)
+		call_rcu(&rcu_sheaf->rcu_head, rcu_free_sheaf);
+
+	stat(s, FREE_RCU_SHEAF);
+	return true;
+
+fail:
+	stat(s, FREE_RCU_SHEAF_FAIL);
+	return false;
+}
+
 /*
  * Bulk free objects to the percpu sheaves.
  * Unlike free_to_pcs() this includes the calls to all necessary hooks
@@ -5348,10 +5524,8 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
 {
 	struct slub_percpu_sheaves *pcs;
 	struct slab_sheaf *main, *empty;
+	bool init = slab_want_init_on_free(s);
 	unsigned int batch, i = 0;
-	bool init;
-
-	init = slab_want_init_on_free(s);
 
 	while (i < size) {
 		struct slab *slab = virt_to_slab(p[i]);
@@ -6838,6 +7012,11 @@ int __kmem_cache_shutdown(struct kmem_cache *s)
 	struct kmem_cache_node *n;
 
 	flush_all_cpus_locked(s);
+
+	/* we might have rcu sheaves in flight */
+	if (s->cpu_sheaves)
+		rcu_barrier();
+
 	/* Attempt to free all objects */
 	for_each_kmem_cache_node(s, node, n) {
 		if (n->barn)
@@ -8251,6 +8430,8 @@ STAT_ATTR(ALLOC_PCS, alloc_cpu_sheaf);
 STAT_ATTR(ALLOC_FASTPATH, alloc_fastpath);
 STAT_ATTR(ALLOC_SLOWPATH, alloc_slowpath);
 STAT_ATTR(FREE_PCS, free_cpu_sheaf);
+STAT_ATTR(FREE_RCU_SHEAF, free_rcu_sheaf);
+STAT_ATTR(FREE_RCU_SHEAF_FAIL, free_rcu_sheaf_fail);
 STAT_ATTR(FREE_FASTPATH, free_fastpath);
 STAT_ATTR(FREE_SLOWPATH, free_slowpath);
 STAT_ATTR(FREE_FROZEN, free_frozen);
@@ -8349,6 +8530,8 @@ static struct attribute *slab_attrs[] = {
 	&alloc_fastpath_attr.attr,
 	&alloc_slowpath_attr.attr,
 	&free_cpu_sheaf_attr.attr,
+	&free_rcu_sheaf_attr.attr,
+	&free_rcu_sheaf_fail_attr.attr,
 	&free_fastpath_attr.attr,
 	&free_slowpath_attr.attr,
 	&free_frozen_attr.attr,

-- 
2.50.1

On Wed, Jul 23, 2025 at 03:34:35PM +0200, Vlastimil Babka wrote:
> Extend the sheaf infrastructure for more efficient kfree_rcu() handling.
> For caches with sheaves, on each cpu maintain a rcu_free sheaf in
> addition to main and spare sheaves.
> 
> kfree_rcu() operations will try to put objects on this sheaf. Once full,
> the sheaf is detached and submitted to call_rcu() with a handler that
> will try to put it in the barn, or flush to slab pages using bulk free,
> when the barn is full. Then a new empty sheaf must be obtained to put
> more objects there.
> 
> It's possible that no free sheaves are available to use for a new
> rcu_free sheaf, and the allocation in kfree_rcu() context can only use
> GFP_NOWAIT and thus may fail. In that case, fall back to the existing
> kfree_rcu() implementation.
> 
> Expected advantages:
> - batching the kfree_rcu() operations, that could eventually replace the
>   existing batching
> - sheaves can be reused for allocations via barn instead of being
>   flushed to slabs, which is more efficient
>   - this includes cases where only some cpus are allowed to process rcu
>     callbacks (Android)
> 
> Possible disadvantage:
> - objects might be waiting for more than their grace period (it is
>   determined by the last object freed into the sheaf), increasing memory
>   usage - but the existing batching does that too.
> 
> Only implement this for CONFIG_KVFREE_RCU_BATCHED as the tiny
> implementation favors smaller memory footprint over performance.
> 
> Add CONFIG_SLUB_STATS counters free_rcu_sheaf and free_rcu_sheaf_fail to
> count how many kfree_rcu() used the rcu_free sheaf successfully and how
> many had to fall back to the existing implementation.
> 
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/slab.h        |   2 +
>  mm/slab_common.c |  24 +++++++
>  mm/slub.c        | 193 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  3 files changed, 214 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index 1980330c2fcb4a4613a7e4f7efc78b349993fd89..44c9b70eaabbd87c06fb39b79dfb791d515acbde 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -459,6 +459,8 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s)
>  	return !(s->flags & (SLAB_CACHE_DMA|SLAB_ACCOUNT|SLAB_RECLAIM_ACCOUNT));
>  }
>  
> +bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj);
> +
>  #define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | \
>  			 SLAB_CACHE_DMA32 | SLAB_PANIC | \
>  			 SLAB_TYPESAFE_BY_RCU | SLAB_DEBUG_OBJECTS | \
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index e2b197e47866c30acdbd1fee4159f262a751c5a7..2d806e02568532a1000fd3912db6978e945dcfa8 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1608,6 +1608,27 @@ static void kfree_rcu_work(struct work_struct *work)
>  		kvfree_rcu_list(head);
>  }
>  
> +static bool kfree_rcu_sheaf(void *obj)
> +{
> +	struct kmem_cache *s;
> +	struct folio *folio;
> +	struct slab *slab;
> +
> +	if (is_vmalloc_addr(obj))
> +		return false;
> +
> +	folio = virt_to_folio(obj);
> +	if (unlikely(!folio_test_slab(folio)))
> +		return false;
> +
> +	slab = folio_slab(folio);
> +	s = slab->slab_cache;
> +	if (s->cpu_sheaves)
> +		return __kfree_rcu_sheaf(s, obj);
> +
> +	return false;
> +}
> +
>  static bool
>  need_offload_krc(struct kfree_rcu_cpu *krcp)
>  {
> @@ -1952,6 +1973,9 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
>  	if (!head)
>  		might_sleep();
>  
> +	if (kfree_rcu_sheaf(ptr))
> +		return;
> +
>
I have a question here. kfree_rcu_sheaf(ptr) tries to revert freeing
an object over one more newly introduced path. This patch adds infra
for such purpose whereas we already have a main path over which we
free memory.

Why do not we use existing logic? As i see you can do:

   if (unlikely(!slab_free_hook(s, p[i], init, true))) {
        p[i] = p[--sheaf->size];
        continue;
   }

in the kfree_rcu_work() function where we process all ready to free objects.
I mean, for slab objects we can replace kfree_bulk() and scan all pointers
and free them over slab_free_hook().

Also we do use a pooled API and other improvements to speed up freeing.

Thanks!

--
Uladzislau Rezki

On 7/23/25 18:39, Uladzislau Rezki wrote:
> On Wed, Jul 23, 2025 at 03:34:35PM +0200, Vlastimil Babka wrote:
>>  static bool
>>  need_offload_krc(struct kfree_rcu_cpu *krcp)
>>  {
>> @@ -1952,6 +1973,9 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
>>  	if (!head)
>>  		might_sleep();
>>  
>> +	if (kfree_rcu_sheaf(ptr))
>> +		return;
>> +
>>
> I have a question here. kfree_rcu_sheaf(ptr) tries to revert freeing
> an object over one more newly introduced path. This patch adds infra
> for such purpose whereas we already have a main path over which we
> free memory.
> 
> Why do not we use existing logic? As i see you can do:
> 
>    if (unlikely(!slab_free_hook(s, p[i], init, true))) {
>         p[i] = p[--sheaf->size];
>         continue;
>    }
> 
> in the kfree_rcu_work() function where we process all ready to free objects.

I'm not sure I understand. In kfree_rcu_work() we process individual
objects. There is no sheaf that you reference in the code above?
Or are you suggesting we add e.g. a "channel" of sheaves to process in
addition to the existing channels of objects?

> I mean, for slab objects we can replace kfree_bulk() and scan all pointers
> and free them over slab_free_hook().

The desired outcome after __rcu_free_sheaf_prepare() is to take the whole
sheaf and have it reused, not free individual objects. So we call
slab_free_hook() in __rcu_free_sheaf_prepare() but don't actually free
individual objects as necessary.

> Also we do use a pooled API and other improvements to speed up freeing.

It could be useful to know the details as in Suren's measurements there's
issues with kfree_rcu() using sheaves when lazy rcu is used. Is the
kfree_rcu() infra avoiding being too lazy somehow? We could use the same
techniques for sheaves.

> Thanks!
> 
> --
> Uladzislau Rezki

On Thu, Jul 24, 2025 at 04:30:49PM +0200, Vlastimil Babka wrote:
> On 7/23/25 18:39, Uladzislau Rezki wrote:
> > On Wed, Jul 23, 2025 at 03:34:35PM +0200, Vlastimil Babka wrote:
> >>  static bool
> >>  need_offload_krc(struct kfree_rcu_cpu *krcp)
> >>  {
> >> @@ -1952,6 +1973,9 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
> >>  	if (!head)
> >>  		might_sleep();
> >>  
> >> +	if (kfree_rcu_sheaf(ptr))
> >> +		return;
> >> +
> >>
> > I have a question here. kfree_rcu_sheaf(ptr) tries to revert freeing
> > an object over one more newly introduced path. This patch adds infra
> > for such purpose whereas we already have a main path over which we
> > free memory.
> > 
> > Why do not we use existing logic? As i see you can do:
> > 
> >    if (unlikely(!slab_free_hook(s, p[i], init, true))) {
> >         p[i] = p[--sheaf->size];
> >         continue;
> >    }
> > 
> > in the kfree_rcu_work() function where we process all ready to free objects.
> 
> I'm not sure I understand. In kfree_rcu_work() we process individual
> objects. There is no sheaf that you reference in the code above?
> Or are you suggesting we add e.g. a "channel" of sheaves to process in
> addition to the existing channels of objects?
> 
There is no that above "sheaf" code. I put it just for reference.
I suggested to put such objects into regular existing channels and
process them. But for that purpose we need to check each SLAB object 
because currently we can free them over kfree_bulk().

A separate channel can also be maintained but it would add more logic
on top but at least it would consolidate the freeing path and use one
RCU machinery.

From the other hand what else can we free? You have a code in your patch:

	if (is_vmalloc_addr(obj))
		return false;

	folio = virt_to_folio(obj);
	if (unlikely(!folio_test_slab(folio)))
		return false;

vmalloc pointers go its own way, others are SLAB. What else can it be?
i.e. folio_test_slab() checks if obj->folio is part of the SLAB objects.
Can it return zero?

> > I mean, for slab objects we can replace kfree_bulk() and scan all pointers
> > and free them over slab_free_hook().
> 
> The desired outcome after __rcu_free_sheaf_prepare() is to take the whole
> sheaf and have it reused, not free individual objects. So we call
> slab_free_hook() in __rcu_free_sheaf_prepare() but don't actually free
> individual objects as necessary.
> 
I see.

> > Also we do use a pooled API and other improvements to speed up freeing.
> 
> It could be useful to know the details as in Suren's measurements there's
> issues with kfree_rcu() using sheaves when lazy rcu is used. Is the
> kfree_rcu() infra avoiding being too lazy somehow? We could use the same
> techniques for sheaves.
> 
I think, it is because your patch uses call_rcu() and not call_rcu_harry().
There is one more tricky part, it is about how long rcu_free_sheaf()
callback executes, because there are other callbacks in a queue which
can wait its time.

kfree_rcu() infra does not use call_rcu() chain because it can be slow.
We can delay a process of freed objects if an array of pointers is not
yet full. When a first object is added we arm the timer to kick the
process in 5 seconds. Once an array becomes full the logic switches into
a fast mode, reprogram a timer to trigger a process asap.

Also, this patch creates a collision because it goes its own way. We
have a kvfree_rcu_barrier() which becomes broken if this patch applied?

--
Uladzislau Rezki