Re: [PATCH RFC 08/19] slab: handle kmalloc sheaves bootstrap

[Harry Yoo]

On Thu, Oct 23, 2025 at 03:52:30PM +0200, Vlastimil Babka wrote:
> Enable sheaves for kmalloc caches. For other types than KMALLOC_NORMAL,
> we can simply allow them in calculate_sizes() as they are created later
> than KMALLOC_NORMAL caches and can allocate sheaves and barns from
> those.
> 
> For KMALLOC_NORMAL caches we perform additional step after first
> creating them without sheaves. Then bootstrap_cache_sheaves() simply
> allocates and initializes barns and sheaves and finally sets
> s->sheaf_capacity to make them actually used.
> 
> Afterwards the only caches left without sheaves (unless SLUB_TINY or
> debugging is enabled) are kmem_cache and kmem_cache_node. These are only
> used when creating or destroying other kmem_caches. Thus they are not
> performance critical and we can simply leave it that way.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  mm/slub.c | 88 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 84 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 5d0b2cf66520..a84027fbca78 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
>
>  static void free_empty_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf)
>  {
>  	kfree(sheaf);
> @@ -8064,8 +8071,11 @@ static int calculate_sizes(struct kmem_cache_args *args, struct kmem_cache *s)
>  	if (s->flags & SLAB_RECLAIM_ACCOUNT)
>  		s->allocflags |= __GFP_RECLAIMABLE;
>  
> -	/* kmalloc caches need extra care to support sheaves */
> -	if (!is_kmalloc_cache(s))
> +	/*
> +	 * For KMALLOC_NORMAL caches we enable sheaves later by
> +	 * bootstrap_kmalloc_sheaves() to avoid recursion
> +	 */
> +	if (!is_kmalloc_normal(s))
>  		s->sheaf_capacity = calculate_sheaf_capacity(s, args);

I was going to say we should differentiate KMALLOC_NORMAL caches that
are created for kmalloc buckets.... but no, they don't have the SLAB_KMALLOC
flag.

>  	/*
> @@ -8549,6 +8559,74 @@ static struct kmem_cache * __init bootstrap(struct kmem_cache *static_cache)
>  	return s;
>  }
>  
> +/*
> + * Finish the sheaves initialization done normally by init_percpu_sheaves() and
> + * init_kmem_cache_nodes(). For normal kmalloc caches we have to bootstrap it
> + * since sheaves and barns are allocated by kmalloc.
> + */
> +static void __init bootstrap_cache_sheaves(struct kmem_cache *s)
> +{
> +	struct kmem_cache_args empty_args = {};
> +	unsigned int capacity;
> +	bool failed = false;
> +	int node, cpu;
> +
> +	capacity = calculate_sheaf_capacity(s, &empty_args);
> +
> +	/* capacity can be 0 due to debugging or SLUB_TINY */
> +	if (!capacity)
> +		return;

I think pcs->main should still be !NULL in this case?

-- 
Cheers,
Harry / Hyeonggon

> +
> +	for_each_node_mask(node, slab_nodes) {
> +		struct node_barn *barn;
> +
> +		barn = kmalloc_node(sizeof(*barn), GFP_KERNEL, node);
> +
> +		if (!barn) {
> +			failed = true;
> +			goto out;
> +		}
> +
> +		barn_init(barn);
> +		get_node(s, node)->barn = barn;
> +	}
> +
> +	for_each_possible_cpu(cpu) {
> +		struct slub_percpu_sheaves *pcs;
> +
> +		pcs = per_cpu_ptr(s->cpu_sheaves, cpu);
> +
> +		pcs->main = __alloc_empty_sheaf(s, GFP_KERNEL, capacity);
> +
> +		if (!pcs->main) {
> +			failed = true;
> +			break;
> +		}
> +	}
> +
> +out:
> +	/*
> +	 * It's still early in boot so treat this like same as a failure to
> +	 * create the kmalloc cache in the first place
> +	 */
> +	if (failed)
> +		panic("Out of memory when creating kmem_cache %s\n", s->name);
> +
> +	s->sheaf_capacity = capacity;
> +}