Re: [PATCH 1/3] slab: decouple pointer to barn from kmem_cache_node

[Harry Yoo <harry.yoo@oracle.com>]

On Wed, Mar 11, 2026 at 09:25:55AM +0100, Vlastimil Babka (SUSE) wrote:
> The pointer to barn currently exists in struct kmem_cache_node. That
> struct is instantiated for every NUMA node with memory, but we want to
> have a barn for every online node (including memoryless).
> 
> Thus decouple the two structures. In struct kmem_cache we have an array
> for kmem_cache_node pointers that appears to be sized MAX_NUMNODES but
> the actual size calculation in kmem_cache_init() uses nr_node_ids.
> Therefore we can't just add another array of barn pointers. Instead
> change the array to newly introduced struct kmem_cache_per_node_ptrs
> holding both kmem_cache_node and barn pointer.
> 
> Adjust barn accessor and allocation/initialization code accordingly. For
> now no functional change intended, barns are created 1:1 together with
> kmem_cache_nodes.
> 
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> ---
>  mm/slab.h |   7 +++-
>  mm/slub.c | 128 +++++++++++++++++++++++++++++++++++---------------------------
>  2 files changed, 78 insertions(+), 57 deletions(-)
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index e9ab292acd22..c735e6b4dddb 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -247,7 +252,7 @@ struct kmem_cache {
>  	struct kmem_cache_stats __percpu *cpu_stats;
>  #endif
>  
> -	struct kmem_cache_node *node[MAX_NUMNODES];
> +	struct kmem_cache_per_node_ptrs per_node[MAX_NUMNODES];
>  };

We should probably turn this into a true flexible array at some point,
but that's out of scope for this patchset.

> diff --git a/mm/slub.c b/mm/slub.c
> index 20cb4f3b636d..609a183f8533 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -436,26 +436,24 @@ struct kmem_cache_node {
>  /*
> - * Get the barn of the current cpu's closest memory node. It may not exist on
> - * systems with memoryless nodes but without CONFIG_HAVE_MEMORYLESS_NODES
> + * Get the barn of the current cpu's memory node. It may be a memoryless node.
>   */
>  static inline struct node_barn *get_barn(struct kmem_cache *s)
>  {
> -	struct kmem_cache_node *n = get_node(s, numa_mem_id());
> -
> -	if (!n)
> -		return NULL;
> -
> -	return n->barn;
> +	return get_barn_node(s, numa_node_id());
>  }

Previously, memoryless nodes on architectures w/ CONFIG_HAVE_MEMORYLESS_NODES
shared the barn of the nearest NUMA node with memory.

But now memoryless nodes will have their own barns (after patch 2)
regardless of CONFIG_HAVE_MEMORYLESS_NODES, and that's intentional, right?

Otherwise LGTM!

-- 
Cheers,
Harry / Hyeonggon