Re: [PATCH] mm/slub: deduplicate NUMA policy calculation in allocation paths

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

On Thu, Jun 18, 2026 at 06:08:52PM +0800, Hao Li wrote:
> Currently, alloc_from_pcs() and __slab_alloc_node() both calculate the
> NUMA policy independently. Since they are called consecutively in paths
> like __kmalloc_nolock_noprof() and slab_alloc_node(), this leads to
> redundant computations.
> 
> Introduce a helper function to resolve the NUMA policy once, eliminating
> the duplicated code and reducing execution overhead.
> 
> Signed-off-by: Hao Li <hao.li@linux.dev>
> ---
>  mm/slub.c | 72 ++++++++++++++++++++++---------------------------------
>  1 file changed, 29 insertions(+), 43 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 62e9cd46916f..45e9f379b7da 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4523,32 +4523,36 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>  	return object;
>  }
>  
> +static __always_inline int apply_numa_policy(int node)
> +{
> +#ifdef CONFIG_NUMA
> +	if (static_branch_unlikely(&strict_numa) &&
> +			node == NUMA_NO_NODE) {
> +
> +		struct mempolicy *mpol = current->mempolicy;
> +
> +		if (mpol) {
> +			/*
> +			 * Special BIND rule support. If the local node
> +			 * is in permitted set then do not redirect
> +			 * to a particular node.
> +			 * Otherwise we apply the memory policy to get
> +			 * the node we need to allocate on.
> +			 */
> +			if (mpol->mode != MPOL_BIND ||
> +					!node_isset(numa_mem_id(), mpol->nodes))
> +				node = mempolicy_slab_node();
> +		}
> +	}
> +#endif
> +	return node;
> +}
> +
>  static void *__slab_alloc_node(struct kmem_cache *s, gfp_t gfpflags, int node,
>  			       const struct slab_alloc_context *ac)
>  {
>  	void *object;
>  
> -#ifdef CONFIG_NUMA
> -	if (static_branch_unlikely(&strict_numa) &&
> -			node == NUMA_NO_NODE) {
> -
> -		struct mempolicy *mpol = current->mempolicy;
> -
> -		if (mpol) {
> -			/*
> -			 * Special BIND rule support. If the local node
> -			 * is in permitted set then do not redirect
> -			 * to a particular node.
> -			 * Otherwise we apply the memory policy to get
> -			 * the node we need to allocate on.
> -			 */
> -			if (mpol->mode != MPOL_BIND ||
> -					!node_isset(numa_mem_id(), mpol->nodes))
> -				node = mempolicy_slab_node();
> -		}
> -	}
> -#endif
> -
>  	object = ___slab_alloc(s, gfpflags, node, ac);
>  
>  	return object;

oh!, __slab_alloc_node is almost an empty wrapper
Maybe I need to eliminate it completely...

> @@ -4756,28 +4760,6 @@ void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp, unsigned int alloc_flags,
>  	bool node_requested;
>  	void *object;
>  
> -#ifdef CONFIG_NUMA
> -	if (static_branch_unlikely(&strict_numa) &&
> -			 node == NUMA_NO_NODE) {
> -
> -		struct mempolicy *mpol = current->mempolicy;
> -
> -		if (mpol) {
> -			/*
> -			 * Special BIND rule support. If the local node
> -			 * is in permitted set then do not redirect
> -			 * to a particular node.
> -			 * Otherwise we apply the memory policy to get
> -			 * the node we need to allocate on.
> -			 */
> -			if (mpol->mode != MPOL_BIND ||
> -					!node_isset(numa_mem_id(), mpol->nodes))
> -
> -				node = mempolicy_slab_node();
> -		}
> -	}
> -#endif
> -
>  	node_requested = IS_ENABLED(CONFIG_NUMA) && node != NUMA_NO_NODE;
>  
>  	/*
> @@ -4927,6 +4909,8 @@ static __fastpath_inline void *slab_alloc_node(struct kmem_cache *s,
>  	if (unlikely(object))
>  		goto out;
>  
> +	node = apply_numa_policy(node);
> +
>  	object = alloc_from_pcs(s, gfpflags, ac->alloc_flags, node);
>  
>  	if (unlikely(!object))
> @@ -5430,6 +5414,8 @@ static void *__kmalloc_nolock_noprof(DECL_TOKEN_PARAMS(size, token), gfp_t gfp_f
>  		 */
>  		return NULL;
>  
> +	node = apply_numa_policy(node);
> +
>  	ret = alloc_from_pcs(s, gfp_flags, ac->alloc_flags, node);
>  	if (ret)
>  		goto success;
> -- 
> 2.54.0
>