Re: [PATCH] nodemask: use nr_node_ids instead of MAX_NUMNODES in __nodemask_pr_numnodes()

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 9 Jul 2026 15:17:39 +0800 lirongqing <lirongqing@baidu.com> wrote:

> From: Li RongQing <lirongqing@baidu.com>
> 
> __nodemask_pr_numnodes() returns MAX_NUMNODES, a compile-time constant,
> as the width for '%*pb[l]' nodemask printing. On systems with few NUMA
> nodes this produces excessive zero-padding, e.g. with MAX_NUMNODES=1024
> but only 2 nodes:
> 
>   Mems_allowed: 00000000,00000000,...,00000003  (32 groups)

Please let's completely describe the effects of this. 
nodemask_pr_args() is used in several places so I believe that a number
of userspace-facing strings will now appear differently?  Let's spell
out which userspace interfaces are affected, and how.

> Use nr_node_ids (the runtime highest-node-id + 1) instead, matching the
> behavior of cpumask_pr_args() which uses nr_cpu_ids:
> 
>   Mems_allowed: 00000003
> 
> Move the nr_node_ids declaration earlier in the file (guarded by
> __nodemask_pr_numnodes(), and remove the now-duplicate declarations.
> 
> ...
>
> --- a/include/linux/nodemask.h
> +++ b/include/linux/nodemask.h
> @@ -95,6 +95,12 @@
>  
>  extern nodemask_t _unused_nodemask_arg_;
>  
> +#if MAX_NUMNODES > 1
> +extern unsigned int nr_node_ids;
> +#else
> +#define nr_node_ids		1U
> +#endif
> +
>  /**
>   * nodemask_pr_args - printf args to output a nodemask
>   * @maskp: nodemask to be printed
> @@ -105,7 +111,7 @@ extern nodemask_t _unused_nodemask_arg_;
>  				__nodemask_pr_bits(maskp)
>  static __always_inline unsigned int __nodemask_pr_numnodes(const nodemask_t *m)
>  {
> -	return m ? MAX_NUMNODES : 0;
> +	return m ? nr_node_ids : 0;
>  }
>  static __always_inline const unsigned long *__nodemask_pr_bits(const nodemask_t *m)
>  {
> @@ -438,7 +444,6 @@ static __always_inline unsigned int next_memory_node(int nid)
>  	return next_node(nid, node_states[N_MEMORY]);
>  }
>  
> -extern unsigned int nr_node_ids;
>  extern unsigned int nr_online_nodes;

Let's relocate nr_online_nodes also?  To keep them together.

>  static __always_inline void node_set_online(int nid)
> @@ -480,7 +485,6 @@ static __always_inline int num_node_state(enum node_states state)
>  #define first_memory_node	0
>  #define next_online_node(nid)	(MAX_NUMNODES)
>  #define next_memory_node(nid)	(MAX_NUMNODES)
> -#define nr_node_ids		1U
>  #define nr_online_nodes		1U
>  
>  #define node_set_online(node)	   node_set_state((node), N_ONLINE)
> -- 
> 2.9.4