Re: [PATCH v2] nodemask: reduce bitmap width to nr_node_ids in __nodemask_pr_numnodes()

[Yury Norov]

On Mon, Jul 13, 2026 at 03:51:04PM +0800, lirongqing wrote:
> From: Li RongQing <lirongqing@xxxxxxxxx>
> 
> __nodemask_pr_numnodes() currently returns MAX_NUMNODES as the field
> width for '%*pb[l]' nodemask printing. MAX_NUMNODES is a compile-time
> upper bound and can be much larger than the runtime node id range,
> resulting in excessive zero padding in bitmap-form output.
> 
> For example, /proc/<pid>/status prints Mems_allowed with '%*pb' using
> the nodemask_pr_args() helper. On systems built with MAX_NUMNODES=1024
> but booted with a much smaller possible-node range, this produces:
> 
>   Mems_allowed: 00000000,00000000,...,00000003
> 
> Switch to nr_node_ids, matching the behavior of cpumask_pr_args() which
> uses nr_cpu_ids. This reduces the output width from MAX_NUMNODES bits
> to the runtime node id range:
> 
>   Mems_allowed: 3
> 
> Visible impact on in-tree users:
> - Bitmap format ('%*pb') users:
>   * /proc/<pid>/status Mems_allowed (format changes as shown above)

OK, let me take it for testing. If no complains from users, it's a
nice cleanup.
 
> - List format ('%*pbl') users, output is unchanged, as list formatter
>   only prints set bit ranges:
>   * /sys/devices/system/node/{possible,online,has_normal_memory, ...}
>   * NVMe multipath sysfs numa_nodes
>   * memory tier sysfs nodelist
>   * cpuset cgroup mems and effective_mems files
>   * /proc/<pid>/status Mems_allowed_list
>   * mempolicy strings in /proc/<pid>/numa_maps
>   * SLUB debugfs output
>   * Kernel log messages printing nodemasks
> 
> Move nr_node_ids and nr_online_nodes declarations earlier in the file
> to allow __nodemask_pr_numnodes() to use nr_node_ids.
> 
> Cc: Yury Norov <yury.norov@xxxxxxxxx>
> Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
> ---
> Changes from v1:
> - List the unaffected '%*pbl' users explicitly, noting that their
>   visible output is unchanged since the list formatter only prints
>   set bit ranges regardless of field width.
> - Move the nr_node_ids and nr_online_nodes declarations earlier 
> 
> include/linux/nodemask.h | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/nodemask.h b/include/linux/nodemask.h
> index b842aa5..607373b 100644
> --- a/include/linux/nodemask.h
> +++ b/include/linux/nodemask.h
> @@ -95,6 +95,14 @@
>  
>  extern nodemask_t _unused_nodemask_arg_;
>  
> +#if MAX_NUMNODES > 1
> +extern unsigned int nr_node_ids;
> +extern unsigned int nr_online_nodes;
> +#else
> +#define nr_node_ids		1U
> +#define nr_online_nodes		1U
> +#endif
> +
>  /**
>   * nodemask_pr_args - printf args to output a nodemask
>   * @maskp: nodemask to be printed
> @@ -105,7 +113,7 @@ 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)
>  {
> @@ -438,9 +446,6 @@ 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;
> -
>  static __always_inline void node_set_online(int nid)
>  {
>  	node_set_state(nid, N_ONLINE);
> @@ -480,8 +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)
>  #define node_set_offline(node)	   node_clear_state((node), N_ONLINE)
> -- 
> 2.9.4