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

[lirongqing <lirongqing@baidu.com>]

From: Li RongQing <lirongqing@baidu.com>

__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)

- 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@gmail.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Signed-off-by: Li RongQing <lirongqing@baidu.com>
---
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