[RFC PATCH 1/4] mm/numa: introduce nearest_nodes_nodemask()

[Rakie Kim]

Add a new NUMA helper, nearest_nodes_nodemask(), to find all nodes in a
given nodemask that are located at the minimum distance from a specified
source node.

Unlike nearest_node_nodemask(), which returns only a single node, this
function identifies all nodes that share the closest distance value. This
is useful when multiple nodes are equally near in the NUMA topology and
a complete set of nearest candidates is required.

The helper clears the output nodemask and sets all nodes that meet the
minimum distance condition. It returns 0 on success or -EINVAL if the
output argument is invalid.

Signed-off-by: Rakie Kim <rakie.kim@xxxxxx>
---
 include/linux/numa.h |  8 ++++++++
 mm/mempolicy.c       | 41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/include/linux/numa.h b/include/linux/numa.h
index e6baaf6051bc..aa9526e9078b 100644
--- a/include/linux/numa.h
+++ b/include/linux/numa.h
@@ -33,6 +33,8 @@ int numa_nearest_node(int node, unsigned int state);
 
 int nearest_node_nodemask(int node, nodemask_t *mask);
 
+int nearest_nodes_nodemask(int node, const nodemask_t *mask, nodemask_t *out);
+
 #ifndef memory_add_physaddr_to_nid
 int memory_add_physaddr_to_nid(u64 start);
 #endif
@@ -54,6 +56,12 @@ static inline int nearest_node_nodemask(int node, nodemask_t *mask)
 	return NUMA_NO_NODE;
 }
 
+static inline int nearest_nodes_nodemask(int node, const nodemask_t *mask,
+					 nodemask_t *out)
+{
+	return 0;
+}
+
 static inline int memory_add_physaddr_to_nid(u64 start)
 {
 	return 0;
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 68a98ba57882..a3f0fde6c626 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -338,6 +338,47 @@ int nearest_node_nodemask(int node, nodemask_t *mask)
 }
 EXPORT_SYMBOL_GPL(nearest_node_nodemask);
 
+/**
+ * nearest_nodes_nodemask - Find all nodes in @mask that are nearest to @node
+ * @node: The reference node ID to measure distance from
+ * @mask: The set of candidate nodes to compare against
+ * @out:  Pointer to a nodemask that will store the nearest node(s)
+ *
+ * This function iterates over all nodes in @mask and measures the distance
+ * between each candidate node and the given @node using node_distance().
+ * It finds the minimum distance and then records all nodes in @mask that
+ * share that same minimum distance into the output mask @out.
+ *
+ * For example, if multiple nodes have equal minimal distance to @node, all
+ * of them are included in @out.
+ *
+ * Return: 0 on success, or -EINVAL if @out is NULL.
+ */
+int nearest_nodes_nodemask(int node, const nodemask_t *mask, nodemask_t *out)
+{
+	int dist, n, min_dist = INT_MAX;
+
+	if (!out)
+		return -EINVAL;
+
+	nodes_clear(*out);
+
+	for_each_node_mask(n, *mask) {
+		dist = node_distance(node, n);
+
+		if (dist < min_dist) {
+			min_dist = dist;
+			nodes_clear(*out);
+			node_set(n, *out);
+		} else if (dist == min_dist) {
+			node_set(n, *out);
+		}
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(nearest_nodes_nodemask);
+
 struct mempolicy *get_task_policy(struct task_struct *p)
 {
 	struct mempolicy *pol = p->mempolicy;
-- 
2.34.1