[PATCH 5/6] mm/migration: export demotion_path of a node via sysfs

[Davidlohr Bueso <dave@stgolabs.net>]

Add a /sys/devices/system/node/nodeX/demotion_path file
to export the possible target(s) in node_demotion[node].

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
---
 Documentation/ABI/stable/sysfs-devices-node |  6 ++++
 drivers/base/node.c                         | 39 +++++++++++++++++++++
 include/linux/migrate.h                     | 15 ++++++++
 mm/migrate.c                                | 15 +-------
 4 files changed, 61 insertions(+), 14 deletions(-)

diff --git a/Documentation/ABI/stable/sysfs-devices-node b/Documentation/ABI/stable/sysfs-devices-node
index 3c935e1334f7..f620c6ae013c 100644
--- a/Documentation/ABI/stable/sysfs-devices-node
+++ b/Documentation/ABI/stable/sysfs-devices-node
@@ -192,3 +192,9 @@ Description:
 		When it completes successfully, the specified amount or more memory
 		will have been reclaimed, and -EAGAIN if less bytes are reclaimed
 		than the specified amount.
+
+What:		/sys/devices/system/node/nodeX/demotion_path
+Date:		April 2022
+Contact:	Davidlohr Bueso <dave@stgolabs.net>
+Description:
+		Shows nodes within the next tier of slower memory below this node.
diff --git a/drivers/base/node.c b/drivers/base/node.c
index d80c478e2a6e..ab4bae777535 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -17,6 +17,7 @@
 #include <linux/nodemask.h>
 #include <linux/cpu.h>
 #include <linux/device.h>
+#include <linux/migrate.h>
 #include <linux/pm_runtime.h>
 #include <linux/swap.h>
 #include <linux/slab.h>
@@ -560,11 +561,49 @@ static ssize_t node_read_distance(struct device *dev,
 }
 static DEVICE_ATTR(distance, 0444, node_read_distance, NULL);
 
+static ssize_t node_read_demotion_path(struct device *dev,
+				       struct device_attribute *attr, char *buf)
+{
+	int nid = dev->id;
+	int len = 0;
+	int i;
+	struct demotion_nodes *nd;
+
+	/*
+	 * buf is currently PAGE_SIZE in length and each node needs 4 chars
+	 * at the most (target + space or newline).
+	 */
+	BUILD_BUG_ON(MAX_NUMNODES * 4 > PAGE_SIZE);
+
+	if (!node_demotion) {
+		len += sysfs_emit_at(buf, len, "%d", NUMA_NO_NODE);
+		goto done;
+	}
+
+	nd = &node_demotion[nid];
+
+	rcu_read_lock();
+	if (nd->nr == 0)
+		len += sysfs_emit_at(buf, len, "%d", NUMA_NO_NODE);
+	else {
+		for (i = 0; i < nd->nr; i++) {
+			len += sysfs_emit_at(buf, len, "%s%d",
+					     i ? " " : "", nd->nodes[i]);
+		}
+	}
+	rcu_read_unlock();
+done:
+	len += sysfs_emit_at(buf, len, "\n");
+	return len;
+}
+static DEVICE_ATTR(demotion_path, 0444, node_read_demotion_path, NULL);
+
 static struct attribute *node_dev_attrs[] = {
 	&dev_attr_meminfo.attr,
 	&dev_attr_numastat.attr,
 	&dev_attr_distance.attr,
 	&dev_attr_vmstat.attr,
+	&dev_attr_demotion_path.attr,
 	NULL
 };
 
diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 90e75d5a54d6..b0ac6a717e44 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -111,6 +111,21 @@ static inline int migrate_misplaced_page(struct page *page,
 }
 #endif /* CONFIG_NUMA_BALANCING */
 
+#define DEFAULT_DEMOTION_TARGET_NODES 15
+
+#if MAX_NUMNODES < DEFAULT_DEMOTION_TARGET_NODES
+#define DEMOTION_TARGET_NODES	(MAX_NUMNODES - 1)
+#else
+#define DEMOTION_TARGET_NODES	DEFAULT_DEMOTION_TARGET_NODES
+#endif
+
+struct demotion_nodes {
+	unsigned short nr;
+	short nodes[DEMOTION_TARGET_NODES];
+};
+
+extern struct demotion_nodes *node_demotion __read_mostly;
+
 #ifdef CONFIG_MIGRATION
 
 /*
diff --git a/mm/migrate.c b/mm/migrate.c
index 6c31ee1e1c9b..e47ea25fcfe8 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2172,20 +2172,7 @@ int migrate_misplaced_page(struct page *page, struct vm_area_struct *vma,
  * must be held over all reads to ensure that no cycles are
  * observed.
  */
-#define DEFAULT_DEMOTION_TARGET_NODES 15
-
-#if MAX_NUMNODES < DEFAULT_DEMOTION_TARGET_NODES
-#define DEMOTION_TARGET_NODES	(MAX_NUMNODES - 1)
-#else
-#define DEMOTION_TARGET_NODES	DEFAULT_DEMOTION_TARGET_NODES
-#endif
-
-struct demotion_nodes {
-	unsigned short nr;
-	short nodes[DEMOTION_TARGET_NODES];
-};
-
-static struct demotion_nodes *node_demotion __read_mostly;
+struct demotion_nodes *node_demotion __read_mostly;
 
 /**
  * next_demotion_node() - Get the next node in the demotion path
-- 
2.26.2