[RFC PATCH v4 6/7] mm/demotion: Add support for removing node from demotion memory tiers

["Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx>]

This patch adds the special string "none" as a supported memtier value
that we can use to remove a specific node from being using as demotion target.

For ex:
:/sys/devices/system/node/node1# cat memtier
1
:/sys/devices/system/node/node1# cat ../../memtier/memtier1/nodelist
1-3
:/sys/devices/system/node/node1# echo none > memtier
:/sys/devices/system/node/node1#
:/sys/devices/system/node/node1# cat memtier
:/sys/devices/system/node/node1# cat ../../memtier/memtier1/nodelist
2-3
:/sys/devices/system/node/node1#

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
---
 drivers/base/node.c     |  7 ++++++-
 include/linux/migrate.h |  1 +
 mm/migrate.c            | 15 +++++++++++++--
 3 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/drivers/base/node.c b/drivers/base/node.c
index 892f7c23c94e..5311cf1db500 100644
--- a/drivers/base/node.c
+++ b/drivers/base/node.c
@@ -578,10 +578,15 @@ static ssize_t memtier_store(struct device *dev,
 			     struct device_attribute *attr,
 			     const char *buf, size_t count)
 {
+	int ret;
 	unsigned long tier;
 	int node = dev->id;
 
-	int ret = kstrtoul(buf, 10, &tier);
+	if (!strncmp(buf, "none", strlen("none"))) {
+		node_remove_from_memory_tier(node);
+		return count;
+	}
+	ret = kstrtoul(buf, 10, &tier);
 	if (ret)
 		return ret;
 
diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index fd09fd009a69..77c581f47953 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -178,6 +178,7 @@ enum memory_tier_type {
 #define MAX_MEMORY_TIERS  3
 
 int next_demotion_node(int node);
+void node_remove_from_memory_tier(int node);
 int node_get_memory_tier_id(int node);
 int node_set_memory_tier_rank(int node, int tier);
 int node_reset_memory_tier(int node, int tier);
diff --git a/mm/migrate.c b/mm/migrate.c
index f013d14f77ed..114c7428b9f3 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2354,7 +2354,7 @@ static struct memory_tier *__get_memory_tier_from_id(int id)
 }
 
 
-static void node_remove_from_memory_tier(int node)
+void node_remove_from_memory_tier(int node)
 {
 	struct memory_tier *memtier;
 
@@ -2418,7 +2418,18 @@ int node_reset_memory_tier(int node, int tier)
 	mutex_lock(&memory_tier_lock);
 
 	current_tier = __node_get_memory_tier(node);
-	if (!current_tier || current_tier->dev.id == tier)
+	if (!current_tier) {
+		/*
+		 * If a N_MEMORY node doesn't have a tier index, then
+		 * we removed it from demotion earlier and we are trying
+		 * add it back. Just add the node to requested tier.
+		 */
+		if (node_state(node, N_MEMORY))
+			ret = __node_set_memory_tier(node, tier);
+		goto out;
+	}
+
+	if (current_tier->dev.id == tier)
 		goto out;
 
 	node_clear(node, current_tier->nodelist);
-- 
2.36.1