[PATCH v8 08/12] mm/demotion: Add pg_data_t member to track node memory tier details

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

Also update different helpes to use NODE_DATA()->memtier. Since
node specific memtier can change based on the reassignment of
NUMA node to a different memory tiers, accessing NODE_DATA()->memtier
needs to happen under an rcu read lock or memory_tier_lock.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
---
 include/linux/memory-tiers.h |  11 ++++
 include/linux/mmzone.h       |   3 +
 mm/memory-tiers.c            | 104 +++++++++++++++++++++++++----------
 3 files changed, 89 insertions(+), 29 deletions(-)

diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
index 453f6e5d357c..705b63ee31d5 100644
--- a/include/linux/memory-tiers.h
+++ b/include/linux/memory-tiers.h
@@ -6,6 +6,9 @@
 
 #ifdef CONFIG_NUMA
 
+#include <linux/device.h>
+#include <linux/nodemask.h>
+
 #define MEMORY_TIER_HBM_GPU	300
 #define MEMORY_TIER_DRAM	200
 #define MEMORY_TIER_PMEM	100
@@ -13,6 +16,12 @@
 #define DEFAULT_MEMORY_TIER	MEMORY_TIER_DRAM
 #define MAX_MEMORY_TIER_ID	400
 
+struct memory_tier {
+	struct list_head list;
+	struct device dev;
+	nodemask_t nodelist;
+};
+
 extern bool numa_demotion_enabled;
 int node_create_and_set_memory_tier(int node, int tier);
 #ifdef CONFIG_MIGRATION
@@ -25,6 +34,8 @@ static inline int next_demotion_node(int node)
 #endif
 int node_get_memory_tier_id(int node);
 int node_update_memory_tier(int node, int tier);
+struct memory_tier *node_get_memory_tier(int node);
+void node_put_memory_tier(struct memory_tier *memtier);
 
 #else
 
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index aab70355d64f..353812495a70 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -928,6 +928,9 @@ typedef struct pglist_data {
 	/* Per-node vmstats */
 	struct per_cpu_nodestat __percpu *per_cpu_nodestats;
 	atomic_long_t		vm_stat[NR_VM_NODE_STAT_ITEMS];
+#ifdef CONFIG_NUMA
+	struct memory_tier __rcu *memtier;
+#endif
 } pg_data_t;
 
 #define node_present_pages(nid)	(NODE_DATA(nid)->node_present_pages)
diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
index b7cb368cb9c0..6a2476faf13a 100644
--- a/mm/memory-tiers.c
+++ b/mm/memory-tiers.c
@@ -1,22 +1,15 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/types.h>
-#include <linux/device.h>
-#include <linux/nodemask.h>
 #include <linux/slab.h>
 #include <linux/lockdep.h>
 #include <linux/moduleparam.h>
 #include <linux/memory.h>
 #include <linux/random.h>
+#include <linux/rcupdate.h>
 #include <linux/memory-tiers.h>
 
 #include "internal.h"
 
-struct memory_tier {
-	struct list_head list;
-	struct device dev;
-	nodemask_t nodelist;
-};
-
 struct demotion_nodes {
 	nodemask_t preferred;
 };
@@ -120,7 +113,7 @@ static void memory_tier_device_release(struct device *dev)
 {
 	struct memory_tier *tier = to_memory_tier(dev);
 
-	kfree(tier);
+	kfree_rcu(tier);
 }
 
 static void insert_memory_tier(struct memory_tier *memtier)
@@ -176,13 +169,18 @@ static void unregister_memory_tier(struct memory_tier *memtier)
 
 static struct memory_tier *__node_get_memory_tier(int node)
 {
-	struct memory_tier *memtier;
+	pg_data_t *pgdat;
 
-	list_for_each_entry(memtier, &memory_tiers, list) {
-		if (node_isset(node, memtier->nodelist))
-			return memtier;
-	}
-	return NULL;
+	pgdat = NODE_DATA(node);
+	if (!pgdat)
+		return NULL;
+	/*
+	 * Since we hold memory_tier_lock, we can avoid
+	 * RCU read locks when accessing the details. No
+	 * parallel updates are possible here.
+	 */
+	return rcu_dereference_check(pgdat->memtier,
+				     lockdep_is_held(&memory_tier_lock));
 }
 
 static struct memory_tier *__get_memory_tier_from_id(int id)
@@ -196,6 +194,33 @@ static struct memory_tier *__get_memory_tier_from_id(int id)
 	return NULL;
 }
 
+/*
+ * Called with memory_tier_lock. Hence the device references cannot
+ * be dropped during this function.
+ */
+static void memtier_node_set(int node, struct memory_tier *memtier)
+{
+	pg_data_t *pgdat;
+	struct memory_tier *current_memtier;
+
+	pgdat = NODE_DATA(node);
+	if (!pgdat)
+		return;
+	/*
+	 * Make sure we mark the memtier NULL before we assign the new memory tier
+	 * to the NUMA node. This make sure that anybody looking at NODE_DATA
+	 * finds a NULL memtier or the one which is still valid.
+	 */
+	current_memtier = rcu_dereference_check(pgdat->memtier,
+						lockdep_is_held(&memory_tier_lock));
+	rcu_assign_pointer(pgdat->memtier, NULL);
+	if (current_memtier)
+		node_clear(node, current_memtier->nodelist);
+	synchronize_rcu();
+	node_set(node, memtier->nodelist);
+	rcu_assign_pointer(pgdat->memtier, memtier);
+}
+
 static int __node_create_and_set_memory_tier(int node, int tier)
 {
 	int ret = 0;
@@ -209,7 +234,7 @@ static int __node_create_and_set_memory_tier(int node, int tier)
 			goto out;
 		}
 	}
-	node_set(node, memtier->nodelist);
+	memtier_node_set(node, memtier);
 out:
 	return ret;
 }
@@ -231,14 +256,7 @@ int node_create_and_set_memory_tier(int node, int tier)
 	if (current_tier->dev.id == tier)
 		goto out;
 
-	node_clear(node, current_tier->nodelist);
-
 	ret = __node_create_and_set_memory_tier(node, tier);
-	if (ret) {
-		/* reset it back to older tier */
-		node_set(node, current_tier->nodelist);
-		goto out;
-	}
 	if (nodes_empty(current_tier->nodelist))
 		unregister_memory_tier(current_tier);
 
@@ -260,7 +278,7 @@ static int __node_set_memory_tier(int node, int tier)
 		ret = -EINVAL;
 		goto out;
 	}
-	node_set(node, memtier->nodelist);
+	memtier_node_set(node, memtier);
 out:
 	return ret;
 }
@@ -316,10 +334,7 @@ int node_update_memory_tier(int node, int tier)
 	if (!current_tier || current_tier->dev.id == tier)
 		goto out;
 
-	node_clear(node, current_tier->nodelist);
-
 	ret = __node_create_and_set_memory_tier(node, tier);
-
 	if (nodes_empty(current_tier->nodelist))
 		unregister_memory_tier(current_tier);
 
@@ -330,6 +345,34 @@ int node_update_memory_tier(int node, int tier)
 	return ret;
 }
 
+/*
+ * lockless access to memory tier of a NUMA node.
+ */
+struct memory_tier *node_get_memory_tier(int node)
+{
+	pg_data_t *pgdat;
+	struct memory_tier *memtier;
+
+	pgdat = NODE_DATA(node);
+	if (!pgdat)
+		return NULL;
+
+	rcu_read_lock();
+	memtier = rcu_dereference(pgdat->memtier);
+	if (!memtier)
+		goto out;
+
+	get_device(&memtier->dev);
+out:
+	rcu_read_unlock();
+	return memtier;
+}
+
+void node_put_memory_tier(struct memory_tier *memtier)
+{
+	put_device(&memtier->dev);
+}
+
 #ifdef CONFIG_MIGRATION
 /**
  * next_demotion_node() - Get the next node in the demotion path
@@ -546,7 +589,7 @@ static const struct attribute_group *memory_tier_attr_groups[] = {
 
 static int __init memory_tier_init(void)
 {
-	int ret;
+	int ret, node;
 	struct memory_tier *memtier;
 
 	ret = subsys_system_register(&memory_tier_subsys, memory_tier_attr_groups);
@@ -567,7 +610,10 @@ static int __init memory_tier_init(void)
 		      __func__, PTR_ERR(memtier));
 
 	/* CPU only nodes are not part of memory tiers. */
-	memtier->nodelist = node_states[N_MEMORY];
+	for_each_node_state(node, N_MEMORY) {
+		rcu_assign_pointer(NODE_DATA(node)->memtier, memtier);
+		node_set(node, memtier->nodelist);
+	}
 	mutex_unlock(&memory_tier_lock);
 
 	migrate_on_reclaim_init();
-- 
2.36.1