mm: hugetlb: eliminate memory-less nodes handling

The memory-notify-based approach aims to handle meory-less nodes, however,
it just adds the complexity of code as pointed by David in thread [1]. 
The handling of memory-less nodes is introduced by commit 4faf8d95
("hugetlb: handle memory hot-plug events").  >From its commit message, we
cannot find any necessity of handling this case.  So, we can simply
register/unregister sysfs entries in register_node/unregister_node to
simlify the code.

BTW, hotplug callback added because in hugetlb_register_all_nodes() we
register sysfs nodes only for N_MEMORY nodes, seeing commit 9b5e5d0f,
which said it was a preparation for handling memory-less nodes via memory
hotplug.  Since we want to remove memory hotplug, so make sure we only
register per-node sysfs for online (N_ONLINE) nodes in
hugetlb_register_all_nodes().

https://lore.kernel.org/linux-mm/60933ffc-b850-976c-78a0-0ee6e0ea9ef0@redhat.com/ [1]
Link: https://lkml.kernel.org/r/20220914072603.60293-3-songmuchun@bytedance.comSuggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/base/node.c

@@ -20,6 +20,7 @@
 #include <linux/pm_runtime.h>
 #include <linux/swap.h>
 #include <linux/slab.h>
+#include <linux/hugetlb.h>
 
 static struct bus_type node_subsys = {
 	.name = "node",
@@ -608,10 +609,12 @@ static int register_node(struct node *node, int num)
 	node->dev.groups = node_dev_groups;
 	error = device_register(&node->dev);
 
-	if (error)
+	if (error) {
 		put_device(&node->dev);
-	else
+	} else {
+		hugetlb_register_node(node);
 		compaction_register_node(node);
+	}
 
 	return error;
 }
@@ -625,6 +628,7 @@ static int register_node(struct node *node, int num)
  */
 void unregister_node(struct node *node)
 {
+	hugetlb_unregister_node(node);
 	compaction_unregister_node(node);
 	node_remove_accesses(node);
 	node_remove_caches(node);

include/linux/hugetlb.h

@@ -16,6 +16,7 @@
 struct ctl_table;
 struct user_struct;
 struct mmu_gather;
+struct node;
 
 #ifndef is_hugepd
 typedef struct { unsigned long pd; } hugepd_t;
@@ -935,6 +936,11 @@ static inline void huge_ptep_modify_prot_commit(struct vm_area_struct *vma,
 }
 #endif
 
+#ifdef CONFIG_NUMA
+void hugetlb_register_node(struct node *node);
+void hugetlb_unregister_node(struct node *node);
+#endif
+
 #else	/* CONFIG_HUGETLB_PAGE */
 struct hstate {};
 
@@ -1109,6 +1115,14 @@ static inline void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
 				   pte_t *ptep, pte_t pte)
 {
 }
+
+static inline void hugetlb_register_node(struct node *node)
+{
+}
+
+static inline void hugetlb_unregister_node(struct node *node)
+{
+}
 #endif	/* CONFIG_HUGETLB_PAGE */
 
 static inline spinlock_t *huge_pte_lock(struct hstate *h,

mm/hugetlb.c

@@ -3871,24 +3871,8 @@ static int hugetlb_sysfs_add_hstate(struct hstate *h, struct kobject *parent,
 	return 0;
 }
 
-static void __init hugetlb_sysfs_init(void)
-{
-	struct hstate *h;
-	int err;
-
-	hugepages_kobj = kobject_create_and_add("hugepages", mm_kobj);
-	if (!hugepages_kobj)
-		return;
-
-	for_each_hstate(h) {
-		err = hugetlb_sysfs_add_hstate(h, hugepages_kobj,
-					 hstate_kobjs, &hstate_attr_group);
-		if (err)
-			pr_err("HugeTLB: Unable to add hstate %s", h->name);
-	}
-}
-
 #ifdef CONFIG_NUMA
+static bool hugetlb_sysfs_initialized __ro_after_init;
 
 /*
  * node_hstate/s - associate per node hstate attributes, via their kobjects,
@@ -3944,7 +3928,7 @@ static struct hstate *kobj_to_node_hstate(struct kobject *kobj, int *nidp)
  * Unregister hstate attributes from a single node device.
  * No-op if no hstate attributes attached.
  */
-static void hugetlb_unregister_node(struct node *node)
+void hugetlb_unregister_node(struct node *node)
 {
 	struct hstate *h;
 	struct node_hstate *nhs = &node_hstates[node->dev.id];
@@ -3974,12 +3958,15 @@ static void hugetlb_unregister_node(struct node *node)
  * Register hstate attributes for a single node device.
  * No-op if attributes already registered.
  */
-static void hugetlb_register_node(struct node *node)
+void hugetlb_register_node(struct node *node)
 {
 	struct hstate *h;
 	struct node_hstate *nhs = &node_hstates[node->dev.id];
 	int err;
 
+	if (!hugetlb_sysfs_initialized)
+		return;
+
 	if (nhs->hugepages_kobj)
 		return;		/* already allocated */
 
@@ -4001,23 +3988,6 @@ static void hugetlb_register_node(struct node *node)
 	}
 }
 
-static int __meminit hugetlb_memory_callback(struct notifier_block *self,
-					     unsigned long action, void *arg)
-{
-	struct memory_notify *mnb = arg;
-	int nid = mnb->status_change_nid;
-
-	if (nid == NUMA_NO_NODE)
-		return NOTIFY_DONE;
-
-	if (action == MEM_GOING_ONLINE)
-		hugetlb_register_node(node_devices[nid]);
-	else if (action == MEM_CANCEL_ONLINE || action == MEM_OFFLINE)
-		hugetlb_unregister_node(node_devices[nid]);
-
-	return NOTIFY_OK;
-}
-
 /*
  * hugetlb init time:  register hstate attributes for all registered node
  * devices of nodes that have memory.  All on-line nodes should have
@@ -4027,11 +3997,8 @@ static void __init hugetlb_register_all_nodes(void)
 {
 	int nid;
 
-	get_online_mems();
-	hotplug_memory_notifier(hugetlb_memory_callback, 0);
-	for_each_node_state(nid, N_MEMORY)
+	for_each_online_node(nid)
 		hugetlb_register_node(node_devices[nid]);
-	put_online_mems();
 }
 #else	/* !CONFIG_NUMA */
 
@@ -4055,6 +4022,28 @@ static inline __init void hugetlb_cma_check(void)
 }
 #endif
 
+static void __init hugetlb_sysfs_init(void)
+{
+	struct hstate *h;
+	int err;
+
+	hugepages_kobj = kobject_create_and_add("hugepages", mm_kobj);
+	if (!hugepages_kobj)
+		return;
+
+	for_each_hstate(h) {
+		err = hugetlb_sysfs_add_hstate(h, hugepages_kobj,
+					 hstate_kobjs, &hstate_attr_group);
+		if (err)
+			pr_err("HugeTLB: Unable to add hstate %s", h->name);
+	}
+
+#ifdef CONFIG_NUMA
+	hugetlb_sysfs_initialized = true;
+#endif
+	hugetlb_register_all_nodes();
+}
+
 static int __init hugetlb_init(void)
 {
 	int i;
@@ -4109,7 +4098,6 @@ static int __init hugetlb_init(void)
 	report_hugepages();
 
 	hugetlb_sysfs_init();
-	hugetlb_register_all_nodes();
 	hugetlb_cgroup_file_init();
 
 #ifdef CONFIG_SMP