mm: add build-time option for hotplug memory default online type

Memory hotplug presently auto-onlines memory into a zone the kernel deems
appropriate if CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE=y.

The memhp_default_state boot param enables runtime config, but it's not
possible to do this at build-time.

Remove CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE, and replace it with
CONFIG_MHP_DEFAULT_ONLINE_TYPE_* choices that sync with the boot param.

Selections:
  CONFIG_MHP_DEFAULT_ONLINE_TYPE_OFFLINE
    => mhp_default_online_type = "offline"
       Memory will not be onlined automatically.

  CONFIG_MHP_DEFAULT_ONLINE_TYPE_ONLINE_AUTO
    => mhp_default_online_type = "online"
       Memory will be onlined automatically in a zone deemed.
       appropriate by the kernel.

  CONFIG_MHP_DEFAULT_ONLINE_TYPE_ONLINE_KERNEL
    => mhp_default_online_type = "online_kernel"
       Memory will be onlined automatically.
       The zone may allow kernel data (e.g. ZONE_NORMAL).

  CONFIG_MHP_DEFAULT_ONLINE_TYPE_ONLINE_MOVABLE
    => mhp_default_online_type = "online_movable"
       Memory will be onlined automatically.
       The zone will be ZONE_MOVABLE.

Default to CONFIG_MHP_DEFAULT_ONLINE_TYPE_OFFLINE to match the existing
default CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE=n behavior.

Existing users of CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE=y should use
CONFIG_MHP_DEFAULT_ONLINE_TYPE_ONLINE_AUTO.

[gourry@gourry.net: update KConfig comments]
  Link: https://lkml.kernel.org/r/20241226182918.648799-1-gourry@gourry.net
Link: https://lkml.kernel.org/r/20241220210709.300066-1-gourry@gourry.net
Signed-off-by: Gregory Price <gourry@gourry.net>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: WANG Xuerui <kernel@xen0n.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 76 insertions, 14 deletions

diff --git a/mm/Kconfig b/mm/Kconfig
index 7949ab121070..1b501db06417 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -550,20 +550,63 @@ menuconfig MEMORY_HOTPLUG
 
 if MEMORY_HOTPLUG
 
-config MEMORY_HOTPLUG_DEFAULT_ONLINE
-	bool "Online the newly added memory blocks by default"
-	depends on MEMORY_HOTPLUG
+choice
+	prompt "Memory Hotplug Default Online Type"
+	default MHP_DEFAULT_ONLINE_TYPE_OFFLINE
 	help
+	  Default memory type for hotplugged memory.
+
 	  This option sets the default policy setting for memory hotplug
 	  onlining policy (/sys/devices/system/memory/auto_online_blocks) which
 	  determines what happens to newly added memory regions. Policy setting
 	  can always be changed at runtime.
+
+	  The default is 'offline'.
+
+	  Select offline to defer onlining to drivers and user policy.
+	  Select auto to let the kernel choose what zones to utilize.
+	  Select online_kernel to generally allow kernel usage of this memory.
+	  Select online_movable to generally disallow kernel usage of this memory.
+
+	  Example kernel usage would be page structs and page tables.
+
 	  See Documentation/admin-guide/mm/memory-hotplug.rst for more information.
 
-	  Say Y here if you want all hot-plugged memory blocks to appear in
-	  'online' state by default.
-	  Say N here if you want the default policy to keep all hot-plugged
-	  memory blocks in 'offline' state.
+config MHP_DEFAULT_ONLINE_TYPE_OFFLINE
+	bool "offline"
+	help
+	  Hotplugged memory will not be onlined by default.
+	  Choose this for systems with drivers and user policy that
+	  handle onlining of hotplug memory policy.
+
+config MHP_DEFAULT_ONLINE_TYPE_ONLINE_AUTO
+	bool "auto"
+	help
+	  Select this if you want the kernel to automatically online
+	  hotplugged memory into the zone it thinks is reasonable.
+	  This memory may be utilized for kernel data.
+
+config MHP_DEFAULT_ONLINE_TYPE_ONLINE_KERNEL
+	bool "kernel"
+	help
+	  Select this if you want the kernel to automatically online
+	  hotplugged memory into a zone capable of being used for kernel
+	  data. This typically means ZONE_NORMAL.
+
+config MHP_DEFAULT_ONLINE_TYPE_ONLINE_MOVABLE
+	bool "movable"
+	help
+	  Select this if you want the kernel to automatically online
+	  hotplug memory into ZONE_MOVABLE. This memory will generally
+	  not be utilized for kernel data.
+
+	  This should only be used when the admin knows sufficient
+	  ZONE_NORMAL memory is available to describe hotplug memory,
+	  otherwise hotplug memory may fail to online. For example,
+	  sufficient kernel-capable memory (ZONE_NORMAL) must be
+	  available to allocate page structs to describe ZONE_MOVABLE.
+
+endchoice
 
 config MEMORY_HOTREMOVE
 	bool "Allow for memory hot remove"
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 3b6f93962481..e3655f07dd6e 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -219,11 +219,30 @@ void put_online_mems(void)
 
 bool movable_node_enabled = false;
 
-#ifndef CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE
-int mhp_default_online_type = MMOP_OFFLINE;
-#else
-int mhp_default_online_type = MMOP_ONLINE;
-#endif
+static int mhp_default_online_type = -1;
+int mhp_get_default_online_type(void)
+{
+	if (mhp_default_online_type >= 0)
+		return mhp_default_online_type;
+
+	if (IS_ENABLED(CONFIG_MHP_DEFAULT_ONLINE_TYPE_OFFLINE))
+		mhp_default_online_type = MMOP_OFFLINE;
+	else if (IS_ENABLED(CONFIG_MHP_DEFAULT_ONLINE_TYPE_ONLINE_AUTO))
+		mhp_default_online_type = MMOP_ONLINE;
+	else if (IS_ENABLED(CONFIG_MHP_DEFAULT_ONLINE_TYPE_ONLINE_KERNEL))
+		mhp_default_online_type = MMOP_ONLINE_KERNEL;
+	else if (IS_ENABLED(CONFIG_MHP_DEFAULT_ONLINE_TYPE_ONLINE_MOVABLE))
+		mhp_default_online_type = MMOP_ONLINE_MOVABLE;
+	else
+		mhp_default_online_type = MMOP_OFFLINE;
+
+	return mhp_default_online_type;
+}
+
+void mhp_set_default_online_type(int online_type)
+{
+	mhp_default_online_type = online_type;
+}
 
 static int __init setup_memhp_default_state(char *str)
 {
@@ -1328,7 +1347,7 @@ static int check_hotplug_memory_range(u64 start, u64 size)
 
 static int online_memory_block(struct memory_block *mem, void *arg)
 {
-	mem->online_type = mhp_default_online_type;
+	mem->online_type = mhp_get_default_online_type();
 	return device_online(&mem->dev);
 }
 
@@ -1575,7 +1594,7 @@ int add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
 		merge_system_ram_resource(res);
 
 	/* online pages if requested */
-	if (mhp_default_online_type != MMOP_OFFLINE)
+	if (mhp_get_default_online_type() != MMOP_OFFLINE)
 		walk_memory_blocks(start, size, NULL, online_memory_block);
 
 	return ret;