[PATCH v1 5/5] mm/memory_hotplug: allow to specify a default online_type

[David Hildenbrand <david@xxxxxxxxxx>]

For now, distributions implement advanced udev rules to essentially
- Don't online any hotplugged memory (s390x)
- Online all memory to ZONE_NORMAL (e.g., most virt environments like
  hyperv)
- Online all memory to ZONE_MOVABLE in case the zone imbalance is taken
  care of (e.g., bare metal, special virt environments)

In summary: All memory is usually onlined the same way, however, the
kernel always has to ask userspace to come up with the same answer.
E.g., HyperV always waits for a memory block to get onlined before
continuing, otherwise it might end up adding memory faster than
hotplugging it, which can result in strange OOM situations.

Let's allow to specify a default online_type, not just "online" and
"offline". This allows distributions to configure the default online_type
when booting up and be done with it.

We can now specify "offline", "online", "online_movable" and
"online_kernel" via
- "memhp_default_state=" on the kernel cmdline
- /sys/devices/systemn/memory/auto_online_blocks
just like we are able to specify for a single memory block via
/sys/devices/systemn/memory/memoryX/state

Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
Cc: Baoquan He <bhe@xxxxxxxxxx>
Cc: Wei Yang <richard.weiyang@xxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 drivers/base/memory.c          | 11 +++++------
 include/linux/memory_hotplug.h |  2 ++
 mm/memory_hotplug.c            |  8 ++++----
 3 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index 8d3e16dab69f..2b09b68b9f78 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -35,7 +35,7 @@ static const char *const online_type_to_str[] = {
 	[MMOP_ONLINE_MOVABLE] = "online_movable",
 };
 
-static int memhp_online_type_from_str(const char *str)
+int memhp_online_type_from_str(const char *str)
 {
 	int i;
 
@@ -394,13 +394,12 @@ static ssize_t auto_online_blocks_store(struct device *dev,
 					struct device_attribute *attr,
 					const char *buf, size_t count)
 {
-	if (sysfs_streq(buf, "online"))
-		memhp_default_online_type = MMOP_ONLINE;
-	else if (sysfs_streq(buf, "offline"))
-		memhp_default_online_type = MMOP_OFFLINE;
-	else
+	const int online_type = memhp_online_type_from_str(buf);
+
+	if (online_type < 0)
 		return -EINVAL;
 
+	memhp_default_online_type = online_type;
 	return count;
 }
 
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index c6e090b34c4b..ef55115320fb 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -117,6 +117,8 @@ extern int arch_add_memory(int nid, u64 start, u64 size,
 			struct mhp_restrictions *restrictions);
 extern u64 max_mem_size;
 
+extern int memhp_online_type_from_str(const char *str);
+
 /* Default online_type (MMOP_*) when new memory blocks are added. */
 extern int memhp_default_online_type;
 /* If movable_node boot option specified */
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 01443c70aa27..4a96273eafa7 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -75,10 +75,10 @@ EXPORT_SYMBOL_GPL(memhp_default_online_type);
 
 static int __init setup_memhp_default_state(char *str)
 {
-	if (!strcmp(str, "online"))
-		memhp_default_online_type = MMOP_ONLINE;
-	else if (!strcmp(str, "offline"))
-		memhp_default_online_type = MMOP_OFFLINE;
+	const int online_type = memhp_online_type_from_str(str);
+
+	if (online_type >= 0)
+		memhp_default_online_type = online_type;
 
 	return 1;
 }
-- 
2.24.1