[RFC PATCH v3 4/4] mm,memory_hotplug: Add mhp_memmap_on_memory boot option

[Oscar Salvador]

Self stored memmap leads to a sparse memory situation which is unsuitable
for workloads that requires large contiguous memory chunks.

Make this an opt-in which needs to be explicitly enabled.

Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
---
 mm/memory_hotplug.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 4b4708512f82..858d6161e915 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -42,6 +42,8 @@
 #include "internal.h"
 #include "shuffle.h"
 
+static bool memmap_on_memory_enabled __read_mostly = false;
+
 /*
  * online_page_callback contains pointer to current page onlining function.
  * Initially it is generic_online_page(). If it is required it could be
@@ -1034,7 +1036,7 @@ bool __weak arch_support_memmap_on_memory(void)
 
 bool mhp_supports_memmap_on_memory(unsigned long size)
 {
-	if (!arch_support_memmap_on_memory() ||
+	if (!memmap_on_memory_enabled || !arch_support_memmap_on_memory() ||
 	    !IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP) ||
 	    size > memory_block_size_bytes())
 		return false;
@@ -1422,6 +1424,13 @@ static int __init cmdline_parse_movable_node(char *p)
 }
 early_param("movable_node", cmdline_parse_movable_node);
 
+static int __init cmdline_parse_memmap_on_memory(char *p)
+{
+	memmap_on_memory_enabled = true;
+	return 0;
+}
+early_param("mhp_memmap_on_memory", cmdline_parse_memmap_on_memory);
+
 /* check which state of node_states will be changed when offline memory */
 static void node_states_check_changes_offline(unsigned long nr_pages,
 		struct zone *zone, struct memory_notify *arg)
-- 
2.26.2