Self stored memmap leads to a sparse memory situation which is unsuitable
for workloads that requires large contiguous memory chunks, so make this
an opt-in which needs to be explicitly enabled.

To control this, let memory_hotplug have its own memory space, as suggested
by David, so we can add memory_hotplug.memmap_on_memory parameter.

Signed-off-by: Oscar Salvador <osalvador@suse.de>
---
  Documentation/admin-guide/kernel-parameters.txt | 14 ++++++++++++++
  mm/Makefile                                     |  5 ++++-
  mm/memory_hotplug.c                             |  8 +++++++-
  3 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 04545725f187..e626dab39c60 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -2794,6 +2794,20 @@
  			seconds.  Use this parameter to check at some
  			other rate.  0 disables periodic checking.
  
+	memory_hotplug.memmap_on_memory
+			[KNL,X86,ARM] Boolean flag to enable this feature.

+
  	memtest=	[KNL,X86,ARM,PPC] Enable memtest
  			Format: <integer>
  			default : 0 <disable>

diff --git a/mm/Makefile b/mm/Makefile
index 72227b24a616..82ae9482f5e3 100644
--- a/mm/Makefile
+++ b/mm/Makefile

@@ -58,9 +58,13 @@ obj-y			:= filemap.o mempool.o oom_kill.o fadvise.o \
  page-alloc-y := page_alloc.o
  page-alloc-$(CONFIG_SHUFFLE_PAGE_ALLOCATOR) += shuffle.o
  
+# Give 'memory_hotplug' its own module-parameter namespace
+memory-hotplug-$(CONFIG_MEMORY_HOTPLUG) += memory_hotplug.o
+
  obj-y += page-alloc.o
  obj-y += init-mm.o
  obj-y += memblock.o
+obj-y += $(memory-hotplug-y)
  
  ifdef CONFIG_MMU
  	obj-$(CONFIG_ADVISE_SYSCALLS)	+= madvise.o

@@ -83,7 +87,6 @@ obj-$(CONFIG_SLUB) += slub.o
  obj-$(CONFIG_KASAN)	+= kasan/
  obj-$(CONFIG_KFENCE) += kfence/
  obj-$(CONFIG_FAILSLAB) += failslab.o
-obj-$(CONFIG_MEMORY_HOTPLUG) += memory_hotplug.o
  obj-$(CONFIG_MEMTEST)		+= memtest.o
  obj-$(CONFIG_MIGRATION) += migrate.o
  obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o khugepaged.o

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 63e5a0e9a6f3..94b0ec3d2ff2 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c

@@ -42,7 +42,13 @@
  #include "internal.h"
  #include "shuffle.h"
  
-static bool memmap_on_memory;
+
+/*
+ * memory_hotplug.memmap_on_memory parameter
+ */
+static bool memmap_on_memory __ro_after_init;
+module_param(memmap_on_memory, bool, 0444);
+MODULE_PARM_DESC(memmap_on_memory, "Enable memmap on memory for memory hotplug");