[PATCH 5/5] mm: hugetlb: introduce CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON

[Muchun Song <songmuchun@bytedance.com>]

When using HUGETLB_PAGE_FREE_VMEMMAP, the freeing unused vmemmap pages
associated with each HugeTLB page is default off. Now the vmemmap is PMD
mapped. So there is no side effect when this feature is enabled with no
HugeTLB pages in the system. Someone may want to enable this feature in
the compiler time instead of using boot command line. So add a config to
make it default on when someone do not want to enable it via command line.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  3 +++
 fs/Kconfig                                      | 10 ++++++++++
 mm/hugetlb_vmemmap.c                            |  6 ++++--
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a01aadafee38..8eee439d943c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1604,6 +1604,9 @@
 			on:  enable the feature
 			off: disable the feature
 
+			Built with CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON=y,
+			the default is on.
+
 			This is not compatible with memory_hotplug.memmap_on_memory.
 			If both parameters are enabled, hugetlb_free_vmemmap takes
 			precedence over memory_hotplug.memmap_on_memory.
diff --git a/fs/Kconfig b/fs/Kconfig
index f40b5b98f7ba..e78bc5daf7b0 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -245,6 +245,16 @@ config HUGETLB_PAGE_FREE_VMEMMAP
 	depends on X86_64
 	depends on SPARSEMEM_VMEMMAP
 
+config HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON
+	bool "Default freeing vmemmap pages of HugeTLB to on"
+	default n
+	depends on HUGETLB_PAGE_FREE_VMEMMAP
+	help
+	  When using HUGETLB_PAGE_FREE_VMEMMAP, the freeing unused vmemmap
+	  pages associated with each HugeTLB page is default off. Say Y here
+	  to enable freeing vmemmap pages of HugeTLB by default. It can then
+	  be disabled on the command line via hugetlb_free_vmemmap=off.
+
 config MEMFD_CREATE
 	def_bool TMPFS || HUGETLBFS
 
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 01f3652fa359..b5f4f29e042a 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -186,7 +186,7 @@
 #define RESERVE_VMEMMAP_NR		2U
 #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 
-bool hugetlb_free_vmemmap_enabled;
+bool hugetlb_free_vmemmap_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
 
 static int __init early_hugetlb_free_vmemmap_param(char *buf)
 {
@@ -201,7 +201,9 @@ static int __init early_hugetlb_free_vmemmap_param(char *buf)
 
 	if (!strcmp(buf, "on"))
 		hugetlb_free_vmemmap_enabled = true;
-	else if (strcmp(buf, "off"))
+	else if (!strcmp(buf, "off"))
+		hugetlb_free_vmemmap_enabled = false;
+	else
 		return -EINVAL;
 
 	return 0;
-- 
2.11.0