[PATCH v10 02/11] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

[Muchun Song <songmuchun@bytedance.com>]

The HUGETLB_PAGE_FREE_VMEMMAP option is used to enable the freeing
of unnecessary vmemmap associated with HugeTLB pages. The config
option is introduced early so that supporting code can be written
to depend on the option. The initial version of the code only
provides support for x86-64.

Like other code which frees vmemmap, this config option depends on
HAVE_BOOTMEM_INFO_NODE. The routine register_page_bootmem_info() is
used to register bootmem info. Therefore, make sure
register_page_bootmem_info is enabled if HUGETLB_PAGE_FREE_VMEMMAP
is defined.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 arch/x86/mm/init_64.c |  2 +-
 fs/Kconfig            | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 0a45f062826e..0435bee2e172 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1225,7 +1225,7 @@ static struct kcore_list kcore_vsyscall;
 
 static void __init register_page_bootmem_info(void)
 {
-#ifdef CONFIG_NUMA
+#if defined(CONFIG_NUMA) || defined(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP)
 	int i;
 
 	for_each_online_node(i)
diff --git a/fs/Kconfig b/fs/Kconfig
index 976e8b9033c4..e7c4c2a79311 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -245,6 +245,24 @@ config HUGETLBFS
 config HUGETLB_PAGE
 	def_bool HUGETLBFS
 
+config HUGETLB_PAGE_FREE_VMEMMAP
+	def_bool HUGETLB_PAGE
+	depends on X86_64
+	depends on SPARSEMEM_VMEMMAP
+	depends on HAVE_BOOTMEM_INFO_NODE
+	help
+	  The option HUGETLB_PAGE_FREE_VMEMMAP allows for the freeing of
+	  some vmemmap pages associated with pre-allocated HugeTLB pages.
+	  For example, on X86_64 6 vmemmap pages of size 4KB each can be
+	  saved for each 2MB HugeTLB page.  4094 vmemmap pages of size 4KB
+	  each can be saved for each 1GB HugeTLB page.
+
+	  When a HugeTLB page is allocated or freed, the vmemmap array
+	  representing the range associated with the page will need to be
+	  remapped.  When a page is allocated, vmemmap pages are freed
+	  after remapping.  When a page is freed, previously discarded
+	  vmemmap pages must be allocated before remapping.
+
 config MEMFD_CREATE
 	def_bool TMPFS || HUGETLBFS
 
-- 
2.11.0