[PATCH 3/3] mm: hugetlb: add hugetlb_free_vmemmap sysctl

[Muchun Song <songmuchun@bytedance.com>]

We must add "hugetlb_free_vmemmap=on" to boot cmdline and reboot the
server to enable the feature of freeing vmemmap pages of HugeTLB
pages. Rebooting usually taske a long time. Add a sysctl to enable
the feature at runtime and do not need to reboot.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 Documentation/admin-guide/sysctl/vm.rst | 13 +++++++++++++
 include/linux/hugetlb.h                 |  5 +++++
 include/linux/memory_hotplug.h          |  1 +
 kernel/sysctl.c                         | 11 +++++++++++
 mm/hugetlb_vmemmap.c                    | 23 +++++++++++++++++------
 mm/hugetlb_vmemmap.h                    |  4 +++-
 mm/memory_hotplug.c                     |  2 +-
 7 files changed, 51 insertions(+), 8 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index f4804ce37c58..01f18e6cc227 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -561,6 +561,19 @@ Change the minimum size of the hugepage pool.
 See Documentation/admin-guide/mm/hugetlbpage.rst
 
 
+hugetlb_free_vmemmap
+====================
+
+A toggle value indicating if vmemmap pages are allowed to be optimized.
+If it is off (0), then it can be set true (1).  Once true, the vmemmap
+pages associated with each HugeTLB page will be optimized, and the toggle
+cannot be set back to false.  It only optimizes the subsequent allocation
+of HugeTLB pages from buddy system, while already allocated HugeTLB pages
+will not be optimized.
+
+See Documentation/admin-guide/mm/hugetlbpage.rst
+
+
 nr_hugepages_mempolicy
 ======================
 
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 53c1b6082a4c..cc4ab21892f5 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -1080,6 +1080,11 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
 }
 #endif	/* CONFIG_HUGETLB_PAGE */
 
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+int hugetlb_vmemmap_sysctl_handler(struct ctl_table *table, int write,
+				   void *buffer, size_t *length, loff_t *ppos);
+#endif
+
 static inline spinlock_t *huge_pte_lock(struct hstate *h,
 					struct mm_struct *mm, pte_t *pte)
 {
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index e0b2209ab71c..b30f9fdaed73 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -349,6 +349,7 @@ extern int arch_create_linear_mapping(int nid, u64 start, u64 size,
 				      struct mhp_params *params);
 void arch_remove_linear_mapping(u64 start, u64 size);
 extern bool mhp_supports_memmap_on_memory(unsigned long size);
+extern bool memmap_on_memory;
 #endif /* CONFIG_MEMORY_HOTPLUG */
 
 #endif /* __LINUX_MEMORY_HOTPLUG_H */
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index ab3e9c937268..77f039849b2a 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2223,6 +2223,17 @@ static struct ctl_table vm_table[] = {
 		.mode		= 0644,
 		.proc_handler	= hugetlb_sysctl_handler,
 	},
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+	{
+		.procname	= "hugetlb_free_vmemmap",
+		.data		= &hugetlb_free_vmemmap_enabled_key.key,
+		.maxlen		= sizeof(hugetlb_free_vmemmap_enabled_key.key),
+		.mode		= 0644,
+		/* only handle a transition from default "0" to "1" */
+		.proc_handler	= hugetlb_vmemmap_sysctl_handler,
+		.extra1		= SYSCTL_ONE,
+	},
+#endif
 #ifdef CONFIG_NUMA
 	{
 		.procname       = "nr_hugepages_mempolicy",
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 836d1117f08b..3167021055d6 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -10,6 +10,7 @@
 
 #define pr_fmt(fmt)	"HugeTLB: " fmt
 
+#include <linux/memory_hotplug.h>
 #include "hugetlb_vmemmap.h"
 
 /*
@@ -118,17 +119,14 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 	BUILD_BUG_ON(__NR_USED_SUBPAGE >=
 		     RESERVE_VMEMMAP_SIZE / sizeof(struct page));
 
-	if (!hugetlb_free_vmemmap_enabled())
-		return;
-
-	if (IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON) &&
-	    !is_power_of_2(sizeof(struct page))) {
+	if (!is_power_of_2(sizeof(struct page))) {
 		/*
 		 * The hugetlb_free_vmemmap_enabled_key can be enabled when
 		 * CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON. It should
 		 * be disabled if "struct page" crosses page boundaries.
 		 */
-		static_branch_disable(&hugetlb_free_vmemmap_enabled_key);
+		if (IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON))
+			static_branch_disable(&hugetlb_free_vmemmap_enabled_key);
 		return;
 	}
 
@@ -147,3 +145,16 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 	pr_info("can free %d vmemmap pages for %s\n", h->nr_free_vmemmap_pages,
 		h->name);
 }
+
+int hugetlb_vmemmap_sysctl_handler(struct ctl_table *table, int write,
+				   void *buffer, size_t *length, loff_t *ppos)
+{
+	/*
+	 * The vmemmap pages cannot be optimized if a "struct page" crosses page
+	 * boundaries or memory_hotplug.memmap_on_memory is enabled.
+	 */
+	if (write && (!is_power_of_2(sizeof(struct page)) || memmap_on_memory))
+		return -EPERM;
+
+	return proc_do_static_key(table, write, buffer, length, ppos);
+}
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index cb2bef8f9e73..b67a159027f4 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -21,7 +21,9 @@ void hugetlb_vmemmap_init(struct hstate *h);
  */
 static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
 {
-	return h->nr_free_vmemmap_pages;
+	if (hugetlb_free_vmemmap_enabled())
+		return h->nr_free_vmemmap_pages;
+	return 0;
 }
 #else
 static inline int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index c226a337c1ef..b5cc5abde05a 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -46,7 +46,7 @@
 /*
  * memory_hotplug.memmap_on_memory parameter
  */
-static bool memmap_on_memory __ro_after_init;
+bool memmap_on_memory __ro_after_init;
 #ifdef CONFIG_MHP_MEMMAP_ON_MEMORY
 module_param(memmap_on_memory, bool, 0444);
 MODULE_PARM_DESC(memmap_on_memory, "Enable memmap on memory for memory hotplug");
-- 
2.11.0