mm: hugetlb_vmemmap: optimize vmemmap_optimize_mode handling

We hold an another reference to hugetlb_optimize_vmemmap_key when making
vmemmap_optimize_mode on, because we use static_key to tell memory_hotplug
that memory_hotplug.memmap_on_memory should be overridden. However, this
rule has gone when we have introduced PageVmemmapSelfHosted. Therefore,
we could simplify vmemmap_optimize_mode handling by not holding an another
reference to hugetlb_optimize_vmemmap_key. This also means that we not
incur the extra page_fixed_fake_head checks if there are no vmemmap
optinmized hugetlb pages after this change.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Muchun Song <[e-mail]>
Reviewed-by: Oscar Salvador <[e-mail]>
Reviewed-by: Mike Kravetz <[e-mail]>
Cc: Anshuman Khandual <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Xiongchun Duan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/page-flags.h

···
 #ifndef __GENERATING_BOUNDS_H
 
 #ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
-DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON,
-			 hugetlb_optimize_vmemmap_key);
+DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
 
 /*
  * If the feature of optimizing vmemmap pages associated with each HugeTLB
···
  */
 static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
 {
-	if (!static_branch_maybe(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON,
-				 &hugetlb_optimize_vmemmap_key))
+	if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key))
 		return page;
 
 	/*

mm/hugetlb_vmemmap.c

···
 #define RESERVE_VMEMMAP_NR		1U
 #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 
-enum vmemmap_optimize_mode {
-	VMEMMAP_OPTIMIZE_OFF,
-	VMEMMAP_OPTIMIZE_ON,
-};
-
-DEFINE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON,
-			hugetlb_optimize_vmemmap_key);
+DEFINE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
 EXPORT_SYMBOL(hugetlb_optimize_vmemmap_key);
 
-static enum vmemmap_optimize_mode vmemmap_optimize_mode =
+static bool vmemmap_optimize_enabled =
 	IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON);
-
-static void vmemmap_optimize_mode_switch(enum vmemmap_optimize_mode to)
-{
-	if (vmemmap_optimize_mode == to)
-		return;
-
-	if (to == VMEMMAP_OPTIMIZE_OFF)
-		static_branch_dec(&hugetlb_optimize_vmemmap_key);
-	else
-		static_branch_inc(&hugetlb_optimize_vmemmap_key);
-	WRITE_ONCE(vmemmap_optimize_mode, to);
-}
 
 static int __init hugetlb_vmemmap_early_param(char *buf)
 {
-	bool enable;
-	enum vmemmap_optimize_mode mode;
-
-	if (kstrtobool(buf, &enable))
-		return -EINVAL;
-
-	mode = enable ? VMEMMAP_OPTIMIZE_ON : VMEMMAP_OPTIMIZE_OFF;
-	vmemmap_optimize_mode_switch(mode);
-
-	return 0;
+	return kstrtobool(buf, &vmemmap_optimize_enabled);
 }
 early_param("hugetlb_free_vmemmap", hugetlb_vmemmap_early_param);
 
···
 static unsigned int vmemmap_optimizable_pages(struct hstate *h,
 					      struct page *head)
 {
-	if (READ_ONCE(vmemmap_optimize_mode) == VMEMMAP_OPTIMIZE_OFF)
+	if (!READ_ONCE(vmemmap_optimize_enabled))
 		return 0;
 
 	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) {
···
 
 	if (!is_power_of_2(sizeof(struct page))) {
 		pr_warn_once("cannot optimize vmemmap pages because \"struct page\" crosses page boundaries\n");
-		static_branch_disable(&hugetlb_optimize_vmemmap_key);
 		return;
 	}
 
···
 }
 
 #ifdef CONFIG_PROC_SYSCTL
-static int hugetlb_optimize_vmemmap_handler(struct ctl_table *table, int write,
-					    void *buffer, size_t *length,
-					    loff_t *ppos)
-{
-	int ret;
-	enum vmemmap_optimize_mode mode;
-	static DEFINE_MUTEX(sysctl_mutex);
-
-	if (write && !capable(CAP_SYS_ADMIN))
-		return -EPERM;
-
-	mutex_lock(&sysctl_mutex);
-	mode = vmemmap_optimize_mode;
-	table->data = &mode;
-	ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
-	if (write && !ret)
-		vmemmap_optimize_mode_switch(mode);
-	mutex_unlock(&sysctl_mutex);
-
-	return ret;
-}
-
 static struct ctl_table hugetlb_vmemmap_sysctls[] = {
 	{
 		.procname	= "hugetlb_optimize_vmemmap",
-		.maxlen		= sizeof(enum vmemmap_optimize_mode),
+		.data		= &vmemmap_optimize_enabled,
+		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= hugetlb_optimize_vmemmap_handler,
-		.extra1		= SYSCTL_ZERO,
-		.extra2		= SYSCTL_ONE,
+		.proc_handler	= proc_dobool,
 	},
 	{ }
 };