mm/hugetlb_vmemmap.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 /*
  * HugeTLB Vmemmap Optimization (HVO)
  *
  * Copyright (c) 2020, ByteDance. All rights reserved.
  *
  *     Author: Muchun Song <songmuchun@bytedance.com>
  *
  * See Documentation/mm/vmemmap_dedup.rst
  */
 #define pr_fmt(fmt)	"HugeTLB: " fmt

 #include <linux/pgtable.h>
 #include <linux/moduleparam.h>
 #include <linux/bootmem_info.h>
 #include <asm/pgalloc.h>
 #include <asm/tlbflush.h>
 #include "hugetlb_vmemmap.h"

 /**
  * struct vmemmap_remap_walk - walk vmemmap page table
  *
  * @remap_pte:		called for each lowest-level entry (PTE).
  * @nr_walked:		the number of walked pte.
  * @reuse_page:		the page which is reused for the tail vmemmap pages.
  * @reuse_addr:		the virtual address of the @reuse_page page.
  * @vmemmap_pages:	the list head of the vmemmap pages that can be freed
  *			or is mapped from.
  */
 struct vmemmap_remap_walk {
 	void			(*remap_pte)(pte_t *pte, unsigned long addr,
 					     struct vmemmap_remap_walk *walk);
 	unsigned long		nr_walked;
 	struct page		*reuse_page;
 	unsigned long		reuse_addr;
 	struct list_head	*vmemmap_pages;
 };

 static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
 {
 	pmd_t __pmd;
 	int i;
 	unsigned long addr = start;
 	struct page *head;
 	pte_t *pgtable;

 	spin_lock(&init_mm.page_table_lock);
 	head = pmd_leaf(*pmd) ? pmd_page(*pmd) : NULL;
 	spin_unlock(&init_mm.page_table_lock);

 	if (!head)
 		return 0;

 	pgtable = pte_alloc_one_kernel(&init_mm);
 	if (!pgtable)
 		return -ENOMEM;

 	pmd_populate_kernel(&init_mm, &__pmd, pgtable);

 	for (i = 0; i < PTRS_PER_PTE; i++, addr += PAGE_SIZE) {
 		pte_t entry, *pte;
 		pgprot_t pgprot = PAGE_KERNEL;

 		entry = mk_pte(head + i, pgprot);
 		pte = pte_offset_kernel(&__pmd, addr);
 		set_pte_at(&init_mm, addr, pte, entry);
 	}

 	spin_lock(&init_mm.page_table_lock);
 	if (likely(pmd_leaf(*pmd))) {
 		/*
 		 * Higher order allocations from buddy allocator must be able to
 		 * be treated as indepdenent small pages (as they can be freed
 		 * individually).
 		 */
 		if (!PageReserved(head))
 			split_page(head, get_order(PMD_SIZE));

 		/* Make pte visible before pmd. See comment in pmd_install(). */
 		smp_wmb();
 		pmd_populate_kernel(&init_mm, pmd, pgtable);
 		flush_tlb_kernel_range(start, start + PMD_SIZE);
 	} else {
 		pte_free_kernel(&init_mm, pgtable);
 	}
 	spin_unlock(&init_mm.page_table_lock);

 	return 0;
 }

 static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
 			      unsigned long end,
 			      struct vmemmap_remap_walk *walk)
 {
 	pte_t *pte = pte_offset_kernel(pmd, addr);

 	/*
 	 * The reuse_page is found 'first' in table walk before we start
 	 * remapping (which is calling @walk->remap_pte).
 	 */
 	if (!walk->reuse_page) {
 		walk->reuse_page = pte_page(ptep_get(pte));
 		/*
 		 * Because the reuse address is part of the range that we are
 		 * walking, skip the reuse address range.
 		 */
 		addr += PAGE_SIZE;
 		pte++;
 		walk->nr_walked++;
 	}

 	for (; addr != end; addr += PAGE_SIZE, pte++) {
 		walk->remap_pte(pte, addr, walk);
 		walk->nr_walked++;
 	}
 }

 static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
 			     unsigned long end,
 			     struct vmemmap_remap_walk *walk)
 {
 	pmd_t *pmd;
 	unsigned long next;

 	pmd = pmd_offset(pud, addr);
 	do {
 		int ret;

 		ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK);
 		if (ret)
 			return ret;

 		next = pmd_addr_end(addr, end);
 		vmemmap_pte_range(pmd, addr, next, walk);
 	} while (pmd++, addr = next, addr != end);

 	return 0;
 }

 static int vmemmap_pud_range(p4d_t *p4d, unsigned long addr,
 			     unsigned long end,
 			     struct vmemmap_remap_walk *walk)
 {
 	pud_t *pud;
 	unsigned long next;

 	pud = pud_offset(p4d, addr);
 	do {
 		int ret;

 		next = pud_addr_end(addr, end);
 		ret = vmemmap_pmd_range(pud, addr, next, walk);
 		if (ret)
 			return ret;
 	} while (pud++, addr = next, addr != end);

 	return 0;
 }

 static int vmemmap_p4d_range(pgd_t *pgd, unsigned long addr,
 			     unsigned long end,
 			     struct vmemmap_remap_walk *walk)
 {
 	p4d_t *p4d;
 	unsigned long next;

 	p4d = p4d_offset(pgd, addr);
 	do {
 		int ret;

 		next = p4d_addr_end(addr, end);
 		ret = vmemmap_pud_range(p4d, addr, next, walk);
 		if (ret)
 			return ret;
 	} while (p4d++, addr = next, addr != end);

 	return 0;
 }

 static int vmemmap_remap_range(unsigned long start, unsigned long end,
 			       struct vmemmap_remap_walk *walk)
 {
 	unsigned long addr = start;
 	unsigned long next;
 	pgd_t *pgd;

 	VM_BUG_ON(!PAGE_ALIGNED(start));
 	VM_BUG_ON(!PAGE_ALIGNED(end));

 	pgd = pgd_offset_k(addr);
 	do {
 		int ret;

 		next = pgd_addr_end(addr, end);
 		ret = vmemmap_p4d_range(pgd, addr, next, walk);
 		if (ret)
 			return ret;
 	} while (pgd++, addr = next, addr != end);

 	flush_tlb_kernel_range(start, end);

 	return 0;
 }

 /*
  * Free a vmemmap page. A vmemmap page can be allocated from the memblock
  * allocator or buddy allocator. If the PG_reserved flag is set, it means
  * that it allocated from the memblock allocator, just free it via the
  * free_bootmem_page(). Otherwise, use __free_page().
  */
 static inline void free_vmemmap_page(struct page *page)
 {
 	if (PageReserved(page))
 		free_bootmem_page(page);
 	else
 		__free_page(page);
 }

 /* Free a list of the vmemmap pages */
 static void free_vmemmap_page_list(struct list_head *list)
 {
 	struct page *page, *next;

 	list_for_each_entry_safe(page, next, list, lru)
 		free_vmemmap_page(page);
 }

 static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
 			      struct vmemmap_remap_walk *walk)
 {
 	/*
 	 * Remap the tail pages as read-only to catch illegal write operation
 	 * to the tail pages.
 	 */
 	pgprot_t pgprot = PAGE_KERNEL_RO;
 	struct page *page = pte_page(ptep_get(pte));
 	pte_t entry;

 	/* Remapping the head page requires r/w */
 	if (unlikely(addr == walk->reuse_addr)) {
 		pgprot = PAGE_KERNEL;
 		list_del(&walk->reuse_page->lru);

 		/*
 		 * Makes sure that preceding stores to the page contents from
 		 * vmemmap_remap_free() become visible before the set_pte_at()
 		 * write.
 		 */
 		smp_wmb();
 	}

 	entry = mk_pte(walk->reuse_page, pgprot);
 	list_add_tail(&page->lru, walk->vmemmap_pages);
 	set_pte_at(&init_mm, addr, pte, entry);
 }

 /*
  * How many struct page structs need to be reset. When we reuse the head
  * struct page, the special metadata (e.g. page->flags or page->mapping)
  * cannot copy to the tail struct page structs. The invalid value will be
  * checked in the free_tail_page_prepare(). In order to avoid the message
  * of "corrupted mapping in tail page". We need to reset at least 3 (one
  * head struct page struct and two tail struct page structs) struct page
  * structs.
  */
 #define NR_RESET_STRUCT_PAGE		3

 static inline void reset_struct_pages(struct page *start)
 {
 	struct page *from = start + NR_RESET_STRUCT_PAGE;

 	BUILD_BUG_ON(NR_RESET_STRUCT_PAGE * 2 > PAGE_SIZE / sizeof(struct page));
 	memcpy(start, from, sizeof(*from) * NR_RESET_STRUCT_PAGE);
 }

 static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
 				struct vmemmap_remap_walk *walk)
 {
 	pgprot_t pgprot = PAGE_KERNEL;
 	struct page *page;
 	void *to;

 	BUG_ON(pte_page(ptep_get(pte)) != walk->reuse_page);

 	page = list_first_entry(walk->vmemmap_pages, struct page, lru);
 	list_del(&page->lru);
 	to = page_to_virt(page);
 	copy_page(to, (void *)walk->reuse_addr);
 	reset_struct_pages(to);

 	/*
 	 * Makes sure that preceding stores to the page contents become visible
 	 * before the set_pte_at() write.
 	 */
 	smp_wmb();
 	set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
 }

 /**
  * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
  *			to the page which @reuse is mapped to, then free vmemmap
  *			which the range are mapped to.
  * @start:	start address of the vmemmap virtual address range that we want
  *		to remap.
  * @end:	end address of the vmemmap virtual address range that we want to
  *		remap.
  * @reuse:	reuse address.
  *
  * Return: %0 on success, negative error code otherwise.
  */
 static int vmemmap_remap_free(unsigned long start, unsigned long end,
 			      unsigned long reuse)
 {
 	int ret;
 	LIST_HEAD(vmemmap_pages);
 	struct vmemmap_remap_walk walk = {
 		.remap_pte	= vmemmap_remap_pte,
 		.reuse_addr	= reuse,
 		.vmemmap_pages	= &vmemmap_pages,
 	};
 	int nid = page_to_nid((struct page *)start);
 	gfp_t gfp_mask = GFP_KERNEL | __GFP_THISNODE | __GFP_NORETRY |
 			__GFP_NOWARN;

 	/*
 	 * Allocate a new head vmemmap page to avoid breaking a contiguous
 	 * block of struct page memory when freeing it back to page allocator
 	 * in free_vmemmap_page_list(). This will allow the likely contiguous
 	 * struct page backing memory to be kept contiguous and allowing for
 	 * more allocations of hugepages. Fallback to the currently
 	 * mapped head page in case should it fail to allocate.
 	 */
 	walk.reuse_page = alloc_pages_node(nid, gfp_mask, 0);
 	if (walk.reuse_page) {
 		copy_page(page_to_virt(walk.reuse_page),
 			  (void *)walk.reuse_addr);
 		list_add(&walk.reuse_page->lru, &vmemmap_pages);
 	}

 	/*
 	 * In order to make remapping routine most efficient for the huge pages,
 	 * the routine of vmemmap page table walking has the following rules
 	 * (see more details from the vmemmap_pte_range()):
 	 *
 	 * - The range [@start, @end) and the range [@reuse, @reuse + PAGE_SIZE)
 	 *   should be continuous.
 	 * - The @reuse address is part of the range [@reuse, @end) that we are
 	 *   walking which is passed to vmemmap_remap_range().
 	 * - The @reuse address is the first in the complete range.
 	 *
 	 * So we need to make sure that @start and @reuse meet the above rules.
 	 */
 	BUG_ON(start - reuse != PAGE_SIZE);

 	mmap_read_lock(&init_mm);
 	ret = vmemmap_remap_range(reuse, end, &walk);
 	if (ret && walk.nr_walked) {
 		end = reuse + walk.nr_walked * PAGE_SIZE;
 		/*
 		 * vmemmap_pages contains pages from the previous
 		 * vmemmap_remap_range call which failed.  These
 		 * are pages which were removed from the vmemmap.
 		 * They will be restored in the following call.
 		 */
 		walk = (struct vmemmap_remap_walk) {
 			.remap_pte	= vmemmap_restore_pte,
 			.reuse_addr	= reuse,
 			.vmemmap_pages	= &vmemmap_pages,
 		};

 		vmemmap_remap_range(reuse, end, &walk);
 	}
 	mmap_read_unlock(&init_mm);

 	free_vmemmap_page_list(&vmemmap_pages);

 	return ret;
 }

 static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
 				   struct list_head *list)
 {
 	gfp_t gfp_mask = GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_THISNODE;
 	unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
 	int nid = page_to_nid((struct page *)start);
 	struct page *page, *next;

 	while (nr_pages--) {
 		page = alloc_pages_node(nid, gfp_mask, 0);
 		if (!page)
 			goto out;
 		list_add_tail(&page->lru, list);
 	}

 	return 0;
 out:
 	list_for_each_entry_safe(page, next, list, lru)
 		__free_page(page);
 	return -ENOMEM;
 }

 /**
  * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
  *			 to the page which is from the @vmemmap_pages
  *			 respectively.
  * @start:	start address of the vmemmap virtual address range that we want
  *		to remap.
  * @end:	end address of the vmemmap virtual address range that we want to
  *		remap.
  * @reuse:	reuse address.
  *
  * Return: %0 on success, negative error code otherwise.
  */
 static int vmemmap_remap_alloc(unsigned long start, unsigned long end,
 			       unsigned long reuse)
 {
 	LIST_HEAD(vmemmap_pages);
 	struct vmemmap_remap_walk walk = {
 		.remap_pte	= vmemmap_restore_pte,
 		.reuse_addr	= reuse,
 		.vmemmap_pages	= &vmemmap_pages,
 	};

 	/* See the comment in the vmemmap_remap_free(). */
 	BUG_ON(start - reuse != PAGE_SIZE);

 	if (alloc_vmemmap_page_list(start, end, &vmemmap_pages))
 		return -ENOMEM;

 	mmap_read_lock(&init_mm);
 	vmemmap_remap_range(reuse, end, &walk);
 	mmap_read_unlock(&init_mm);

 	return 0;
 }

 DEFINE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
 EXPORT_SYMBOL(hugetlb_optimize_vmemmap_key);

 static bool vmemmap_optimize_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON);
 core_param(hugetlb_free_vmemmap, vmemmap_optimize_enabled, bool, 0);

 /**
  * hugetlb_vmemmap_restore - restore previously optimized (by
  *			     hugetlb_vmemmap_optimize()) vmemmap pages which
  *			     will be reallocated and remapped.
  * @h:		struct hstate.
  * @head:	the head page whose vmemmap pages will be restored.
  *
  * Return: %0 if @head's vmemmap pages have been reallocated and remapped,
  * negative error code otherwise.
  */
 int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head)
 {
 	int ret;
 	unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
 	unsigned long vmemmap_reuse;

 	if (!HPageVmemmapOptimized(head))
 		return 0;

 	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
 	vmemmap_reuse	= vmemmap_start;
 	vmemmap_start	+= HUGETLB_VMEMMAP_RESERVE_SIZE;

 	/*
 	 * The pages which the vmemmap virtual address range [@vmemmap_start,
 	 * @vmemmap_end) are mapped to are freed to the buddy allocator, and
 	 * the range is mapped to the page which @vmemmap_reuse is mapped to.
 	 * When a HugeTLB page is freed to the buddy allocator, previously
 	 * discarded vmemmap pages must be allocated and remapping.
 	 */
 	ret = vmemmap_remap_alloc(vmemmap_start, vmemmap_end, vmemmap_reuse);
 	if (!ret) {
 		ClearHPageVmemmapOptimized(head);
 		static_branch_dec(&hugetlb_optimize_vmemmap_key);
 	}

 	return ret;
 }

 /* Return true iff a HugeTLB whose vmemmap should and can be optimized. */
 static bool vmemmap_should_optimize(const struct hstate *h, const struct page *head)
 {
 	if (!READ_ONCE(vmemmap_optimize_enabled))
 		return false;

 	if (!hugetlb_vmemmap_optimizable(h))
 		return false;

 	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) {
 		pmd_t *pmdp, pmd;
 		struct page *vmemmap_page;
 		unsigned long vaddr = (unsigned long)head;

 		/*
 		 * Only the vmemmap page's vmemmap page can be self-hosted.
 		 * Walking the page tables to find the backing page of the
 		 * vmemmap page.
 		 */
 		pmdp = pmd_off_k(vaddr);
 		/*
 		 * The READ_ONCE() is used to stabilize *pmdp in a register or
 		 * on the stack so that it will stop changing under the code.
 		 * The only concurrent operation where it can be changed is
 		 * split_vmemmap_huge_pmd() (*pmdp will be stable after this
 		 * operation).
 		 */
 		pmd = READ_ONCE(*pmdp);
 		if (pmd_leaf(pmd))
 			vmemmap_page = pmd_page(pmd) + pte_index(vaddr);
 		else
 			vmemmap_page = pte_page(*pte_offset_kernel(pmdp, vaddr));
 		/*
 		 * Due to HugeTLB alignment requirements and the vmemmap pages
 		 * being at the start of the hotplugged memory region in
 		 * memory_hotplug.memmap_on_memory case. Checking any vmemmap
 		 * page's vmemmap page if it is marked as VmemmapSelfHosted is
 		 * sufficient.
 		 *
 		 * [                  hotplugged memory                  ]
 		 * [        section        ][...][        section        ]
 		 * [ vmemmap ][              usable memory               ]
 		 *   ^   |     |                                        |
 		 *   +---+     |                                        |
 		 *     ^       |                                        |
 		 *     +-------+                                        |
 		 *          ^                                           |
 		 *          +-------------------------------------------+
 		 */
 		if (PageVmemmapSelfHosted(vmemmap_page))
 			return false;
 	}

 	return true;
 }

 /**
  * hugetlb_vmemmap_optimize - optimize @head page's vmemmap pages.
  * @h:		struct hstate.
  * @head:	the head page whose vmemmap pages will be optimized.
  *
  * This function only tries to optimize @head's vmemmap pages and does not
  * guarantee that the optimization will succeed after it returns. The caller
  * can use HPageVmemmapOptimized(@head) to detect if @head's vmemmap pages
  * have been optimized.
  */
 void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
 {
 	unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
 	unsigned long vmemmap_reuse;

 	if (!vmemmap_should_optimize(h, head))
 		return;

 	static_branch_inc(&hugetlb_optimize_vmemmap_key);

 	vmemmap_end	= vmemmap_start + hugetlb_vmemmap_size(h);
 	vmemmap_reuse	= vmemmap_start;
 	vmemmap_start	+= HUGETLB_VMEMMAP_RESERVE_SIZE;

 	/*
 	 * Remap the vmemmap virtual address range [@vmemmap_start, @vmemmap_end)
 	 * to the page which @vmemmap_reuse is mapped to, then free the pages
 	 * which the range [@vmemmap_start, @vmemmap_end] is mapped to.
 	 */
 	if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse))
 		static_branch_dec(&hugetlb_optimize_vmemmap_key);
 	else
 		SetHPageVmemmapOptimized(head);
 }

 static struct ctl_table hugetlb_vmemmap_sysctls[] = {
 	{
 		.procname	= "hugetlb_optimize_vmemmap",
 		.data		= &vmemmap_optimize_enabled,
 		.maxlen		= sizeof(vmemmap_optimize_enabled),
 		.mode		= 0644,
 		.proc_handler	= proc_dobool,
 	},
 	{ }
 };

 static int __init hugetlb_vmemmap_init(void)
 {
 	const struct hstate *h;

 	/* HUGETLB_VMEMMAP_RESERVE_SIZE should cover all used struct pages */
 	BUILD_BUG_ON(__NR_USED_SUBPAGE * sizeof(struct page) > HUGETLB_VMEMMAP_RESERVE_SIZE);

 	for_each_hstate(h) {
 		if (hugetlb_vmemmap_optimizable(h)) {
 			register_sysctl_init("vm", hugetlb_vmemmap_sysctls);
 			break;
 		}
 	}
 	return 0;
 }
 late_initcall(hugetlb_vmemmap_init);
	// SPDX-License-Identifier: GPL-2.0
	/*
	* HugeTLB Vmemmap Optimization (HVO)
	*
	* Copyright (c) 2020, ByteDance. All rights reserved.
	*
	* Author: Muchun Song <songmuchun@bytedance.com>
	*
	* See Documentation/mm/vmemmap_dedup.rst
	*/
	#define pr_fmt(fmt) "HugeTLB: " fmt

	#include <linux/pgtable.h>
	#include <linux/moduleparam.h>
	#include <linux/bootmem_info.h>
	#include <asm/pgalloc.h>
	#include <asm/tlbflush.h>
	#include "hugetlb_vmemmap.h"

	/**
	* struct vmemmap_remap_walk - walk vmemmap page table
	*
	* @remap_pte: called for each lowest-level entry (PTE).
	* @nr_walked: the number of walked pte.
	* @reuse_page: the page which is reused for the tail vmemmap pages.
	* @reuse_addr: the virtual address of the @reuse_page page.
	* @vmemmap_pages: the list head of the vmemmap pages that can be freed
	* or is mapped from.
	*/
	struct vmemmap_remap_walk {
	void (remap_pte)(pte_t pte, unsigned long addr,
	struct vmemmap_remap_walk *walk);
	unsigned long nr_walked;
	struct page *reuse_page;
	unsigned long reuse_addr;
	struct list_head *vmemmap_pages;
	};

	static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
	{
	pmd_t __pmd;
	int i;
	unsigned long addr = start;
	struct page *head;
	pte_t *pgtable;

	spin_lock(&init_mm.page_table_lock);
	head = pmd_leaf(pmd) ? pmd_page(pmd) : NULL;
	spin_unlock(&init_mm.page_table_lock);

	if (!head)
	return 0;

	pgtable = pte_alloc_one_kernel(&init_mm);
	if (!pgtable)
	return -ENOMEM;

	pmd_populate_kernel(&init_mm, &__pmd, pgtable);

	for (i = 0; i < PTRS_PER_PTE; i++, addr += PAGE_SIZE) {
	pte_t entry, *pte;
	pgprot_t pgprot = PAGE_KERNEL;

	entry = mk_pte(head + i, pgprot);
	pte = pte_offset_kernel(&__pmd, addr);
	set_pte_at(&init_mm, addr, pte, entry);
	}

	spin_lock(&init_mm.page_table_lock);
	if (likely(pmd_leaf(*pmd))) {
	/*
	* Higher order allocations from buddy allocator must be able to
	* be treated as indepdenent small pages (as they can be freed
	* individually).
	*/
	if (!PageReserved(head))
	split_page(head, get_order(PMD_SIZE));

	/* Make pte visible before pmd. See comment in pmd_install(). */
	smp_wmb();
	pmd_populate_kernel(&init_mm, pmd, pgtable);
	flush_tlb_kernel_range(start, start + PMD_SIZE);
	} else {
	pte_free_kernel(&init_mm, pgtable);
	}
	spin_unlock(&init_mm.page_table_lock);

	return 0;
	}

	static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
	unsigned long end,
	struct vmemmap_remap_walk *walk)
	{
	pte_t *pte = pte_offset_kernel(pmd, addr);

	/*
	* The reuse_page is found 'first' in table walk before we start
	* remapping (which is calling @walk->remap_pte).
	*/
	if (!walk->reuse_page) {
	walk->reuse_page = pte_page(ptep_get(pte));
	/*
	* Because the reuse address is part of the range that we are
	* walking, skip the reuse address range.
	*/
	addr += PAGE_SIZE;
	pte++;
	walk->nr_walked++;
	}

	for (; addr != end; addr += PAGE_SIZE, pte++) {
	walk->remap_pte(pte, addr, walk);
	walk->nr_walked++;
	}
	}

	static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
	unsigned long end,
	struct vmemmap_remap_walk *walk)
	{
	pmd_t *pmd;
	unsigned long next;

	pmd = pmd_offset(pud, addr);
	do {
	int ret;

	ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK);
	if (ret)
	return ret;

	next = pmd_addr_end(addr, end);
	vmemmap_pte_range(pmd, addr, next, walk);
	} while (pmd++, addr = next, addr != end);

	return 0;
	}

	static int vmemmap_pud_range(p4d_t *p4d, unsigned long addr,
	unsigned long end,
	struct vmemmap_remap_walk *walk)
	{
	pud_t *pud;
	unsigned long next;

	pud = pud_offset(p4d, addr);
	do {
	int ret;

	next = pud_addr_end(addr, end);
	ret = vmemmap_pmd_range(pud, addr, next, walk);
	if (ret)
	return ret;
	} while (pud++, addr = next, addr != end);

	return 0;
	}

	static int vmemmap_p4d_range(pgd_t *pgd, unsigned long addr,
	unsigned long end,
	struct vmemmap_remap_walk *walk)
	{
	p4d_t *p4d;
	unsigned long next;

	p4d = p4d_offset(pgd, addr);
	do {
	int ret;

	next = p4d_addr_end(addr, end);
	ret = vmemmap_pud_range(p4d, addr, next, walk);
	if (ret)
	return ret;
	} while (p4d++, addr = next, addr != end);

	return 0;
	}

	static int vmemmap_remap_range(unsigned long start, unsigned long end,
	struct vmemmap_remap_walk *walk)
	{
	unsigned long addr = start;
	unsigned long next;
	pgd_t *pgd;

	VM_BUG_ON(!PAGE_ALIGNED(start));
	VM_BUG_ON(!PAGE_ALIGNED(end));

	pgd = pgd_offset_k(addr);
	do {
	int ret;

	next = pgd_addr_end(addr, end);
	ret = vmemmap_p4d_range(pgd, addr, next, walk);
	if (ret)
	return ret;
	} while (pgd++, addr = next, addr != end);

	flush_tlb_kernel_range(start, end);

	return 0;
	}

	/*
	* Free a vmemmap page. A vmemmap page can be allocated from the memblock
	* allocator or buddy allocator. If the PG_reserved flag is set, it means
	* that it allocated from the memblock allocator, just free it via the
	* free_bootmem_page(). Otherwise, use __free_page().
	*/
	static inline void free_vmemmap_page(struct page *page)
	{
	if (PageReserved(page))
	free_bootmem_page(page);
	else
	__free_page(page);
	}

	/* Free a list of the vmemmap pages */
	static void free_vmemmap_page_list(struct list_head *list)
	{
	struct page page, next;

	list_for_each_entry_safe(page, next, list, lru)
	free_vmemmap_page(page);
	}

	static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
	struct vmemmap_remap_walk *walk)
	{
	/*
	* Remap the tail pages as read-only to catch illegal write operation
	* to the tail pages.
	*/
	pgprot_t pgprot = PAGE_KERNEL_RO;
	struct page *page = pte_page(ptep_get(pte));
	pte_t entry;

	/* Remapping the head page requires r/w */
	if (unlikely(addr == walk->reuse_addr)) {
	pgprot = PAGE_KERNEL;
	list_del(&walk->reuse_page->lru);

	/*
	* Makes sure that preceding stores to the page contents from
	* vmemmap_remap_free() become visible before the set_pte_at()
	* write.
	*/
	smp_wmb();
	}

	entry = mk_pte(walk->reuse_page, pgprot);
	list_add_tail(&page->lru, walk->vmemmap_pages);
	set_pte_at(&init_mm, addr, pte, entry);
	}

	/*
	* How many struct page structs need to be reset. When we reuse the head
	* struct page, the special metadata (e.g. page->flags or page->mapping)
	* cannot copy to the tail struct page structs. The invalid value will be
	* checked in the free_tail_page_prepare(). In order to avoid the message
	* of "corrupted mapping in tail page". We need to reset at least 3 (one
	* head struct page struct and two tail struct page structs) struct page
	* structs.
	*/
	#define NR_RESET_STRUCT_PAGE 3

	static inline void reset_struct_pages(struct page *start)
	{
	struct page *from = start + NR_RESET_STRUCT_PAGE;

	BUILD_BUG_ON(NR_RESET_STRUCT_PAGE * 2 > PAGE_SIZE / sizeof(struct page));
	memcpy(start, from, sizeof(from) NR_RESET_STRUCT_PAGE);
	}

	static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
	struct vmemmap_remap_walk *walk)
	{
	pgprot_t pgprot = PAGE_KERNEL;
	struct page *page;
	void *to;

	BUG_ON(pte_page(ptep_get(pte)) != walk->reuse_page);

	page = list_first_entry(walk->vmemmap_pages, struct page, lru);
	list_del(&page->lru);
	to = page_to_virt(page);
	copy_page(to, (void *)walk->reuse_addr);
	reset_struct_pages(to);

	/*
	* Makes sure that preceding stores to the page contents become visible
	* before the set_pte_at() write.
	*/
	smp_wmb();
	set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
	}

	/**
	* vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
	* to the page which @reuse is mapped to, then free vmemmap
	* which the range are mapped to.
	* @start: start address of the vmemmap virtual address range that we want
	* to remap.
	* @end: end address of the vmemmap virtual address range that we want to
	* remap.
	* @reuse: reuse address.
	*
	* Return: %0 on success, negative error code otherwise.
	*/
	static int vmemmap_remap_free(unsigned long start, unsigned long end,
	unsigned long reuse)
	{
	int ret;
	LIST_HEAD(vmemmap_pages);
	struct vmemmap_remap_walk walk = {
	.remap_pte = vmemmap_remap_pte,
	.reuse_addr = reuse,
	.vmemmap_pages = &vmemmap_pages,
	};
	int nid = page_to_nid((struct page *)start);
	gfp_t gfp_mask = GFP_KERNEL \| __GFP_THISNODE \| __GFP_NORETRY \|
	__GFP_NOWARN;

	/*
	* Allocate a new head vmemmap page to avoid breaking a contiguous
	* block of struct page memory when freeing it back to page allocator
	* in free_vmemmap_page_list(). This will allow the likely contiguous
	* struct page backing memory to be kept contiguous and allowing for
	* more allocations of hugepages. Fallback to the currently
	* mapped head page in case should it fail to allocate.
	*/
	walk.reuse_page = alloc_pages_node(nid, gfp_mask, 0);
	if (walk.reuse_page) {
	copy_page(page_to_virt(walk.reuse_page),
	(void *)walk.reuse_addr);
	list_add(&walk.reuse_page->lru, &vmemmap_pages);
	}

	/*
	* In order to make remapping routine most efficient for the huge pages,
	* the routine of vmemmap page table walking has the following rules
	* (see more details from the vmemmap_pte_range()):
	*
	* - The range [@start, @end) and the range [@reuse, @reuse + PAGE_SIZE)
	* should be continuous.
	* - The @reuse address is part of the range [@reuse, @end) that we are
	* walking which is passed to vmemmap_remap_range().
	* - The @reuse address is the first in the complete range.
	*
	* So we need to make sure that @start and @reuse meet the above rules.
	*/
	BUG_ON(start - reuse != PAGE_SIZE);

	mmap_read_lock(&init_mm);
	ret = vmemmap_remap_range(reuse, end, &walk);
	if (ret && walk.nr_walked) {
	end = reuse + walk.nr_walked * PAGE_SIZE;
	/*
	* vmemmap_pages contains pages from the previous
	* vmemmap_remap_range call which failed. These
	* are pages which were removed from the vmemmap.
	* They will be restored in the following call.
	*/
	walk = (struct vmemmap_remap_walk) {
	.remap_pte = vmemmap_restore_pte,
	.reuse_addr = reuse,
	.vmemmap_pages = &vmemmap_pages,
	};

	vmemmap_remap_range(reuse, end, &walk);
	}
	mmap_read_unlock(&init_mm);

	free_vmemmap_page_list(&vmemmap_pages);

	return ret;
	}

	static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
	struct list_head *list)
	{
	gfp_t gfp_mask = GFP_KERNEL \| __GFP_RETRY_MAYFAIL \| __GFP_THISNODE;
	unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
	int nid = page_to_nid((struct page *)start);
	struct page page, next;

	while (nr_pages--) {
	page = alloc_pages_node(nid, gfp_mask, 0);
	if (!page)
	goto out;
	list_add_tail(&page->lru, list);
	}

	return 0;
	out:
	list_for_each_entry_safe(page, next, list, lru)
	__free_page(page);
	return -ENOMEM;
	}

	/**
	* vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
	* to the page which is from the @vmemmap_pages
	* respectively.
	* @start: start address of the vmemmap virtual address range that we want
	* to remap.
	* @end: end address of the vmemmap virtual address range that we want to
	* remap.
	* @reuse: reuse address.
	*
	* Return: %0 on success, negative error code otherwise.
	*/
	static int vmemmap_remap_alloc(unsigned long start, unsigned long end,
	unsigned long reuse)
	{
	LIST_HEAD(vmemmap_pages);
	struct vmemmap_remap_walk walk = {
	.remap_pte = vmemmap_restore_pte,
	.reuse_addr = reuse,
	.vmemmap_pages = &vmemmap_pages,
	};

	/* See the comment in the vmemmap_remap_free(). */
	BUG_ON(start - reuse != PAGE_SIZE);

	if (alloc_vmemmap_page_list(start, end, &vmemmap_pages))
	return -ENOMEM;

	mmap_read_lock(&init_mm);
	vmemmap_remap_range(reuse, end, &walk);
	mmap_read_unlock(&init_mm);

	return 0;
	}

	DEFINE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
	EXPORT_SYMBOL(hugetlb_optimize_vmemmap_key);

	static bool vmemmap_optimize_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON);
	core_param(hugetlb_free_vmemmap, vmemmap_optimize_enabled, bool, 0);

	/**
	* hugetlb_vmemmap_restore - restore previously optimized (by
	* hugetlb_vmemmap_optimize()) vmemmap pages which
	* will be reallocated and remapped.
	* @h: struct hstate.
	* @head: the head page whose vmemmap pages will be restored.
	*
	* Return: %0 if @head's vmemmap pages have been reallocated and remapped,
	* negative error code otherwise.
	*/
	int hugetlb_vmemmap_restore(const struct hstate h, struct page head)
	{
	int ret;
	unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
	unsigned long vmemmap_reuse;

	if (!HPageVmemmapOptimized(head))
	return 0;

	vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);
	vmemmap_reuse = vmemmap_start;
	vmemmap_start += HUGETLB_VMEMMAP_RESERVE_SIZE;

	/*
	* The pages which the vmemmap virtual address range [@vmemmap_start,
	* @vmemmap_end) are mapped to are freed to the buddy allocator, and
	* the range is mapped to the page which @vmemmap_reuse is mapped to.
	* When a HugeTLB page is freed to the buddy allocator, previously
	* discarded vmemmap pages must be allocated and remapping.
	*/
	ret = vmemmap_remap_alloc(vmemmap_start, vmemmap_end, vmemmap_reuse);
	if (!ret) {
	ClearHPageVmemmapOptimized(head);
	static_branch_dec(&hugetlb_optimize_vmemmap_key);
	}

	return ret;
	}

	/* Return true iff a HugeTLB whose vmemmap should and can be optimized. */
	static bool vmemmap_should_optimize(const struct hstate h, const struct page head)
	{
	if (!READ_ONCE(vmemmap_optimize_enabled))
	return false;

	if (!hugetlb_vmemmap_optimizable(h))
	return false;

	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) {
	pmd_t *pmdp, pmd;
	struct page *vmemmap_page;
	unsigned long vaddr = (unsigned long)head;

	/*
	* Only the vmemmap page's vmemmap page can be self-hosted.
	* Walking the page tables to find the backing page of the
	* vmemmap page.
	*/
	pmdp = pmd_off_k(vaddr);
	/*
	* The READ_ONCE() is used to stabilize *pmdp in a register or
	* on the stack so that it will stop changing under the code.
	* The only concurrent operation where it can be changed is
	* split_vmemmap_huge_pmd() (*pmdp will be stable after this
	* operation).
	*/
	pmd = READ_ONCE(*pmdp);
	if (pmd_leaf(pmd))
	vmemmap_page = pmd_page(pmd) + pte_index(vaddr);
	else
	vmemmap_page = pte_page(*pte_offset_kernel(pmdp, vaddr));
	/*
	* Due to HugeTLB alignment requirements and the vmemmap pages
	* being at the start of the hotplugged memory region in
	* memory_hotplug.memmap_on_memory case. Checking any vmemmap
	* page's vmemmap page if it is marked as VmemmapSelfHosted is
	* sufficient.
	*
	* [ hotplugged memory ]
	* [ section ][...][ section ]
	* [ vmemmap ][ usable memory ]
	* ^ \| \| \|
	* +---+ \| \|
	* ^ \| \|
	* +-------+ \|
	* ^ \|
	* +-------------------------------------------+
	*/
	if (PageVmemmapSelfHosted(vmemmap_page))
	return false;
	}

	return true;
	}

	/**
	* hugetlb_vmemmap_optimize - optimize @head page's vmemmap pages.
	* @h: struct hstate.
	* @head: the head page whose vmemmap pages will be optimized.
	*
	* This function only tries to optimize @head's vmemmap pages and does not
	* guarantee that the optimization will succeed after it returns. The caller
	* can use HPageVmemmapOptimized(@head) to detect if @head's vmemmap pages
	* have been optimized.
	*/
	void hugetlb_vmemmap_optimize(const struct hstate h, struct page head)
	{
	unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
	unsigned long vmemmap_reuse;

	if (!vmemmap_should_optimize(h, head))
	return;

	static_branch_inc(&hugetlb_optimize_vmemmap_key);

	vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);
	vmemmap_reuse = vmemmap_start;
	vmemmap_start += HUGETLB_VMEMMAP_RESERVE_SIZE;

	/*
	* Remap the vmemmap virtual address range [@vmemmap_start, @vmemmap_end)
	* to the page which @vmemmap_reuse is mapped to, then free the pages
	* which the range [@vmemmap_start, @vmemmap_end] is mapped to.
	*/
	if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse))
	static_branch_dec(&hugetlb_optimize_vmemmap_key);
	else
	SetHPageVmemmapOptimized(head);
	}

	static struct ctl_table hugetlb_vmemmap_sysctls[] = {
	{
	.procname = "hugetlb_optimize_vmemmap",
	.data = &vmemmap_optimize_enabled,
	.maxlen = sizeof(vmemmap_optimize_enabled),
	.mode = 0644,
	.proc_handler = proc_dobool,
	},
	{ }
	};

	static int __init hugetlb_vmemmap_init(void)
	{
	const struct hstate *h;

	/* HUGETLB_VMEMMAP_RESERVE_SIZE should cover all used struct pages */
	BUILD_BUG_ON(__NR_USED_SUBPAGE * sizeof(struct page) > HUGETLB_VMEMMAP_RESERVE_SIZE);

	for_each_hstate(h) {
	if (hugetlb_vmemmap_optimizable(h)) {
	register_sysctl_init("vm", hugetlb_vmemmap_sysctls);
	break;
	}
	}
	return 0;
	}
	late_initcall(hugetlb_vmemmap_init);