// SPDX-License-Identifier: GPL-2.0

  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
  
  #include <linux/mm.h>
  #include <linux/sched.h>

  #include <linux/sched/mm.h>

  #include <linux/sched/coredump.h>

  #include <linux/mmu_notifier.h>
  #include <linux/rmap.h>
  #include <linux/swap.h>
  #include <linux/mm_inline.h>
  #include <linux/kthread.h>
  #include <linux/khugepaged.h>
  #include <linux/freezer.h>
  #include <linux/mman.h>
  #include <linux/hashtable.h>
  #include <linux/userfaultfd_k.h>
  #include <linux/page_idle.h>
  #include <linux/swapops.h>

  #include <linux/shmem_fs.h>

  
  #include <asm/tlb.h>
  #include <asm/pgalloc.h>
  #include "internal.h"
  
  enum scan_result {
  	SCAN_FAIL,
  	SCAN_SUCCEED,
  	SCAN_PMD_NULL,
  	SCAN_EXCEED_NONE_PTE,

  	SCAN_EXCEED_SWAP_PTE,
  	SCAN_EXCEED_SHARED_PTE,

  	SCAN_PTE_NON_PRESENT,

  	SCAN_PTE_UFFD_WP,

  	SCAN_PAGE_RO,

  	SCAN_LACK_REFERENCED_PAGE,

  	SCAN_PAGE_NULL,
  	SCAN_SCAN_ABORT,
  	SCAN_PAGE_COUNT,
  	SCAN_PAGE_LRU,
  	SCAN_PAGE_LOCK,
  	SCAN_PAGE_ANON,
  	SCAN_PAGE_COMPOUND,
  	SCAN_ANY_PROCESS,
  	SCAN_VMA_NULL,
  	SCAN_VMA_CHECK,
  	SCAN_ADDRESS_RANGE,
  	SCAN_SWAP_CACHE_PAGE,
  	SCAN_DEL_PAGE_LRU,
  	SCAN_ALLOC_HUGE_PAGE_FAIL,
  	SCAN_CGROUP_CHARGE_FAIL,

  	SCAN_TRUNCATED,

  	SCAN_PAGE_HAS_PRIVATE,

  };
  
  #define CREATE_TRACE_POINTS
  #include <trace/events/huge_memory.h>

  static struct task_struct *khugepaged_thread __read_mostly;
  static DEFINE_MUTEX(khugepaged_mutex);

  /* default scan 8*512 pte (or vmas) every 30 second */
  static unsigned int khugepaged_pages_to_scan __read_mostly;
  static unsigned int khugepaged_pages_collapsed;
  static unsigned int khugepaged_full_scans;
  static unsigned int khugepaged_scan_sleep_millisecs __read_mostly = 10000;
  /* during fragmentation poll the hugepage allocator once every minute */
  static unsigned int khugepaged_alloc_sleep_millisecs __read_mostly = 60000;
  static unsigned long khugepaged_sleep_expire;
  static DEFINE_SPINLOCK(khugepaged_mm_lock);
  static DECLARE_WAIT_QUEUE_HEAD(khugepaged_wait);
  /*
   * default collapse hugepages if there is at least one pte mapped like
   * it would have happened if the vma was large enough during page
   * fault.
   */
  static unsigned int khugepaged_max_ptes_none __read_mostly;
  static unsigned int khugepaged_max_ptes_swap __read_mostly;

  static unsigned int khugepaged_max_ptes_shared __read_mostly;

  
  #define MM_SLOTS_HASH_BITS 10
  static __read_mostly DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
  
  static struct kmem_cache *mm_slot_cache __read_mostly;

  #define MAX_PTE_MAPPED_THP 8

  /**
   * struct mm_slot - hash lookup from mm to mm_slot
   * @hash: hash collision list
   * @mm_node: khugepaged scan list headed in khugepaged_scan.mm_head
   * @mm: the mm that this information is valid for
   */
  struct mm_slot {
  	struct hlist_node hash;
  	struct list_head mm_node;
  	struct mm_struct *mm;

  
  	/* pte-mapped THP in this mm */
  	int nr_pte_mapped_thp;
  	unsigned long pte_mapped_thp[MAX_PTE_MAPPED_THP];

  };
  
  /**
   * struct khugepaged_scan - cursor for scanning
   * @mm_head: the head of the mm list to scan
   * @mm_slot: the current mm_slot we are scanning
   * @address: the next address inside that to be scanned
   *
   * There is only the one khugepaged_scan instance of this cursor structure.
   */
  struct khugepaged_scan {
  	struct list_head mm_head;
  	struct mm_slot *mm_slot;
  	unsigned long address;
  };
  
  static struct khugepaged_scan khugepaged_scan = {
  	.mm_head = LIST_HEAD_INIT(khugepaged_scan.mm_head),
  };

  #ifdef CONFIG_SYSFS

  static ssize_t scan_sleep_millisecs_show(struct kobject *kobj,
  					 struct kobj_attribute *attr,
  					 char *buf)
  {
  	return sprintf(buf, "%u
  ", khugepaged_scan_sleep_millisecs);
  }
  
  static ssize_t scan_sleep_millisecs_store(struct kobject *kobj,
  					  struct kobj_attribute *attr,
  					  const char *buf, size_t count)
  {
  	unsigned long msecs;
  	int err;
  
  	err = kstrtoul(buf, 10, &msecs);
  	if (err || msecs > UINT_MAX)
  		return -EINVAL;
  
  	khugepaged_scan_sleep_millisecs = msecs;
  	khugepaged_sleep_expire = 0;
  	wake_up_interruptible(&khugepaged_wait);
  
  	return count;
  }
  static struct kobj_attribute scan_sleep_millisecs_attr =
  	__ATTR(scan_sleep_millisecs, 0644, scan_sleep_millisecs_show,
  	       scan_sleep_millisecs_store);
  
  static ssize_t alloc_sleep_millisecs_show(struct kobject *kobj,
  					  struct kobj_attribute *attr,
  					  char *buf)
  {
  	return sprintf(buf, "%u
  ", khugepaged_alloc_sleep_millisecs);
  }
  
  static ssize_t alloc_sleep_millisecs_store(struct kobject *kobj,
  					   struct kobj_attribute *attr,
  					   const char *buf, size_t count)
  {
  	unsigned long msecs;
  	int err;
  
  	err = kstrtoul(buf, 10, &msecs);
  	if (err || msecs > UINT_MAX)
  		return -EINVAL;
  
  	khugepaged_alloc_sleep_millisecs = msecs;
  	khugepaged_sleep_expire = 0;
  	wake_up_interruptible(&khugepaged_wait);
  
  	return count;
  }
  static struct kobj_attribute alloc_sleep_millisecs_attr =
  	__ATTR(alloc_sleep_millisecs, 0644, alloc_sleep_millisecs_show,
  	       alloc_sleep_millisecs_store);
  
  static ssize_t pages_to_scan_show(struct kobject *kobj,
  				  struct kobj_attribute *attr,
  				  char *buf)
  {
  	return sprintf(buf, "%u
  ", khugepaged_pages_to_scan);
  }
  static ssize_t pages_to_scan_store(struct kobject *kobj,
  				   struct kobj_attribute *attr,
  				   const char *buf, size_t count)
  {
  	int err;
  	unsigned long pages;
  
  	err = kstrtoul(buf, 10, &pages);
  	if (err || !pages || pages > UINT_MAX)
  		return -EINVAL;
  
  	khugepaged_pages_to_scan = pages;
  
  	return count;
  }
  static struct kobj_attribute pages_to_scan_attr =
  	__ATTR(pages_to_scan, 0644, pages_to_scan_show,
  	       pages_to_scan_store);
  
  static ssize_t pages_collapsed_show(struct kobject *kobj,
  				    struct kobj_attribute *attr,
  				    char *buf)
  {
  	return sprintf(buf, "%u
  ", khugepaged_pages_collapsed);
  }
  static struct kobj_attribute pages_collapsed_attr =
  	__ATTR_RO(pages_collapsed);
  
  static ssize_t full_scans_show(struct kobject *kobj,
  			       struct kobj_attribute *attr,
  			       char *buf)
  {
  	return sprintf(buf, "%u
  ", khugepaged_full_scans);
  }
  static struct kobj_attribute full_scans_attr =
  	__ATTR_RO(full_scans);
  
  static ssize_t khugepaged_defrag_show(struct kobject *kobj,
  				      struct kobj_attribute *attr, char *buf)
  {
  	return single_hugepage_flag_show(kobj, attr, buf,
  				TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG);
  }
  static ssize_t khugepaged_defrag_store(struct kobject *kobj,
  				       struct kobj_attribute *attr,
  				       const char *buf, size_t count)
  {
  	return single_hugepage_flag_store(kobj, attr, buf, count,
  				 TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG);
  }
  static struct kobj_attribute khugepaged_defrag_attr =
  	__ATTR(defrag, 0644, khugepaged_defrag_show,
  	       khugepaged_defrag_store);
  
  /*
   * max_ptes_none controls if khugepaged should collapse hugepages over
   * any unmapped ptes in turn potentially increasing the memory
   * footprint of the vmas. When max_ptes_none is 0 khugepaged will not
   * reduce the available free memory in the system as it
   * runs. Increasing max_ptes_none will instead potentially reduce the
   * free memory in the system during the khugepaged scan.
   */
  static ssize_t khugepaged_max_ptes_none_show(struct kobject *kobj,
  					     struct kobj_attribute *attr,
  					     char *buf)
  {
  	return sprintf(buf, "%u
  ", khugepaged_max_ptes_none);
  }
  static ssize_t khugepaged_max_ptes_none_store(struct kobject *kobj,
  					      struct kobj_attribute *attr,
  					      const char *buf, size_t count)
  {
  	int err;
  	unsigned long max_ptes_none;
  
  	err = kstrtoul(buf, 10, &max_ptes_none);
  	if (err || max_ptes_none > HPAGE_PMD_NR-1)
  		return -EINVAL;
  
  	khugepaged_max_ptes_none = max_ptes_none;
  
  	return count;
  }
  static struct kobj_attribute khugepaged_max_ptes_none_attr =
  	__ATTR(max_ptes_none, 0644, khugepaged_max_ptes_none_show,
  	       khugepaged_max_ptes_none_store);
  
  static ssize_t khugepaged_max_ptes_swap_show(struct kobject *kobj,
  					     struct kobj_attribute *attr,
  					     char *buf)
  {
  	return sprintf(buf, "%u
  ", khugepaged_max_ptes_swap);
  }
  
  static ssize_t khugepaged_max_ptes_swap_store(struct kobject *kobj,
  					      struct kobj_attribute *attr,
  					      const char *buf, size_t count)
  {
  	int err;
  	unsigned long max_ptes_swap;
  
  	err  = kstrtoul(buf, 10, &max_ptes_swap);
  	if (err || max_ptes_swap > HPAGE_PMD_NR-1)
  		return -EINVAL;
  
  	khugepaged_max_ptes_swap = max_ptes_swap;
  
  	return count;
  }
  
  static struct kobj_attribute khugepaged_max_ptes_swap_attr =
  	__ATTR(max_ptes_swap, 0644, khugepaged_max_ptes_swap_show,
  	       khugepaged_max_ptes_swap_store);

  static ssize_t khugepaged_max_ptes_shared_show(struct kobject *kobj,
  					     struct kobj_attribute *attr,
  					     char *buf)
  {
  	return sprintf(buf, "%u
  ", khugepaged_max_ptes_shared);
  }
  
  static ssize_t khugepaged_max_ptes_shared_store(struct kobject *kobj,
  					      struct kobj_attribute *attr,
  					      const char *buf, size_t count)
  {
  	int err;
  	unsigned long max_ptes_shared;
  
  	err  = kstrtoul(buf, 10, &max_ptes_shared);
  	if (err || max_ptes_shared > HPAGE_PMD_NR-1)
  		return -EINVAL;
  
  	khugepaged_max_ptes_shared = max_ptes_shared;
  
  	return count;
  }
  
  static struct kobj_attribute khugepaged_max_ptes_shared_attr =
  	__ATTR(max_ptes_shared, 0644, khugepaged_max_ptes_shared_show,
  	       khugepaged_max_ptes_shared_store);

  static struct attribute *khugepaged_attr[] = {
  	&khugepaged_defrag_attr.attr,
  	&khugepaged_max_ptes_none_attr.attr,

  	&khugepaged_max_ptes_swap_attr.attr,
  	&khugepaged_max_ptes_shared_attr.attr,

  	&pages_to_scan_attr.attr,
  	&pages_collapsed_attr.attr,
  	&full_scans_attr.attr,
  	&scan_sleep_millisecs_attr.attr,
  	&alloc_sleep_millisecs_attr.attr,

  	NULL,
  };
  
  struct attribute_group khugepaged_attr_group = {
  	.attrs = khugepaged_attr,
  	.name = "khugepaged",
  };

  #endif /* CONFIG_SYSFS */

  int hugepage_madvise(struct vm_area_struct *vma,
  		     unsigned long *vm_flags, int advice)
  {
  	switch (advice) {
  	case MADV_HUGEPAGE:
  #ifdef CONFIG_S390
  		/*
  		 * qemu blindly sets MADV_HUGEPAGE on all allocations, but s390
  		 * can't handle this properly after s390_enable_sie, so we simply
  		 * ignore the madvise to prevent qemu from causing a SIGSEGV.
  		 */
  		if (mm_has_pgste(vma->vm_mm))
  			return 0;
  #endif
  		*vm_flags &= ~VM_NOHUGEPAGE;
  		*vm_flags |= VM_HUGEPAGE;
  		/*
  		 * If the vma become good for khugepaged to scan,
  		 * register it here without waiting a page fault that
  		 * may not happen any time soon.
  		 */
  		if (!(*vm_flags & VM_NO_KHUGEPAGED) &&
  				khugepaged_enter_vma_merge(vma, *vm_flags))
  			return -ENOMEM;
  		break;
  	case MADV_NOHUGEPAGE:
  		*vm_flags &= ~VM_HUGEPAGE;
  		*vm_flags |= VM_NOHUGEPAGE;
  		/*
  		 * Setting VM_NOHUGEPAGE will prevent khugepaged from scanning
  		 * this vma even if we leave the mm registered in khugepaged if
  		 * it got registered before VM_NOHUGEPAGE was set.
  		 */
  		break;
  	}
  
  	return 0;
  }
  
  int __init khugepaged_init(void)
  {
  	mm_slot_cache = kmem_cache_create("khugepaged_mm_slot",
  					  sizeof(struct mm_slot),
  					  __alignof__(struct mm_slot), 0, NULL);
  	if (!mm_slot_cache)
  		return -ENOMEM;
  
  	khugepaged_pages_to_scan = HPAGE_PMD_NR * 8;
  	khugepaged_max_ptes_none = HPAGE_PMD_NR - 1;
  	khugepaged_max_ptes_swap = HPAGE_PMD_NR / 8;

  	khugepaged_max_ptes_shared = HPAGE_PMD_NR / 2;

  
  	return 0;
  }
  
  void __init khugepaged_destroy(void)
  {
  	kmem_cache_destroy(mm_slot_cache);
  }
  
  static inline struct mm_slot *alloc_mm_slot(void)
  {
  	if (!mm_slot_cache)	/* initialization failed */
  		return NULL;
  	return kmem_cache_zalloc(mm_slot_cache, GFP_KERNEL);
  }
  
  static inline void free_mm_slot(struct mm_slot *mm_slot)
  {
  	kmem_cache_free(mm_slot_cache, mm_slot);
  }
  
  static struct mm_slot *get_mm_slot(struct mm_struct *mm)
  {
  	struct mm_slot *mm_slot;
  
  	hash_for_each_possible(mm_slots_hash, mm_slot, hash, (unsigned long)mm)
  		if (mm == mm_slot->mm)
  			return mm_slot;
  
  	return NULL;
  }
  
  static void insert_to_mm_slots_hash(struct mm_struct *mm,
  				    struct mm_slot *mm_slot)
  {
  	mm_slot->mm = mm;
  	hash_add(mm_slots_hash, &mm_slot->hash, (long)mm);
  }
  
  static inline int khugepaged_test_exit(struct mm_struct *mm)
  {

  	return atomic_read(&mm->mm_users) == 0;

  }

  static bool hugepage_vma_check(struct vm_area_struct *vma,
  			       unsigned long vm_flags)

  {

  	if ((!(vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
  	    (vm_flags & VM_NOHUGEPAGE) ||

  	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
  		return false;

  
  	if (shmem_file(vma->vm_file) ||
  	    (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
  	     vma->vm_file &&
  	     (vm_flags & VM_DENYWRITE))) {

  		return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
  				HPAGE_PMD_NR);
  	}
  	if (!vma->anon_vma || vma->vm_ops)
  		return false;

  	if (vma_is_temporary_stack(vma))

  		return false;

  	return !(vm_flags & VM_NO_KHUGEPAGED);

  }

  int __khugepaged_enter(struct mm_struct *mm)
  {
  	struct mm_slot *mm_slot;
  	int wakeup;
  
  	mm_slot = alloc_mm_slot();
  	if (!mm_slot)
  		return -ENOMEM;
  
  	/* __khugepaged_exit() must not run from under us */

  	VM_BUG_ON_MM(atomic_read(&mm->mm_users) == 0, mm);

  	if (unlikely(test_and_set_bit(MMF_VM_HUGEPAGE, &mm->flags))) {
  		free_mm_slot(mm_slot);
  		return 0;
  	}
  
  	spin_lock(&khugepaged_mm_lock);
  	insert_to_mm_slots_hash(mm, mm_slot);
  	/*
  	 * Insert just behind the scanning cursor, to let the area settle
  	 * down a little.
  	 */
  	wakeup = list_empty(&khugepaged_scan.mm_head);
  	list_add_tail(&mm_slot->mm_node, &khugepaged_scan.mm_head);
  	spin_unlock(&khugepaged_mm_lock);

  	mmgrab(mm);

  	if (wakeup)
  		wake_up_interruptible(&khugepaged_wait);
  
  	return 0;
  }
  
  int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
  			       unsigned long vm_flags)
  {
  	unsigned long hstart, hend;

  
  	/*

  	 * khugepaged only supports read-only files for non-shmem files.
  	 * khugepaged does not yet work on special mappings. And
  	 * file-private shmem THP is not supported.

  	 */

  	if (!hugepage_vma_check(vma, vm_flags))

  		return 0;

  	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
  	hend = vma->vm_end & HPAGE_PMD_MASK;
  	if (hstart < hend)
  		return khugepaged_enter(vma, vm_flags);
  	return 0;
  }
  
  void __khugepaged_exit(struct mm_struct *mm)
  {
  	struct mm_slot *mm_slot;
  	int free = 0;
  
  	spin_lock(&khugepaged_mm_lock);
  	mm_slot = get_mm_slot(mm);
  	if (mm_slot && khugepaged_scan.mm_slot != mm_slot) {
  		hash_del(&mm_slot->hash);
  		list_del(&mm_slot->mm_node);
  		free = 1;
  	}
  	spin_unlock(&khugepaged_mm_lock);
  
  	if (free) {
  		clear_bit(MMF_VM_HUGEPAGE, &mm->flags);
  		free_mm_slot(mm_slot);
  		mmdrop(mm);
  	} else if (mm_slot) {
  		/*
  		 * This is required to serialize against
  		 * khugepaged_test_exit() (which is guaranteed to run
  		 * under mmap sem read mode). Stop here (after we
  		 * return all pagetables will be destroyed) until
  		 * khugepaged has finished working on the pagetables

  		 * under the mmap_lock.

  		 */

  		mmap_write_lock(mm);
  		mmap_write_unlock(mm);

  	}
  }
  
  static void release_pte_page(struct page *page)
  {

  	mod_node_page_state(page_pgdat(page),
  			NR_ISOLATED_ANON + page_is_file_lru(page),
  			-compound_nr(page));

  	unlock_page(page);
  	putback_lru_page(page);
  }

  static void release_pte_pages(pte_t *pte, pte_t *_pte,
  		struct list_head *compound_pagelist)

  {

  	struct page *page, *tmp;

  	while (--_pte >= pte) {
  		pte_t pteval = *_pte;

  
  		page = pte_page(pteval);
  		if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)) &&
  				!PageCompound(page))
  			release_pte_page(page);
  	}
  
  	list_for_each_entry_safe(page, tmp, compound_pagelist, lru) {
  		list_del(&page->lru);
  		release_pte_page(page);

  	}
  }

  static bool is_refcount_suitable(struct page *page)
  {
  	int expected_refcount;
  
  	expected_refcount = total_mapcount(page);
  	if (PageSwapCache(page))
  		expected_refcount += compound_nr(page);
  
  	return page_count(page) == expected_refcount;
  }

  static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
  					unsigned long address,

  					pte_t *pte,
  					struct list_head *compound_pagelist)

  {
  	struct page *page = NULL;
  	pte_t *_pte;

  	int none_or_zero = 0, shared = 0, result = 0, referenced = 0;

  	bool writable = false;

  
  	for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
  	     _pte++, address += PAGE_SIZE) {
  		pte_t pteval = *_pte;
  		if (pte_none(pteval) || (pte_present(pteval) &&
  				is_zero_pfn(pte_pfn(pteval)))) {
  			if (!userfaultfd_armed(vma) &&
  			    ++none_or_zero <= khugepaged_max_ptes_none) {
  				continue;
  			} else {
  				result = SCAN_EXCEED_NONE_PTE;
  				goto out;
  			}
  		}
  		if (!pte_present(pteval)) {
  			result = SCAN_PTE_NON_PRESENT;
  			goto out;
  		}
  		page = vm_normal_page(vma, address, pteval);
  		if (unlikely(!page)) {
  			result = SCAN_PAGE_NULL;
  			goto out;
  		}

  		VM_BUG_ON_PAGE(!PageAnon(page), page);

  		if (page_mapcount(page) > 1 &&
  				++shared > khugepaged_max_ptes_shared) {
  			result = SCAN_EXCEED_SHARED_PTE;
  			goto out;
  		}

  		if (PageCompound(page)) {

  			struct page *p;
  			page = compound_head(page);

  			/*
  			 * Check if we have dealt with the compound page
  			 * already
  			 */
  			list_for_each_entry(p, compound_pagelist, lru) {
  				if (page == p)
  					goto next;
  			}
  		}

  
  		/*
  		 * We can do it before isolate_lru_page because the
  		 * page can't be freed from under us. NOTE: PG_lock
  		 * is needed to serialize against split_huge_page
  		 * when invoked from the VM.
  		 */
  		if (!trylock_page(page)) {
  			result = SCAN_PAGE_LOCK;
  			goto out;
  		}
  
  		/*

  		 * Check if the page has any GUP (or other external) pins.
  		 *
  		 * The page table that maps the page has been already unlinked
  		 * from the page table tree and this process cannot get
  		 * an additinal pin on the page.
  		 *
  		 * New pins can come later if the page is shared across fork,
  		 * but not from this process. The other process cannot write to
  		 * the page, only trigger CoW.

  		 */

  		if (!is_refcount_suitable(page)) {

  			unlock_page(page);
  			result = SCAN_PAGE_COUNT;
  			goto out;
  		}

  		if (!pte_write(pteval) && PageSwapCache(page) &&
  				!reuse_swap_page(page, NULL)) {

  			/*

  			 * Page is in the swap cache and cannot be re-used.
  			 * It cannot be collapsed into a THP.

  			 */

  			unlock_page(page);
  			result = SCAN_SWAP_CACHE_PAGE;
  			goto out;

  		}
  
  		/*
  		 * Isolate the page to avoid collapsing an hugepage
  		 * currently in use by the VM.
  		 */
  		if (isolate_lru_page(page)) {
  			unlock_page(page);
  			result = SCAN_DEL_PAGE_LRU;
  			goto out;
  		}

  		mod_node_page_state(page_pgdat(page),
  				NR_ISOLATED_ANON + page_is_file_lru(page),
  				compound_nr(page));

  		VM_BUG_ON_PAGE(!PageLocked(page), page);
  		VM_BUG_ON_PAGE(PageLRU(page), page);

  		if (PageCompound(page))
  			list_add_tail(&page->lru, compound_pagelist);
  next:

  		/* There should be enough young pte to collapse the page */

  		if (pte_young(pteval) ||
  		    page_is_young(page) || PageReferenced(page) ||
  		    mmu_notifier_test_young(vma->vm_mm, address))

  			referenced++;

  
  		if (pte_write(pteval))
  			writable = true;

  	}
  	if (likely(writable)) {
  		if (likely(referenced)) {
  			result = SCAN_SUCCEED;
  			trace_mm_collapse_huge_page_isolate(page, none_or_zero,
  							    referenced, writable, result);
  			return 1;
  		}
  	} else {
  		result = SCAN_PAGE_RO;
  	}
  
  out:

  	release_pte_pages(pte, _pte, compound_pagelist);

  	trace_mm_collapse_huge_page_isolate(page, none_or_zero,
  					    referenced, writable, result);
  	return 0;
  }
  
  static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
  				      struct vm_area_struct *vma,
  				      unsigned long address,

  				      spinlock_t *ptl,
  				      struct list_head *compound_pagelist)

  {

  	struct page *src_page, *tmp;

  	pte_t *_pte;

  	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
  				_pte++, page++, address += PAGE_SIZE) {

  		pte_t pteval = *_pte;

  
  		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
  			clear_user_highpage(page, address);
  			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
  			if (is_zero_pfn(pte_pfn(pteval))) {
  				/*
  				 * ptl mostly unnecessary.
  				 */
  				spin_lock(ptl);
  				/*
  				 * paravirt calls inside pte_clear here are
  				 * superfluous.
  				 */
  				pte_clear(vma->vm_mm, address, _pte);
  				spin_unlock(ptl);
  			}
  		} else {
  			src_page = pte_page(pteval);
  			copy_user_highpage(page, src_page, address, vma);

  			if (!PageCompound(src_page))
  				release_pte_page(src_page);

  			/*
  			 * ptl mostly unnecessary, but preempt has to
  			 * be disabled to update the per-cpu stats
  			 * inside page_remove_rmap().
  			 */
  			spin_lock(ptl);
  			/*
  			 * paravirt calls inside pte_clear here are
  			 * superfluous.
  			 */
  			pte_clear(vma->vm_mm, address, _pte);
  			page_remove_rmap(src_page, false);
  			spin_unlock(ptl);
  			free_page_and_swap_cache(src_page);
  		}

  	}

  
  	list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
  		list_del(&src_page->lru);
  		release_pte_page(src_page);
  	}

  }
  
  static void khugepaged_alloc_sleep(void)
  {
  	DEFINE_WAIT(wait);
  
  	add_wait_queue(&khugepaged_wait, &wait);
  	freezable_schedule_timeout_interruptible(
  		msecs_to_jiffies(khugepaged_alloc_sleep_millisecs));
  	remove_wait_queue(&khugepaged_wait, &wait);
  }
  
  static int khugepaged_node_load[MAX_NUMNODES];
  
  static bool khugepaged_scan_abort(int nid)
  {
  	int i;
  
  	/*

  	 * If node_reclaim_mode is disabled, then no extra effort is made to

  	 * allocate memory locally.
  	 */

  	if (!node_reclaim_mode)

  		return false;
  
  	/* If there is a count for this node already, it must be acceptable */
  	if (khugepaged_node_load[nid])
  		return false;
  
  	for (i = 0; i < MAX_NUMNODES; i++) {
  		if (!khugepaged_node_load[i])
  			continue;

  		if (node_distance(nid, i) > node_reclaim_distance)

  			return true;
  	}
  	return false;
  }
  
  /* Defrag for khugepaged will enter direct reclaim/compaction if necessary */
  static inline gfp_t alloc_hugepage_khugepaged_gfpmask(void)
  {

  	return khugepaged_defrag() ? GFP_TRANSHUGE : GFP_TRANSHUGE_LIGHT;

  }
  
  #ifdef CONFIG_NUMA
  static int khugepaged_find_target_node(void)
  {
  	static int last_khugepaged_target_node = NUMA_NO_NODE;
  	int nid, target_node = 0, max_value = 0;
  
  	/* find first node with max normal pages hit */
  	for (nid = 0; nid < MAX_NUMNODES; nid++)
  		if (khugepaged_node_load[nid] > max_value) {
  			max_value = khugepaged_node_load[nid];
  			target_node = nid;
  		}
  
  	/* do some balance if several nodes have the same hit record */
  	if (target_node <= last_khugepaged_target_node)
  		for (nid = last_khugepaged_target_node + 1; nid < MAX_NUMNODES;
  				nid++)
  			if (max_value == khugepaged_node_load[nid]) {
  				target_node = nid;
  				break;
  			}
  
  	last_khugepaged_target_node = target_node;
  	return target_node;
  }
  
  static bool khugepaged_prealloc_page(struct page **hpage, bool *wait)
  {
  	if (IS_ERR(*hpage)) {
  		if (!*wait)
  			return false;
  
  		*wait = false;
  		*hpage = NULL;
  		khugepaged_alloc_sleep();
  	} else if (*hpage) {
  		put_page(*hpage);
  		*hpage = NULL;
  	}
  
  	return true;
  }
  
  static struct page *

  khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)

  {
  	VM_BUG_ON_PAGE(*hpage, *hpage);

  	*hpage = __alloc_pages_node(node, gfp, HPAGE_PMD_ORDER);
  	if (unlikely(!*hpage)) {
  		count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
  		*hpage = ERR_PTR(-ENOMEM);
  		return NULL;
  	}
  
  	prep_transhuge_page(*hpage);
  	count_vm_event(THP_COLLAPSE_ALLOC);
  	return *hpage;
  }
  #else
  static int khugepaged_find_target_node(void)
  {
  	return 0;
  }
  
  static inline struct page *alloc_khugepaged_hugepage(void)
  {
  	struct page *page;
  
  	page = alloc_pages(alloc_hugepage_khugepaged_gfpmask(),
  			   HPAGE_PMD_ORDER);
  	if (page)
  		prep_transhuge_page(page);
  	return page;
  }
  
  static struct page *khugepaged_alloc_hugepage(bool *wait)
  {
  	struct page *hpage;
  
  	do {
  		hpage = alloc_khugepaged_hugepage();
  		if (!hpage) {
  			count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
  			if (!*wait)
  				return NULL;
  
  			*wait = false;
  			khugepaged_alloc_sleep();
  		} else
  			count_vm_event(THP_COLLAPSE_ALLOC);
  	} while (unlikely(!hpage) && likely(khugepaged_enabled()));
  
  	return hpage;
  }
  
  static bool khugepaged_prealloc_page(struct page **hpage, bool *wait)
  {

  	/*
  	 * If the hpage allocated earlier was briefly exposed in page cache
  	 * before collapse_file() failed, it is possible that racing lookups
  	 * have not yet completed, and would then be unpleasantly surprised by
  	 * finding the hpage reused for the same mapping at a different offset.
  	 * Just release the previous allocation if there is any danger of that.
  	 */
  	if (*hpage && page_count(*hpage) > 1) {
  		put_page(*hpage);
  		*hpage = NULL;
  	}

  	if (!*hpage)
  		*hpage = khugepaged_alloc_hugepage(wait);
  
  	if (unlikely(!*hpage))
  		return false;
  
  	return true;
  }
  
  static struct page *

  khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)

  {

  	VM_BUG_ON(!*hpage);
  
  	return  *hpage;
  }
  #endif

  /*

   * If mmap_lock temporarily dropped, revalidate vma
   * before taking mmap_lock.

   * Return 0 if succeeds, otherwise return none-zero
   * value (scan code).
   */

  static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
  		struct vm_area_struct **vmap)

  {
  	struct vm_area_struct *vma;
  	unsigned long hstart, hend;
  
  	if (unlikely(khugepaged_test_exit(mm)))
  		return SCAN_ANY_PROCESS;

  	*vmap = vma = find_vma(mm, address);

  	if (!vma)
  		return SCAN_VMA_NULL;
  
  	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
  	hend = vma->vm_end & HPAGE_PMD_MASK;
  	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
  		return SCAN_ADDRESS_RANGE;

  	if (!hugepage_vma_check(vma, vma->vm_flags))

  		return SCAN_VMA_CHECK;

  	/* Anon VMA expected */
  	if (!vma->anon_vma || vma->vm_ops)
  		return SCAN_VMA_CHECK;

  	return 0;
  }
  
  /*
   * Bring missing pages in from swap, to complete THP collapse.
   * Only done if khugepaged_scan_pmd believes it is worthwhile.
   *
   * Called and returns without pte mapped or spinlocks held,

   * but with mmap_lock held to protect against vma changes.

   */
  
  static bool __collapse_huge_page_swapin(struct mm_struct *mm,
  					struct vm_area_struct *vma,

  					unsigned long address, pmd_t *pmd,
  					int referenced)

  {

  	int swapped_in = 0;
  	vm_fault_t ret = 0;

  	struct vm_fault vmf = {

  		.vma = vma,
  		.address = address,
  		.flags = FAULT_FLAG_ALLOW_RETRY,
  		.pmd = pmd,

  		.pgoff = linear_page_index(vma, address),

  	};

  	vmf.pte = pte_offset_map(pmd, address);
  	for (; vmf.address < address + HPAGE_PMD_NR*PAGE_SIZE;
  			vmf.pte++, vmf.address += PAGE_SIZE) {

  		vmf.orig_pte = *vmf.pte;
  		if (!is_swap_pte(vmf.orig_pte))

  			continue;
  		swapped_in++;

  		ret = do_swap_page(&vmf);

  		/* do_swap_page returns VM_FAULT_RETRY with released mmap_lock */

  		if (ret & VM_FAULT_RETRY) {

  			mmap_read_lock(mm);

  			if (hugepage_vma_revalidate(mm, address, &vmf.vma)) {

  				/* vma is no longer available, don't continue to swapin */

  				trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);

  				return false;

  			}

  			/* check if the pmd is still valid */

  			if (mm_find_pmd(mm, address) != pmd) {
  				trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);

  				return false;

  			}

  		}
  		if (ret & VM_FAULT_ERROR) {

  			trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);

  			return false;
  		}
  		/* pte is unmapped now, we need to map it */

  		vmf.pte = pte_offset_map(pmd, vmf.address);

  	}

  	vmf.pte--;
  	pte_unmap(vmf.pte);

  
  	/* Drain LRU add pagevec to remove extra pin on the swapped in pages */
  	if (swapped_in)
  		lru_add_drain();

  	trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 1);

  	return true;
  }
  
  static void collapse_huge_page(struct mm_struct *mm,
  				   unsigned long address,
  				   struct page **hpage,

  				   int node, int referenced, int unmapped)

  {

  	LIST_HEAD(compound_pagelist);

  	pmd_t *pmd, _pmd;
  	pte_t *pte;
  	pgtable_t pgtable;
  	struct page *new_page;
  	spinlock_t *pmd_ptl, *pte_ptl;
  	int isolated = 0, result = 0;

  	struct vm_area_struct *vma;

  	struct mmu_notifier_range range;

  	gfp_t gfp;
  
  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
  
  	/* Only allocate from the target node */

  	gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;

  	/*

  	 * Before allocating the hugepage, release the mmap_lock read lock.

  	 * The allocation can take potentially a long time if it involves

  	 * sync compaction, and we do not need to hold the mmap_lock during

  	 * that. We will recheck the vma after taking it again in write mode.
  	 */

  	mmap_read_unlock(mm);

  	new_page = khugepaged_alloc_page(hpage, gfp, node);

  	if (!new_page) {
  		result = SCAN_ALLOC_HUGE_PAGE_FAIL;
  		goto out_nolock;
  	}

  	if (unlikely(mem_cgroup_charge(new_page, mm, gfp))) {

  		result = SCAN_CGROUP_CHARGE_FAIL;
  		goto out_nolock;
  	}

  	count_memcg_page_event(new_page, THP_COLLAPSE_ALLOC);

  	mmap_read_lock(mm);

  	result = hugepage_vma_revalidate(mm, address, &vma);

  	if (result) {

  		mmap_read_unlock(mm);

  		goto out_nolock;
  	}
  
  	pmd = mm_find_pmd(mm, address);
  	if (!pmd) {
  		result = SCAN_PMD_NULL;

  		mmap_read_unlock(mm);

  		goto out_nolock;
  	}
  
  	/*

  	 * __collapse_huge_page_swapin always returns with mmap_lock locked.
  	 * If it fails, we release mmap_lock and jump out_nolock.

  	 * Continuing to collapse causes inconsistency.
  	 */

  	if (unmapped && !__collapse_huge_page_swapin(mm, vma, address,
  						     pmd, referenced)) {

  		mmap_read_unlock(mm);

  		goto out_nolock;
  	}

  	mmap_read_unlock(mm);

  	/*
  	 * Prevent all access to pagetables with the exception of
  	 * gup_fast later handled by the ptep_clear_flush and the VM
  	 * handled by the anon_vma lock + PG_lock.
  	 */

  	mmap_write_lock(mm);

  	result = hugepage_vma_revalidate(mm, address, &vma);

  	if (result)
  		goto out;
  	/* check if the pmd is still valid */
  	if (mm_find_pmd(mm, address) != pmd)
  		goto out;
  
  	anon_vma_lock_write(vma->anon_vma);

  	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, NULL, mm,

  				address, address + HPAGE_PMD_SIZE);

  	mmu_notifier_invalidate_range_start(&range);

  
  	pte = pte_offset_map(pmd, address);
  	pte_ptl = pte_lockptr(mm, pmd);

  	pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
  	/*
  	 * After this gup_fast can't run anymore. This also removes
  	 * any huge TLB entry from the CPU so we won't allow
  	 * huge and small TLB entries for the same virtual address
  	 * to avoid the risk of CPU bugs in that area.
  	 */
  	_pmd = pmdp_collapse_flush(vma, address, pmd);
  	spin_unlock(pmd_ptl);

  	mmu_notifier_invalidate_range_end(&range);

  
  	spin_lock(pte_ptl);

  	isolated = __collapse_huge_page_isolate(vma, address, pte,
  			&compound_pagelist);

  	spin_unlock(pte_ptl);
  
  	if (unlikely(!isolated)) {
  		pte_unmap(pte);
  		spin_lock(pmd_ptl);
  		BUG_ON(!pmd_none(*pmd));
  		/*
  		 * We can only use set_pmd_at when establishing
  		 * hugepmds and never for establishing regular pmds that
  		 * points to regular pagetables. Use pmd_populate for that
  		 */
  		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
  		spin_unlock(pmd_ptl);
  		anon_vma_unlock_write(vma->anon_vma);
  		result = SCAN_FAIL;
  		goto out;
  	}
  
  	/*
  	 * All pages are isolated and locked so anon_vma rmap
  	 * can't run anymore.
  	 */
  	anon_vma_unlock_write(vma->anon_vma);

  	__collapse_huge_page_copy(pte, new_page, vma, address, pte_ptl,
  			&compound_pagelist);

  	pte_unmap(pte);
  	__SetPageUptodate(new_page);
  	pgtable = pmd_pgtable(_pmd);
  
  	_pmd = mk_huge_pmd(new_page, vma->vm_page_prot);

  	_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);

  
  	/*
  	 * spin_lock() below is not the equivalent of smp_wmb(), so
  	 * this is needed to avoid the copy_huge_page writes to become
  	 * visible after the set_pmd_at() write.
  	 */
  	smp_wmb();
  
  	spin_lock(pmd_ptl);
  	BUG_ON(!pmd_none(*pmd));

  	page_add_new_anon_rmap(new_page, vma, address, true);

  	lru_cache_add_inactive_or_unevictable(new_page, vma);

  	pgtable_trans_huge_deposit(mm, pmd, pgtable);
  	set_pmd_at(mm, address, pmd, _pmd);
  	update_mmu_cache_pmd(vma, address, pmd);
  	spin_unlock(pmd_ptl);
  
  	*hpage = NULL;
  
  	khugepaged_pages_collapsed++;
  	result = SCAN_SUCCEED;
  out_up_write:

  	mmap_write_unlock(mm);

  out_nolock:

  	if (!IS_ERR_OR_NULL(*hpage))
  		mem_cgroup_uncharge(*hpage);

  	trace_mm_collapse_huge_page(mm, isolated, result);
  	return;
  out:

  	goto out_up_write;
  }
  
  static int khugepaged_scan_pmd(struct mm_struct *mm,
  			       struct vm_area_struct *vma,
  			       unsigned long address,
  			       struct page **hpage)
  {
  	pmd_t *pmd;
  	pte_t *pte, *_pte;

  	int ret = 0, result = 0, referenced = 0;
  	int none_or_zero = 0, shared = 0;

  	struct page *page = NULL;
  	unsigned long _address;
  	spinlock_t *ptl;
  	int node = NUMA_NO_NODE, unmapped = 0;

  	bool writable = false;

  
  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
  
  	pmd = mm_find_pmd(mm, address);
  	if (!pmd) {
  		result = SCAN_PMD_NULL;
  		goto out;
  	}
  
  	memset(khugepaged_node_load, 0, sizeof(khugepaged_node_load));
  	pte = pte_offset_map_lock(mm, pmd, address, &ptl);
  	for (_address = address, _pte = pte; _pte < pte+HPAGE_PMD_NR;
  	     _pte++, _address += PAGE_SIZE) {
  		pte_t pteval = *_pte;
  		if (is_swap_pte(pteval)) {
  			if (++unmapped <= khugepaged_max_ptes_swap) {

  				/*
  				 * Always be strict with uffd-wp
  				 * enabled swap entries.  Please see
  				 * comment below for pte_uffd_wp().
  				 */
  				if (pte_swp_uffd_wp(pteval)) {
  					result = SCAN_PTE_UFFD_WP;
  					goto out_unmap;
  				}

  				continue;
  			} else {
  				result = SCAN_EXCEED_SWAP_PTE;
  				goto out_unmap;
  			}
  		}
  		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
  			if (!userfaultfd_armed(vma) &&
  			    ++none_or_zero <= khugepaged_max_ptes_none) {
  				continue;
  			} else {
  				result = SCAN_EXCEED_NONE_PTE;
  				goto out_unmap;
  			}
  		}
  		if (!pte_present(pteval)) {
  			result = SCAN_PTE_NON_PRESENT;
  			goto out_unmap;
  		}

  		if (pte_uffd_wp(pteval)) {
  			/*
  			 * Don't collapse the page if any of the small
  			 * PTEs are armed with uffd write protection.
  			 * Here we can also mark the new huge pmd as
  			 * write protected if any of the small ones is
  			 * marked but that could bring uknown
  			 * userfault messages that falls outside of
  			 * the registered range.  So, just be simple.
  			 */
  			result = SCAN_PTE_UFFD_WP;
  			goto out_unmap;
  		}

  		if (pte_write(pteval))
  			writable = true;
  
  		page = vm_normal_page(vma, _address, pteval);
  		if (unlikely(!page)) {
  			result = SCAN_PAGE_NULL;
  			goto out_unmap;
  		}

  		if (page_mapcount(page) > 1 &&
  				++shared > khugepaged_max_ptes_shared) {
  			result = SCAN_EXCEED_SHARED_PTE;
  			goto out_unmap;
  		}

  		page = compound_head(page);

  
  		/*
  		 * Record which node the original page is from and save this
  		 * information to khugepaged_node_load[].
  		 * Khupaged will allocate hugepage from the node has the max
  		 * hit record.
  		 */
  		node = page_to_nid(page);
  		if (khugepaged_scan_abort(node)) {
  			result = SCAN_SCAN_ABORT;
  			goto out_unmap;
  		}
  		khugepaged_node_load[node]++;
  		if (!PageLRU(page)) {
  			result = SCAN_PAGE_LRU;
  			goto out_unmap;
  		}
  		if (PageLocked(page)) {
  			result = SCAN_PAGE_LOCK;
  			goto out_unmap;
  		}
  		if (!PageAnon(page)) {
  			result = SCAN_PAGE_ANON;
  			goto out_unmap;
  		}
  
  		/*

  		 * Check if the page has any GUP (or other external) pins.
  		 *
  		 * Here the check is racy it may see totmal_mapcount > refcount
  		 * in some cases.
  		 * For example, one process with one forked child process.
  		 * The parent has the PMD split due to MADV_DONTNEED, then
  		 * the child is trying unmap the whole PMD, but khugepaged
  		 * may be scanning the parent between the child has
  		 * PageDoubleMap flag cleared and dec the mapcount.  So
  		 * khugepaged may see total_mapcount > refcount.
  		 *
  		 * But such case is ephemeral we could always retry collapse
  		 * later.  However it may report false positive if the page
  		 * has excessive GUP pins (i.e. 512).  Anyway the same check
  		 * will be done again later the risk seems low.

  		 */

  		if (!is_refcount_suitable(page)) {

  			result = SCAN_PAGE_COUNT;
  			goto out_unmap;
  		}
  		if (pte_young(pteval) ||
  		    page_is_young(page) || PageReferenced(page) ||
  		    mmu_notifier_test_young(vma->vm_mm, address))

  			referenced++;

  	}

  	if (!writable) {

  		result = SCAN_PAGE_RO;

  	} else if (!referenced || (unmapped && referenced < HPAGE_PMD_NR/2)) {
  		result = SCAN_LACK_REFERENCED_PAGE;
  	} else {
  		result = SCAN_SUCCEED;
  		ret = 1;

  	}
  out_unmap:
  	pte_unmap_unlock(pte, ptl);
  	if (ret) {
  		node = khugepaged_find_target_node();

  		/* collapse_huge_page will return with the mmap_lock released */

  		collapse_huge_page(mm, address, hpage, node,
  				referenced, unmapped);

  	}
  out:
  	trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,
  				     none_or_zero, result, unmapped);
  	return ret;
  }
  
  static void collect_mm_slot(struct mm_slot *mm_slot)
  {
  	struct mm_struct *mm = mm_slot->mm;

  	lockdep_assert_held(&khugepaged_mm_lock);

  
  	if (khugepaged_test_exit(mm)) {
  		/* free mm_slot */
  		hash_del(&mm_slot->hash);
  		list_del(&mm_slot->mm_node);
  
  		/*
  		 * Not strictly needed because the mm exited already.
  		 *
  		 * clear_bit(MMF_VM_HUGEPAGE, &mm->flags);
  		 */
  
  		/* khugepaged_mm_lock actually not necessary for the below */
  		free_mm_slot(mm_slot);
  		mmdrop(mm);
  	}
  }

  #ifdef CONFIG_SHMEM

  /*
   * Notify khugepaged that given addr of the mm is pte-mapped THP. Then
   * khugepaged should try to collapse the page table.
   */
  static int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
  					 unsigned long addr)
  {
  	struct mm_slot *mm_slot;
  
  	VM_BUG_ON(addr & ~HPAGE_PMD_MASK);
  
  	spin_lock(&khugepaged_mm_lock);
  	mm_slot = get_mm_slot(mm);
  	if (likely(mm_slot && mm_slot->nr_pte_mapped_thp < MAX_PTE_MAPPED_THP))
  		mm_slot->pte_mapped_thp[mm_slot->nr_pte_mapped_thp++] = addr;
  	spin_unlock(&khugepaged_mm_lock);
  	return 0;
  }
  
  /**
   * Try to collapse a pte-mapped THP for mm at address haddr.
   *
   * This function checks whether all the PTEs in the PMD are pointing to the
   * right THP. If so, retract the page table so the THP can refault in with
   * as pmd-mapped.
   */
  void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
  {
  	unsigned long haddr = addr & HPAGE_PMD_MASK;
  	struct vm_area_struct *vma = find_vma(mm, haddr);

  	struct page *hpage;

  	pte_t *start_pte, *pte;
  	pmd_t *pmd, _pmd;
  	spinlock_t *ptl;
  	int count = 0;
  	int i;
  
  	if (!vma || !vma->vm_file ||
  	    vma->vm_start > haddr || vma->vm_end < haddr + HPAGE_PMD_SIZE)
  		return;
  
  	/*
  	 * This vm_flags may not have VM_HUGEPAGE if the page was not
  	 * collapsed by this mm. But we can still collapse if the page is
  	 * the valid THP. Add extra VM_HUGEPAGE so hugepage_vma_check()
  	 * will not fail the vma for missing VM_HUGEPAGE
  	 */
  	if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE))
  		return;

  	hpage = find_lock_page(vma->vm_file->f_mapping,
  			       linear_page_index(vma, haddr));
  	if (!hpage)
  		return;
  
  	if (!PageHead(hpage))
  		goto drop_hpage;

  	pmd = mm_find_pmd(mm, haddr);
  	if (!pmd)

  		goto drop_hpage;

  
  	start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl);
  
  	/* step 1: check all mapped PTEs are to the right huge page */
  	for (i = 0, addr = haddr, pte = start_pte;
  	     i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {
  		struct page *page;
  
  		/* empty pte, skip */
  		if (pte_none(*pte))
  			continue;
  
  		/* page swapped out, abort */
  		if (!pte_present(*pte))
  			goto abort;
  
  		page = vm_normal_page(vma, addr, *pte);

  		/*

  		 * Note that uprobe, debugger, or MAP_PRIVATE may change the
  		 * page table, but the new page will not be a subpage of hpage.

  		 */

  		if (hpage + i != page)

  			goto abort;
  		count++;
  	}
  
  	/* step 2: adjust rmap */
  	for (i = 0, addr = haddr, pte = start_pte;
  	     i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {
  		struct page *page;
  
  		if (pte_none(*pte))
  			continue;
  		page = vm_normal_page(vma, addr, *pte);
  		page_remove_rmap(page, false);
  	}
  
  	pte_unmap_unlock(start_pte, ptl);
  
  	/* step 3: set proper refcount and mm_counters. */

  	if (count) {

  		page_ref_sub(hpage, count);
  		add_mm_counter(vma->vm_mm, mm_counter_file(hpage), -count);
  	}
  
  	/* step 4: collapse pmd */
  	ptl = pmd_lock(vma->vm_mm, pmd);

  	_pmd = pmdp_collapse_flush(vma, haddr, pmd);

  	spin_unlock(ptl);
  	mm_dec_nr_ptes(mm);
  	pte_free(mm, pmd_pgtable(_pmd));

  
  drop_hpage:
  	unlock_page(hpage);
  	put_page(hpage);

  	return;
  
  abort:
  	pte_unmap_unlock(start_pte, ptl);

  	goto drop_hpage;

  }
  
  static int khugepaged_collapse_pte_mapped_thps(struct mm_slot *mm_slot)
  {
  	struct mm_struct *mm = mm_slot->mm;
  	int i;
  
  	if (likely(mm_slot->nr_pte_mapped_thp == 0))
  		return 0;

  	if (!mmap_write_trylock(mm))

  		return -EBUSY;
  
  	if (unlikely(khugepaged_test_exit(mm)))
  		goto out;
  
  	for (i = 0; i < mm_slot->nr_pte_mapped_thp; i++)
  		collapse_pte_mapped_thp(mm, mm_slot->pte_mapped_thp[i]);
  
  out:
  	mm_slot->nr_pte_mapped_thp = 0;

  	mmap_write_unlock(mm);

  	return 0;
  }

  static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
  {
  	struct vm_area_struct *vma;

  	struct mm_struct *mm;

  	unsigned long addr;
  	pmd_t *pmd, _pmd;
  
  	i_mmap_lock_write(mapping);
  	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {

  		/*
  		 * Check vma->anon_vma to exclude MAP_PRIVATE mappings that
  		 * got written to. These VMAs are likely not worth investing

  		 * mmap_write_lock(mm) as PMD-mapping is likely to be split

  		 * later.
  		 *
  		 * Not that vma->anon_vma check is racy: it can be set up after

  		 * the check but before we took mmap_lock by the fault path.

  		 * But page lock would prevent establishing any new ptes of the
  		 * page, so we are safe.
  		 *
  		 * An alternative would be drop the check, but check that page
  		 * table is clear before calling pmdp_collapse_flush() under
  		 * ptl. It has higher chance to recover THP for the VMA, but
  		 * has higher cost too.
  		 */

  		if (vma->anon_vma)
  			continue;
  		addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
  		if (addr & ~HPAGE_PMD_MASK)
  			continue;
  		if (vma->vm_end < addr + HPAGE_PMD_SIZE)
  			continue;

  		mm = vma->vm_mm;
  		pmd = mm_find_pmd(mm, addr);

  		if (!pmd)
  			continue;
  		/*

  		 * We need exclusive mmap_lock to retract page table.

  		 *
  		 * We use trylock due to lock inversion: we need to acquire

  		 * mmap_lock while holding page lock. Fault path does it in

  		 * reverse order. Trylock is a way to avoid deadlock.

  		 */

  		if (mmap_write_trylock(mm)) {
  			if (!khugepaged_test_exit(mm)) {
  				spinlock_t *ptl = pmd_lock(mm, pmd);
  				/* assume page table is clear */
  				_pmd = pmdp_collapse_flush(vma, addr, pmd);
  				spin_unlock(ptl);
  				mm_dec_nr_ptes(mm);
  				pte_free(mm, pmd_pgtable(_pmd));
  			}
  			mmap_write_unlock(mm);

  		} else {
  			/* Try again later */

  			khugepaged_add_pte_mapped_thp(mm, addr);

  		}
  	}
  	i_mmap_unlock_write(mapping);
  }
  
  /**

   * collapse_file - collapse filemap/tmpfs/shmem pages into huge one.

   *
   * Basic scheme is simple, details are more complex:

   *  - allocate and lock a new huge page;

   *  - scan page cache replacing old pages with the new one

   *    + swap/gup in pages if necessary;

   *    + fill in gaps;

   *    + keep old pages around in case rollback is required;
   *  - if replacing succeeds:

   *    + copy data over;
   *    + free old pages;

   *    + unlock huge page;

   *  - if replacing failed;
   *    + put all pages back and unfreeze them;

   *    + restore gaps in the page cache;

   *    + unlock and free huge page;

   */

  static void collapse_file(struct mm_struct *mm,
  		struct file *file, pgoff_t start,

  		struct page **hpage, int node)
  {

  	struct address_space *mapping = file->f_mapping;

  	gfp_t gfp;

  	struct page *new_page;

  	pgoff_t index, end = start + HPAGE_PMD_NR;
  	LIST_HEAD(pagelist);

  	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);

  	int nr_none = 0, result = SCAN_SUCCEED;

  	bool is_shmem = shmem_file(file);

  	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);

  	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
  
  	/* Only allocate from the target node */

  	gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;

  
  	new_page = khugepaged_alloc_page(hpage, gfp, node);
  	if (!new_page) {
  		result = SCAN_ALLOC_HUGE_PAGE_FAIL;
  		goto out;
  	}

  	if (unlikely(mem_cgroup_charge(new_page, mm, gfp))) {

  		result = SCAN_CGROUP_CHARGE_FAIL;
  		goto out;
  	}

  	count_memcg_page_event(new_page, THP_COLLAPSE_ALLOC);

  	/* This will be less messy when we use multi-index entries */
  	do {
  		xas_lock_irq(&xas);
  		xas_create_range(&xas);
  		if (!xas_error(&xas))
  			break;
  		xas_unlock_irq(&xas);
  		if (!xas_nomem(&xas, GFP_KERNEL)) {

  			result = SCAN_FAIL;
  			goto out;
  		}
  	} while (1);

  	__SetPageLocked(new_page);

  	if (is_shmem)
  		__SetPageSwapBacked(new_page);

  	new_page->index = start;
  	new_page->mapping = mapping;

  	/*

  	 * At this point the new_page is locked and not up-to-date.
  	 * It's safe to insert it into the page cache, because nobody would
  	 * be able to map it or use it in another way until we unlock it.

  	 */

  	xas_set(&xas, start);
  	for (index = start; index < end; index++) {
  		struct page *page = xas_next(&xas);
  
  		VM_BUG_ON(index != xas.xa_index);

  		if (is_shmem) {
  			if (!page) {
  				/*
  				 * Stop if extent has been truncated or
  				 * hole-punched, and is now completely
  				 * empty.
  				 */
  				if (index == start) {
  					if (!xas_next_entry(&xas, end - 1)) {
  						result = SCAN_TRUNCATED;
  						goto xa_locked;
  					}
  					xas_set(&xas, index);
  				}
  				if (!shmem_charge(mapping->host, 1)) {
  					result = SCAN_FAIL;

  					goto xa_locked;

  				}

  				xas_store(&xas, new_page);
  				nr_none++;
  				continue;

  			}

  
  			if (xa_is_value(page) || !PageUptodate(page)) {
  				xas_unlock_irq(&xas);
  				/* swap in or instantiate fallocated page */
  				if (shmem_getpage(mapping->host, index, &page,
  						  SGP_NOHUGE)) {
  					result = SCAN_FAIL;
  					goto xa_unlocked;
  				}
  			} else if (trylock_page(page)) {
  				get_page(page);
  				xas_unlock_irq(&xas);
  			} else {
  				result = SCAN_PAGE_LOCK;

  				goto xa_locked;

  			}

  		} else {	/* !is_shmem */
  			if (!page || xa_is_value(page)) {
  				xas_unlock_irq(&xas);
  				page_cache_sync_readahead(mapping, &file->f_ra,
  							  file, index,

  							  end - index);

  				/* drain pagevecs to help isolate_lru_page() */
  				lru_add_drain();
  				page = find_lock_page(mapping, index);
  				if (unlikely(page == NULL)) {
  					result = SCAN_FAIL;
  					goto xa_unlocked;
  				}

  			} else if (PageDirty(page)) {
  				/*
  				 * khugepaged only works on read-only fd,
  				 * so this page is dirty because it hasn't
  				 * been flushed since first write. There
  				 * won't be new dirty pages.
  				 *
  				 * Trigger async flush here and hope the
  				 * writeback is done when khugepaged
  				 * revisits this page.
  				 *
  				 * This is a one-off situation. We are not
  				 * forcing writeback in loop.
  				 */
  				xas_unlock_irq(&xas);
  				filemap_flush(mapping);
  				result = SCAN_FAIL;
  				goto xa_unlocked;

  			} else if (trylock_page(page)) {
  				get_page(page);
  				xas_unlock_irq(&xas);
  			} else {
  				result = SCAN_PAGE_LOCK;
  				goto xa_locked;

  			}

  		}
  
  		/*

  		 * The page must be locked, so we can drop the i_pages lock

  		 * without racing with truncate.
  		 */
  		VM_BUG_ON_PAGE(!PageLocked(page), page);

  
  		/* make sure the page is up to date */
  		if (unlikely(!PageUptodate(page))) {
  			result = SCAN_FAIL;
  			goto out_unlock;
  		}

  
  		/*
  		 * If file was truncated then extended, or hole-punched, before
  		 * we locked the first page, then a THP might be there already.
  		 */
  		if (PageTransCompound(page)) {
  			result = SCAN_PAGE_COMPOUND;
  			goto out_unlock;
  		}

  
  		if (page_mapping(page) != mapping) {
  			result = SCAN_TRUNCATED;
  			goto out_unlock;
  		}

  		if (!is_shmem && PageDirty(page)) {
  			/*
  			 * khugepaged only works on read-only fd, so this
  			 * page is dirty because it hasn't been flushed
  			 * since first write.
  			 */
  			result = SCAN_FAIL;
  			goto out_unlock;
  		}

  		if (isolate_lru_page(page)) {
  			result = SCAN_DEL_PAGE_LRU;

  			goto out_unlock;

  		}

  		if (page_has_private(page) &&
  		    !try_to_release_page(page, GFP_KERNEL)) {
  			result = SCAN_PAGE_HAS_PRIVATE;

  			putback_lru_page(page);

  			goto out_unlock;
  		}

  		if (page_mapped(page))

  			unmap_mapping_pages(mapping, index, 1, false);

  		xas_lock_irq(&xas);
  		xas_set(&xas, index);

  		VM_BUG_ON_PAGE(page != xas_load(&xas), page);

  		VM_BUG_ON_PAGE(page_mapped(page), page);
  
  		/*
  		 * The page is expected to have page_count() == 3:
  		 *  - we hold a pin on it;

  		 *  - one reference from page cache;

  		 *  - one from isolate_lru_page;
  		 */
  		if (!page_ref_freeze(page, 3)) {
  			result = SCAN_PAGE_COUNT;

  			xas_unlock_irq(&xas);
  			putback_lru_page(page);
  			goto out_unlock;

  		}
  
  		/*
  		 * Add the page to the list to be able to undo the collapse if
  		 * something go wrong.
  		 */
  		list_add_tail(&page->lru, &pagelist);
  
  		/* Finally, replace with the new page. */

  		xas_store(&xas, new_page);

  		continue;

  out_unlock:
  		unlock_page(page);
  		put_page(page);

  		goto xa_unlocked;

  	}

  	if (is_shmem)
  		__inc_node_page_state(new_page, NR_SHMEM_THPS);

  	else {

  		__inc_node_page_state(new_page, NR_FILE_THPS);

  		filemap_nr_thps_inc(mapping);
  	}

  	if (nr_none) {

  		__mod_lruvec_page_state(new_page, NR_FILE_PAGES, nr_none);

  		if (is_shmem)

  			__mod_lruvec_page_state(new_page, NR_SHMEM, nr_none);

  	}
  
  xa_locked:
  	xas_unlock_irq(&xas);

  xa_unlocked:

  	if (result == SCAN_SUCCEED) {

  		struct page *page, *tmp;

  
  		/*

  		 * Replacing old pages with new one has succeeded, now we
  		 * need to copy the content and free the old pages.

  		 */

  		index = start;

  		list_for_each_entry_safe(page, tmp, &pagelist, lru) {

  			while (index < page->index) {
  				clear_highpage(new_page + (index % HPAGE_PMD_NR));
  				index++;
  			}

  			copy_highpage(new_page + (page->index % HPAGE_PMD_NR),
  					page);
  			list_del(&page->lru);

  			page->mapping = NULL;

  			page_ref_unfreeze(page, 1);

  			ClearPageActive(page);
  			ClearPageUnevictable(page);

  			unlock_page(page);

  			put_page(page);

  			index++;
  		}
  		while (index < end) {
  			clear_highpage(new_page + (index % HPAGE_PMD_NR));
  			index++;

  		}

  		SetPageUptodate(new_page);

  		page_ref_add(new_page, HPAGE_PMD_NR - 1);

  		if (is_shmem)

  			set_page_dirty(new_page);

  		lru_cache_add(new_page);

  		/*
  		 * Remove pte page tables, so we can re-fault the page as huge.
  		 */
  		retract_page_tables(mapping, start);

  		*hpage = NULL;

  
  		khugepaged_pages_collapsed++;

  	} else {

  		struct page *page;

  		/* Something went wrong: roll back page cache changes */

  		xas_lock_irq(&xas);

  		mapping->nrpages -= nr_none;

  
  		if (is_shmem)
  			shmem_uncharge(mapping->host, nr_none);

  		xas_set(&xas, start);
  		xas_for_each(&xas, page, end - 1) {

  			page = list_first_entry_or_null(&pagelist,
  					struct page, lru);

  			if (!page || xas.xa_index < page->index) {

  				if (!nr_none)
  					break;

  				nr_none--;

  				/* Put holes back where they were */

  				xas_store(&xas, NULL);

  				continue;
  			}

  			VM_BUG_ON_PAGE(page->index != xas.xa_index, page);

  
  			/* Unfreeze the page. */
  			list_del(&page->lru);
  			page_ref_unfreeze(page, 2);

  			xas_store(&xas, page);
  			xas_pause(&xas);
  			xas_unlock_irq(&xas);

  			unlock_page(page);

  			putback_lru_page(page);

  			xas_lock_irq(&xas);

  		}
  		VM_BUG_ON(nr_none);

  		xas_unlock_irq(&xas);

  		new_page->mapping = NULL;
  	}

  
  	unlock_page(new_page);

  out:
  	VM_BUG_ON(!list_empty(&pagelist));

  	if (!IS_ERR_OR_NULL(*hpage))
  		mem_cgroup_uncharge(*hpage);

  	/* TODO: tracepoints */
  }

  static void khugepaged_scan_file(struct mm_struct *mm,
  		struct file *file, pgoff_t start, struct page **hpage)

  {
  	struct page *page = NULL;

  	struct address_space *mapping = file->f_mapping;

  	XA_STATE(xas, &mapping->i_pages, start);

  	int present, swap;
  	int node = NUMA_NO_NODE;
  	int result = SCAN_SUCCEED;
  
  	present = 0;
  	swap = 0;
  	memset(khugepaged_node_load, 0, sizeof(khugepaged_node_load));
  	rcu_read_lock();

  	xas_for_each(&xas, page, start + HPAGE_PMD_NR - 1) {
  		if (xas_retry(&xas, page))

  			continue;

  		if (xa_is_value(page)) {

  			if (++swap > khugepaged_max_ptes_swap) {
  				result = SCAN_EXCEED_SWAP_PTE;
  				break;
  			}
  			continue;
  		}
  
  		if (PageTransCompound(page)) {
  			result = SCAN_PAGE_COMPOUND;
  			break;
  		}
  
  		node = page_to_nid(page);
  		if (khugepaged_scan_abort(node)) {
  			result = SCAN_SCAN_ABORT;
  			break;
  		}
  		khugepaged_node_load[node]++;
  
  		if (!PageLRU(page)) {
  			result = SCAN_PAGE_LRU;
  			break;
  		}

  		if (page_count(page) !=
  		    1 + page_mapcount(page) + page_has_private(page)) {

  			result = SCAN_PAGE_COUNT;
  			break;
  		}
  
  		/*
  		 * We probably should check if the page is referenced here, but
  		 * nobody would transfer pte_young() to PageReferenced() for us.
  		 * And rmap walk here is just too costly...
  		 */
  
  		present++;
  
  		if (need_resched()) {

  			xas_pause(&xas);

  			cond_resched_rcu();

  		}
  	}
  	rcu_read_unlock();
  
  	if (result == SCAN_SUCCEED) {
  		if (present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
  			result = SCAN_EXCEED_NONE_PTE;
  		} else {
  			node = khugepaged_find_target_node();

  			collapse_file(mm, file, start, hpage, node);

  		}
  	}
  
  	/* TODO: tracepoints */
  }
  #else

  static void khugepaged_scan_file(struct mm_struct *mm,
  		struct file *file, pgoff_t start, struct page **hpage)

  {
  	BUILD_BUG();
  }

  
  static int khugepaged_collapse_pte_mapped_thps(struct mm_slot *mm_slot)
  {
  	return 0;
  }

  #endif

  static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
  					    struct page **hpage)
  	__releases(&khugepaged_mm_lock)
  	__acquires(&khugepaged_mm_lock)
  {
  	struct mm_slot *mm_slot;
  	struct mm_struct *mm;
  	struct vm_area_struct *vma;
  	int progress = 0;
  
  	VM_BUG_ON(!pages);

  	lockdep_assert_held(&khugepaged_mm_lock);

  
  	if (khugepaged_scan.mm_slot)
  		mm_slot = khugepaged_scan.mm_slot;
  	else {
  		mm_slot = list_entry(khugepaged_scan.mm_head.next,
  				     struct mm_slot, mm_node);
  		khugepaged_scan.address = 0;
  		khugepaged_scan.mm_slot = mm_slot;
  	}
  	spin_unlock(&khugepaged_mm_lock);

  	khugepaged_collapse_pte_mapped_thps(mm_slot);

  
  	mm = mm_slot->mm;

  	/*
  	 * Don't wait for semaphore (to avoid long wait times).  Just move to
  	 * the next mm on the list.
  	 */
  	vma = NULL;

  	if (unlikely(!mmap_read_trylock(mm)))

  		goto breakouterloop_mmap_lock;

  	if (likely(!khugepaged_test_exit(mm)))

  		vma = find_vma(mm, khugepaged_scan.address);
  
  	progress++;
  	for (; vma; vma = vma->vm_next) {
  		unsigned long hstart, hend;
  
  		cond_resched();
  		if (unlikely(khugepaged_test_exit(mm))) {
  			progress++;
  			break;
  		}

  		if (!hugepage_vma_check(vma, vma->vm_flags)) {

  skip:
  			progress++;
  			continue;
  		}
  		hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
  		hend = vma->vm_end & HPAGE_PMD_MASK;
  		if (hstart >= hend)
  			goto skip;
  		if (khugepaged_scan.address > hend)
  			goto skip;
  		if (khugepaged_scan.address < hstart)
  			khugepaged_scan.address = hstart;
  		VM_BUG_ON(khugepaged_scan.address & ~HPAGE_PMD_MASK);

  		if (shmem_file(vma->vm_file) && !shmem_huge_enabled(vma))
  			goto skip;

  
  		while (khugepaged_scan.address < hend) {
  			int ret;
  			cond_resched();
  			if (unlikely(khugepaged_test_exit(mm)))
  				goto breakouterloop;
  
  			VM_BUG_ON(khugepaged_scan.address < hstart ||
  				  khugepaged_scan.address + HPAGE_PMD_SIZE >
  				  hend);

  			if (IS_ENABLED(CONFIG_SHMEM) && vma->vm_file) {

  				struct file *file = get_file(vma->vm_file);

  				pgoff_t pgoff = linear_page_index(vma,
  						khugepaged_scan.address);

  				mmap_read_unlock(mm);

  				ret = 1;

  				khugepaged_scan_file(mm, file, pgoff, hpage);

  				fput(file);
  			} else {
  				ret = khugepaged_scan_pmd(mm, vma,
  						khugepaged_scan.address,
  						hpage);
  			}

  			/* move to next address */
  			khugepaged_scan.address += HPAGE_PMD_SIZE;
  			progress += HPAGE_PMD_NR;
  			if (ret)

  				/* we released mmap_lock so break loop */
  				goto breakouterloop_mmap_lock;

  			if (progress >= pages)
  				goto breakouterloop;
  		}
  	}
  breakouterloop:

  	mmap_read_unlock(mm); /* exit_mmap will destroy ptes after this */

  breakouterloop_mmap_lock:

  
  	spin_lock(&khugepaged_mm_lock);
  	VM_BUG_ON(khugepaged_scan.mm_slot != mm_slot);
  	/*
  	 * Release the current mm_slot if this mm is about to die, or
  	 * if we scanned all vmas of this mm.
  	 */
  	if (khugepaged_test_exit(mm) || !vma) {
  		/*
  		 * Make sure that if mm_users is reaching zero while
  		 * khugepaged runs here, khugepaged_exit will find
  		 * mm_slot not pointing to the exiting mm.
  		 */
  		if (mm_slot->mm_node.next != &khugepaged_scan.mm_head) {
  			khugepaged_scan.mm_slot = list_entry(
  				mm_slot->mm_node.next,
  				struct mm_slot, mm_node);
  			khugepaged_scan.address = 0;
  		} else {
  			khugepaged_scan.mm_slot = NULL;
  			khugepaged_full_scans++;
  		}
  
  		collect_mm_slot(mm_slot);
  	}
  
  	return progress;
  }
  
  static int khugepaged_has_work(void)
  {
  	return !list_empty(&khugepaged_scan.mm_head) &&
  		khugepaged_enabled();
  }
  
  static int khugepaged_wait_event(void)
  {
  	return !list_empty(&khugepaged_scan.mm_head) ||
  		kthread_should_stop();
  }
  
  static void khugepaged_do_scan(void)
  {
  	struct page *hpage = NULL;
  	unsigned int progress = 0, pass_through_head = 0;
  	unsigned int pages = khugepaged_pages_to_scan;
  	bool wait = true;
  
  	barrier(); /* write khugepaged_pages_to_scan to local stack */

  	lru_add_drain_all();

  	while (progress < pages) {
  		if (!khugepaged_prealloc_page(&hpage, &wait))
  			break;
  
  		cond_resched();
  
  		if (unlikely(kthread_should_stop() || try_to_freeze()))
  			break;
  
  		spin_lock(&khugepaged_mm_lock);
  		if (!khugepaged_scan.mm_slot)
  			pass_through_head++;
  		if (khugepaged_has_work() &&
  		    pass_through_head < 2)
  			progress += khugepaged_scan_mm_slot(pages - progress,
  							    &hpage);
  		else
  			progress = pages;
  		spin_unlock(&khugepaged_mm_lock);
  	}
  
  	if (!IS_ERR_OR_NULL(hpage))
  		put_page(hpage);
  }
  
  static bool khugepaged_should_wakeup(void)
  {
  	return kthread_should_stop() ||
  	       time_after_eq(jiffies, khugepaged_sleep_expire);
  }
  
  static void khugepaged_wait_work(void)
  {
  	if (khugepaged_has_work()) {
  		const unsigned long scan_sleep_jiffies =
  			msecs_to_jiffies(khugepaged_scan_sleep_millisecs);
  
  		if (!scan_sleep_jiffies)
  			return;
  
  		khugepaged_sleep_expire = jiffies + scan_sleep_jiffies;
  		wait_event_freezable_timeout(khugepaged_wait,
  					     khugepaged_should_wakeup(),
  					     scan_sleep_jiffies);
  		return;
  	}
  
  	if (khugepaged_enabled())
  		wait_event_freezable(khugepaged_wait, khugepaged_wait_event());
  }
  
  static int khugepaged(void *none)
  {
  	struct mm_slot *mm_slot;
  
  	set_freezable();
  	set_user_nice(current, MAX_NICE);
  
  	while (!kthread_should_stop()) {
  		khugepaged_do_scan();
  		khugepaged_wait_work();
  	}
  
  	spin_lock(&khugepaged_mm_lock);
  	mm_slot = khugepaged_scan.mm_slot;
  	khugepaged_scan.mm_slot = NULL;
  	if (mm_slot)
  		collect_mm_slot(mm_slot);
  	spin_unlock(&khugepaged_mm_lock);
  	return 0;
  }
  
  static void set_recommended_min_free_kbytes(void)
  {
  	struct zone *zone;
  	int nr_zones = 0;
  	unsigned long recommended_min;

  	for_each_populated_zone(zone) {
  		/*
  		 * We don't need to worry about fragmentation of
  		 * ZONE_MOVABLE since it only has movable pages.
  		 */
  		if (zone_idx(zone) > gfp_zone(GFP_USER))
  			continue;

  		nr_zones++;

  	}

  
  	/* Ensure 2 pageblocks are free to assist fragmentation avoidance */
  	recommended_min = pageblock_nr_pages * nr_zones * 2;
  
  	/*
  	 * Make sure that on average at least two pageblocks are almost free
  	 * of another type, one for a migratetype to fall back to and a
  	 * second to avoid subsequent fallbacks of other types There are 3
  	 * MIGRATE_TYPES we care about.
  	 */
  	recommended_min += pageblock_nr_pages * nr_zones *
  			   MIGRATE_PCPTYPES * MIGRATE_PCPTYPES;
  
  	/* don't ever allow to reserve more than 5% of the lowmem */
  	recommended_min = min(recommended_min,
  			      (unsigned long) nr_free_buffer_pages() / 20);
  	recommended_min <<= (PAGE_SHIFT-10);
  
  	if (recommended_min > min_free_kbytes) {
  		if (user_min_free_kbytes >= 0)
  			pr_info("raising min_free_kbytes from %d to %lu to help transparent hugepage allocations
  ",
  				min_free_kbytes, recommended_min);
  
  		min_free_kbytes = recommended_min;
  	}
  	setup_per_zone_wmarks();
  }
  
  int start_stop_khugepaged(void)
  {

  	int err = 0;
  
  	mutex_lock(&khugepaged_mutex);
  	if (khugepaged_enabled()) {
  		if (!khugepaged_thread)
  			khugepaged_thread = kthread_run(khugepaged, NULL,
  							"khugepaged");
  		if (IS_ERR(khugepaged_thread)) {
  			pr_err("khugepaged: kthread_run(khugepaged) failed
  ");
  			err = PTR_ERR(khugepaged_thread);
  			khugepaged_thread = NULL;
  			goto fail;
  		}
  
  		if (!list_empty(&khugepaged_scan.mm_head))
  			wake_up_interruptible(&khugepaged_wait);
  
  		set_recommended_min_free_kbytes();
  	} else if (khugepaged_thread) {
  		kthread_stop(khugepaged_thread);
  		khugepaged_thread = NULL;
  	}
  fail:
  	mutex_unlock(&khugepaged_mutex);
  	return err;
  }

  
  void khugepaged_min_free_kbytes_update(void)
  {
  	mutex_lock(&khugepaged_mutex);
  	if (khugepaged_enabled() && khugepaged_thread)
  		set_recommended_min_free_kbytes();
  	mutex_unlock(&khugepaged_mutex);
  }