Adding upstream version 5.10.209.upstream/5.10.209

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2400 insertions, 0 deletions

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
new file mode 100644
index 000000000..28e18777e
--- /dev/null
+++ b/mm/khugepaged.c
@@ -0,0 +1,2400 @@
+// 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\n", 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\n", 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\n", 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\n", 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\n", 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\n", 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\n", 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\n", 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 (!transhuge_vma_enabled(vma, vm_flags))
+		return false;
+
+	if (vma->vm_file && !IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) -
+				vma->vm_pgoff, HPAGE_PMD_NR))
+		return false;
+
+	/* Enabled via shmem mount options or sysfs settings. */
+	if (shmem_file(vma->vm_file))
+		return shmem_huge_enabled(vma);
+
+	/* THP settings require madvise. */
+	if (!(vm_flags & VM_HUGEPAGE) && !khugepaged_always())
+		return false;
+
+	/* Only regular file is valid */
+	if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && vma->vm_file &&
+	    (vm_flags & VM_DENYWRITE)) {
+		struct inode *inode = vma->vm_file->f_inode;
+
+		return S_ISREG(inode->i_mode);
+	}
+
+	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;
+		}
+		if (pte_uffd_wp(pteval)) {
+			result = SCAN_PTE_UFFD_WP;
+			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 (unlikely(!writable)) {
+		result = SCAN_PAGE_RO;
+	} else if (unlikely(!referenced)) {
+		result = SCAN_LACK_REFERENCED_PAGE;
+	} else {
+		result = SCAN_SUCCEED;
+		trace_mm_collapse_huge_page_isolate(page, none_or_zero,
+						    referenced, writable, result);
+		return 1;
+	}
+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 */
+	/*
+	 * This 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.
+	 *
+	 * Parallel fast GUP is fine since fast GUP will back off when
+	 * it detects PMD is changed.
+	 */
+	_pmd = pmdp_collapse_flush(vma, address, pmd);
+	spin_unlock(pmd_ptl);
+	mmu_notifier_invalidate_range_end(&range);
+	tlb_remove_table_sync_one();
+
+	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;
+	struct mmu_notifier_range range;
+
+	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;
+
+	/*
+	 * We need to lock the mapping so that from here on, only GUP-fast and
+	 * hardware page walks can access the parts of the page tables that
+	 * we're operating on.
+	 */
+	i_mmap_lock_write(vma->vm_file->f_mapping);
+
+	/*
+	 * This spinlock should be unnecessary: Nobody else should be accessing
+	 * the page tables under spinlock protection here, only
+	 * lockless_pages_from_mm() and the hardware page walker can access page
+	 * tables while all the high-level locks are held in write mode.
+	 */
+	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 */
+	/* we make no change to anon, but protect concurrent anon page lookup */
+	if (vma->anon_vma)
+		anon_vma_lock_write(vma->anon_vma);
+
+	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, NULL, mm, haddr,
+				haddr + HPAGE_PMD_SIZE);
+	mmu_notifier_invalidate_range_start(&range);
+	_pmd = pmdp_collapse_flush(vma, haddr, pmd);
+	mm_dec_nr_ptes(mm);
+	tlb_remove_table_sync_one();
+	mmu_notifier_invalidate_range_end(&range);
+	pte_free(mm, pmd_pgtable(_pmd));
+
+	if (vma->anon_vma)
+		anon_vma_unlock_write(vma->anon_vma);
+	i_mmap_unlock_write(vma->vm_file->f_mapping);
+
+drop_hpage:
+	unlock_page(hpage);
+	put_page(hpage);
+	return;
+
+abort:
+	pte_unmap_unlock(start_pte, ptl);
+	i_mmap_unlock_write(vma->vm_file->f_mapping);
+	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. It would also probably require locking
+		 * the anon_vma.
+		 */
+		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)) {
+				struct mmu_notifier_range range;
+
+				mmu_notifier_range_init(&range,
+							MMU_NOTIFY_CLEAR, 0,
+							NULL, mm, addr,
+							addr + HPAGE_PMD_SIZE);
+				mmu_notifier_invalidate_range_start(&range);
+				/* assume page table is clear */
+				_pmd = pmdp_collapse_flush(vma, addr, pmd);
+				mm_dec_nr_ptes(mm);
+				tlb_remove_table_sync_one();
+				pte_free(mm, pmd_pgtable(_pmd));
+				mmu_notifier_invalidate_range_end(&range);
+			}
+			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 (PageWriteback(page)) {
+				xas_unlock_irq(&xas);
+				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) ||
+				  PageWriteback(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\n",
+				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\n");
+			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);
+}