Adding upstream version 6.6.15.upstream/6.6.15

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

1 files changed, 2803 insertions, 0 deletions

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
new file mode 100644
index 0000000000..88433cc25d
--- /dev/null
+++ b/mm/khugepaged.c
@@ -0,0 +1,2803 @@
+// 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/page_table_check.h>
+#include <linux/swapops.h>
+#include <linux/shmem_fs.h>
+#include <linux/ksm.h>
+
+#include <asm/tlb.h>
+#include <asm/pgalloc.h>
+#include "internal.h"
+#include "mm_slot.h"
+
+enum scan_result {
+	SCAN_FAIL,
+	SCAN_SUCCEED,
+	SCAN_PMD_NULL,
+	SCAN_PMD_NONE,
+	SCAN_PMD_MAPPED,
+	SCAN_EXCEED_NONE_PTE,
+	SCAN_EXCEED_SWAP_PTE,
+	SCAN_EXCEED_SHARED_PTE,
+	SCAN_PTE_NON_PRESENT,
+	SCAN_PTE_UFFD_WP,
+	SCAN_PTE_MAPPED_HUGEPAGE,
+	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_DEL_PAGE_LRU,
+	SCAN_ALLOC_HUGE_PAGE_FAIL,
+	SCAN_CGROUP_CHARGE_FAIL,
+	SCAN_TRUNCATED,
+	SCAN_PAGE_HAS_PRIVATE,
+	SCAN_STORE_FAILED,
+	SCAN_COPY_MC,
+	SCAN_PAGE_FILLED,
+};
+
+#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.
+ *
+ * Note that these are only respected if collapse was initiated by khugepaged.
+ */
+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 DEFINE_READ_MOSTLY_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
+
+static struct kmem_cache *mm_slot_cache __read_mostly;
+
+struct collapse_control {
+	bool is_khugepaged;
+
+	/* Num pages scanned per node */
+	u32 node_load[MAX_NUMNODES];
+
+	/* nodemask for allocation fallback */
+	nodemask_t alloc_nmask;
+};
+
+/**
+ * struct khugepaged_mm_slot - khugepaged information per mm that is being scanned
+ * @slot: hash lookup from mm to mm_slot
+ */
+struct khugepaged_mm_slot {
+	struct mm_slot slot;
+};
+
+/**
+ * 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 khugepaged_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 sysfs_emit(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 int msecs;
+	int err;
+
+	err = kstrtouint(buf, 10, &msecs);
+	if (err)
+		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_RW(scan_sleep_millisecs);
+
+static ssize_t alloc_sleep_millisecs_show(struct kobject *kobj,
+					  struct kobj_attribute *attr,
+					  char *buf)
+{
+	return sysfs_emit(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 int msecs;
+	int err;
+
+	err = kstrtouint(buf, 10, &msecs);
+	if (err)
+		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_RW(alloc_sleep_millisecs);
+
+static ssize_t pages_to_scan_show(struct kobject *kobj,
+				  struct kobj_attribute *attr,
+				  char *buf)
+{
+	return sysfs_emit(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)
+{
+	unsigned int pages;
+	int err;
+
+	err = kstrtouint(buf, 10, &pages);
+	if (err || !pages)
+		return -EINVAL;
+
+	khugepaged_pages_to_scan = pages;
+
+	return count;
+}
+static struct kobj_attribute pages_to_scan_attr =
+	__ATTR_RW(pages_to_scan);
+
+static ssize_t pages_collapsed_show(struct kobject *kobj,
+				    struct kobj_attribute *attr,
+				    char *buf)
+{
+	return sysfs_emit(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 sysfs_emit(buf, "%u\n", khugepaged_full_scans);
+}
+static struct kobj_attribute full_scans_attr =
+	__ATTR_RO(full_scans);
+
+static ssize_t 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 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_RW(defrag);
+
+/*
+ * 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 max_ptes_none_show(struct kobject *kobj,
+				  struct kobj_attribute *attr,
+				  char *buf)
+{
+	return sysfs_emit(buf, "%u\n", khugepaged_max_ptes_none);
+}
+static ssize_t 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_RW(max_ptes_none);
+
+static ssize_t max_ptes_swap_show(struct kobject *kobj,
+				  struct kobj_attribute *attr,
+				  char *buf)
+{
+	return sysfs_emit(buf, "%u\n", khugepaged_max_ptes_swap);
+}
+
+static ssize_t 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_RW(max_ptes_swap);
+
+static ssize_t max_ptes_shared_show(struct kobject *kobj,
+				    struct kobj_attribute *attr,
+				    char *buf)
+{
+	return sysfs_emit(buf, "%u\n", khugepaged_max_ptes_shared);
+}
+
+static ssize_t 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_RW(max_ptes_shared);
+
+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.
+		 */
+		khugepaged_enter_vma(vma, *vm_flags);
+		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 khugepaged_mm_slot),
+					  __alignof__(struct khugepaged_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 int hpage_collapse_test_exit(struct mm_struct *mm)
+{
+	return atomic_read(&mm->mm_users) == 0;
+}
+
+void __khugepaged_enter(struct mm_struct *mm)
+{
+	struct khugepaged_mm_slot *mm_slot;
+	struct mm_slot *slot;
+	int wakeup;
+
+	/* __khugepaged_exit() must not run from under us */
+	VM_BUG_ON_MM(hpage_collapse_test_exit(mm), mm);
+	if (unlikely(test_and_set_bit(MMF_VM_HUGEPAGE, &mm->flags)))
+		return;
+
+	mm_slot = mm_slot_alloc(mm_slot_cache);
+	if (!mm_slot)
+		return;
+
+	slot = &mm_slot->slot;
+
+	spin_lock(&khugepaged_mm_lock);
+	mm_slot_insert(mm_slots_hash, 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(&slot->mm_node, &khugepaged_scan.mm_head);
+	spin_unlock(&khugepaged_mm_lock);
+
+	mmgrab(mm);
+	if (wakeup)
+		wake_up_interruptible(&khugepaged_wait);
+}
+
+void khugepaged_enter_vma(struct vm_area_struct *vma,
+			  unsigned long vm_flags)
+{
+	if (!test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags) &&
+	    hugepage_flags_enabled()) {
+		if (hugepage_vma_check(vma, vm_flags, false, false, true))
+			__khugepaged_enter(vma->vm_mm);
+	}
+}
+
+void __khugepaged_exit(struct mm_struct *mm)
+{
+	struct khugepaged_mm_slot *mm_slot;
+	struct mm_slot *slot;
+	int free = 0;
+
+	spin_lock(&khugepaged_mm_lock);
+	slot = mm_slot_lookup(mm_slots_hash, mm);
+	mm_slot = mm_slot_entry(slot, struct khugepaged_mm_slot, slot);
+	if (mm_slot && khugepaged_scan.mm_slot != mm_slot) {
+		hash_del(&slot->hash);
+		list_del(&slot->mm_node);
+		free = 1;
+	}
+	spin_unlock(&khugepaged_mm_lock);
+
+	if (free) {
+		clear_bit(MMF_VM_HUGEPAGE, &mm->flags);
+		mm_slot_free(mm_slot_cache, mm_slot);
+		mmdrop(mm);
+	} else if (mm_slot) {
+		/*
+		 * This is required to serialize against
+		 * hpage_collapse_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_folio(struct folio *folio)
+{
+	node_stat_mod_folio(folio,
+			NR_ISOLATED_ANON + folio_is_file_lru(folio),
+			-folio_nr_pages(folio));
+	folio_unlock(folio);
+	folio_putback_lru(folio);
+}
+
+static void release_pte_page(struct page *page)
+{
+	release_pte_folio(page_folio(page));
+}
+
+static void release_pte_pages(pte_t *pte, pte_t *_pte,
+		struct list_head *compound_pagelist)
+{
+	struct folio *folio, *tmp;
+
+	while (--_pte >= pte) {
+		pte_t pteval = ptep_get(_pte);
+		unsigned long pfn;
+
+		if (pte_none(pteval))
+			continue;
+		pfn = pte_pfn(pteval);
+		if (is_zero_pfn(pfn))
+			continue;
+		folio = pfn_folio(pfn);
+		if (folio_test_large(folio))
+			continue;
+		release_pte_folio(folio);
+	}
+
+	list_for_each_entry_safe(folio, tmp, compound_pagelist, lru) {
+		list_del(&folio->lru);
+		release_pte_folio(folio);
+	}
+}
+
+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 collapse_control *cc,
+					struct list_head *compound_pagelist)
+{
+	struct page *page = NULL;
+	pte_t *_pte;
+	int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
+	bool writable = false;
+
+	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
+	     _pte++, address += PAGE_SIZE) {
+		pte_t pteval = ptep_get(_pte);
+		if (pte_none(pteval) || (pte_present(pteval) &&
+				is_zero_pfn(pte_pfn(pteval)))) {
+			++none_or_zero;
+			if (!userfaultfd_armed(vma) &&
+			    (!cc->is_khugepaged ||
+			     none_or_zero <= khugepaged_max_ptes_none)) {
+				continue;
+			} else {
+				result = SCAN_EXCEED_NONE_PTE;
+				count_vm_event(THP_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) || unlikely(is_zone_device_page(page))) {
+			result = SCAN_PAGE_NULL;
+			goto out;
+		}
+
+		VM_BUG_ON_PAGE(!PageAnon(page), page);
+
+		if (page_mapcount(page) > 1) {
+			++shared;
+			if (cc->is_khugepaged &&
+			    shared > khugepaged_max_ptes_shared) {
+				result = SCAN_EXCEED_SHARED_PTE;
+				count_vm_event(THP_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 additional 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;
+		}
+
+		/*
+		 * 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:
+		/*
+		 * If collapse was initiated by khugepaged, check that there is
+		 * enough young pte to justify collapsing the page
+		 */
+		if (cc->is_khugepaged &&
+		    (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(cc->is_khugepaged && !referenced)) {
+		result = SCAN_LACK_REFERENCED_PAGE;
+	} else {
+		result = SCAN_SUCCEED;
+		trace_mm_collapse_huge_page_isolate(page, none_or_zero,
+						    referenced, writable, result);
+		return result;
+	}
+out:
+	release_pte_pages(pte, _pte, compound_pagelist);
+	trace_mm_collapse_huge_page_isolate(page, none_or_zero,
+					    referenced, writable, result);
+	return result;
+}
+
+static void __collapse_huge_page_copy_succeeded(pte_t *pte,
+						struct vm_area_struct *vma,
+						unsigned long address,
+						spinlock_t *ptl,
+						struct list_head *compound_pagelist)
+{
+	struct page *src_page;
+	struct page *tmp;
+	pte_t *_pte;
+	pte_t pteval;
+
+	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
+	     _pte++, address += PAGE_SIZE) {
+		pteval = ptep_get(_pte);
+		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
+			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
+			if (is_zero_pfn(pte_pfn(pteval))) {
+				/*
+				 * ptl mostly unnecessary.
+				 */
+				spin_lock(ptl);
+				ptep_clear(vma->vm_mm, address, _pte);
+				spin_unlock(ptl);
+				ksm_might_unmap_zero_page(vma->vm_mm, pteval);
+			}
+		} else {
+			src_page = pte_page(pteval);
+			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);
+			ptep_clear(vma->vm_mm, address, _pte);
+			page_remove_rmap(src_page, vma, 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);
+		mod_node_page_state(page_pgdat(src_page),
+				    NR_ISOLATED_ANON + page_is_file_lru(src_page),
+				    -compound_nr(src_page));
+		unlock_page(src_page);
+		free_swap_cache(src_page);
+		putback_lru_page(src_page);
+	}
+}
+
+static void __collapse_huge_page_copy_failed(pte_t *pte,
+					     pmd_t *pmd,
+					     pmd_t orig_pmd,
+					     struct vm_area_struct *vma,
+					     struct list_head *compound_pagelist)
+{
+	spinlock_t *pmd_ptl;
+
+	/*
+	 * Re-establish the PMD to point to the original page table
+	 * entry. Restoring PMD needs to be done prior to releasing
+	 * pages. Since pages are still isolated and locked here,
+	 * acquiring anon_vma_lock_write is unnecessary.
+	 */
+	pmd_ptl = pmd_lock(vma->vm_mm, pmd);
+	pmd_populate(vma->vm_mm, pmd, pmd_pgtable(orig_pmd));
+	spin_unlock(pmd_ptl);
+	/*
+	 * Release both raw and compound pages isolated
+	 * in __collapse_huge_page_isolate.
+	 */
+	release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist);
+}
+
+/*
+ * __collapse_huge_page_copy - attempts to copy memory contents from raw
+ * pages to a hugepage. Cleans up the raw pages if copying succeeds;
+ * otherwise restores the original page table and releases isolated raw pages.
+ * Returns SCAN_SUCCEED if copying succeeds, otherwise returns SCAN_COPY_MC.
+ *
+ * @pte: starting of the PTEs to copy from
+ * @page: the new hugepage to copy contents to
+ * @pmd: pointer to the new hugepage's PMD
+ * @orig_pmd: the original raw pages' PMD
+ * @vma: the original raw pages' virtual memory area
+ * @address: starting address to copy
+ * @ptl: lock on raw pages' PTEs
+ * @compound_pagelist: list that stores compound pages
+ */
+static int __collapse_huge_page_copy(pte_t *pte,
+				     struct page *page,
+				     pmd_t *pmd,
+				     pmd_t orig_pmd,
+				     struct vm_area_struct *vma,
+				     unsigned long address,
+				     spinlock_t *ptl,
+				     struct list_head *compound_pagelist)
+{
+	struct page *src_page;
+	pte_t *_pte;
+	pte_t pteval;
+	unsigned long _address;
+	int result = SCAN_SUCCEED;
+
+	/*
+	 * Copying pages' contents is subject to memory poison at any iteration.
+	 */
+	for (_pte = pte, _address = address; _pte < pte + HPAGE_PMD_NR;
+	     _pte++, page++, _address += PAGE_SIZE) {
+		pteval = ptep_get(_pte);
+		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
+			clear_user_highpage(page, _address);
+			continue;
+		}
+		src_page = pte_page(pteval);
+		if (copy_mc_user_highpage(page, src_page, _address, vma) > 0) {
+			result = SCAN_COPY_MC;
+			break;
+		}
+	}
+
+	if (likely(result == SCAN_SUCCEED))
+		__collapse_huge_page_copy_succeeded(pte, vma, address, ptl,
+						    compound_pagelist);
+	else
+		__collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
+						 compound_pagelist);
+
+	return result;
+}
+
+static void khugepaged_alloc_sleep(void)
+{
+	DEFINE_WAIT(wait);
+
+	add_wait_queue(&khugepaged_wait, &wait);
+	__set_current_state(TASK_INTERRUPTIBLE|TASK_FREEZABLE);
+	schedule_timeout(msecs_to_jiffies(khugepaged_alloc_sleep_millisecs));
+	remove_wait_queue(&khugepaged_wait, &wait);
+}
+
+struct collapse_control khugepaged_collapse_control = {
+	.is_khugepaged = true,
+};
+
+static bool hpage_collapse_scan_abort(int nid, struct collapse_control *cc)
+{
+	int i;
+
+	/*
+	 * If node_reclaim_mode is disabled, then no extra effort is made to
+	 * allocate memory locally.
+	 */
+	if (!node_reclaim_enabled())
+		return false;
+
+	/* If there is a count for this node already, it must be acceptable */
+	if (cc->node_load[nid])
+		return false;
+
+	for (i = 0; i < MAX_NUMNODES; i++) {
+		if (!cc->node_load[i])
+			continue;
+		if (node_distance(nid, i) > node_reclaim_distance)
+			return true;
+	}
+	return false;
+}
+
+#define khugepaged_defrag()					\
+	(transparent_hugepage_flags &				\
+	 (1<<TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG))
+
+/* 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 hpage_collapse_find_target_node(struct collapse_control *cc)
+{
+	int nid, target_node = 0, max_value = 0;
+
+	/* find first node with max normal pages hit */
+	for (nid = 0; nid < MAX_NUMNODES; nid++)
+		if (cc->node_load[nid] > max_value) {
+			max_value = cc->node_load[nid];
+			target_node = nid;
+		}
+
+	for_each_online_node(nid) {
+		if (max_value == cc->node_load[nid])
+			node_set(nid, cc->alloc_nmask);
+	}
+
+	return target_node;
+}
+#else
+static int hpage_collapse_find_target_node(struct collapse_control *cc)
+{
+	return 0;
+}
+#endif
+
+static bool hpage_collapse_alloc_page(struct page **hpage, gfp_t gfp, int node,
+				      nodemask_t *nmask)
+{
+	*hpage = __alloc_pages(gfp, HPAGE_PMD_ORDER, node, nmask);
+	if (unlikely(!*hpage)) {
+		count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
+		return false;
+	}
+
+	folio_prep_large_rmappable((struct folio *)*hpage);
+	count_vm_event(THP_COLLAPSE_ALLOC);
+	return true;
+}
+
+/*
+ * If mmap_lock temporarily dropped, revalidate vma
+ * before taking mmap_lock.
+ * Returns enum scan_result value.
+ */
+
+static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
+				   bool expect_anon,
+				   struct vm_area_struct **vmap,
+				   struct collapse_control *cc)
+{
+	struct vm_area_struct *vma;
+
+	if (unlikely(hpage_collapse_test_exit(mm)))
+		return SCAN_ANY_PROCESS;
+
+	*vmap = vma = find_vma(mm, address);
+	if (!vma)
+		return SCAN_VMA_NULL;
+
+	if (!transhuge_vma_suitable(vma, address))
+		return SCAN_ADDRESS_RANGE;
+	if (!hugepage_vma_check(vma, vma->vm_flags, false, false,
+				cc->is_khugepaged))
+		return SCAN_VMA_CHECK;
+	/*
+	 * Anon VMA expected, the address may be unmapped then
+	 * remapped to file after khugepaged reaquired the mmap_lock.
+	 *
+	 * hugepage_vma_check may return true for qualified file
+	 * vmas.
+	 */
+	if (expect_anon && (!(*vmap)->anon_vma || !vma_is_anonymous(*vmap)))
+		return SCAN_PAGE_ANON;
+	return SCAN_SUCCEED;
+}
+
+static int find_pmd_or_thp_or_none(struct mm_struct *mm,
+				   unsigned long address,
+				   pmd_t **pmd)
+{
+	pmd_t pmde;
+
+	*pmd = mm_find_pmd(mm, address);
+	if (!*pmd)
+		return SCAN_PMD_NULL;
+
+	pmde = pmdp_get_lockless(*pmd);
+	if (pmd_none(pmde))
+		return SCAN_PMD_NONE;
+	if (!pmd_present(pmde))
+		return SCAN_PMD_NULL;
+	if (pmd_trans_huge(pmde))
+		return SCAN_PMD_MAPPED;
+	if (pmd_devmap(pmde))
+		return SCAN_PMD_NULL;
+	if (pmd_bad(pmde))
+		return SCAN_PMD_NULL;
+	return SCAN_SUCCEED;
+}
+
+static int check_pmd_still_valid(struct mm_struct *mm,
+				 unsigned long address,
+				 pmd_t *pmd)
+{
+	pmd_t *new_pmd;
+	int result = find_pmd_or_thp_or_none(mm, address, &new_pmd);
+
+	if (result != SCAN_SUCCEED)
+		return result;
+	if (new_pmd != pmd)
+		return SCAN_FAIL;
+	return SCAN_SUCCEED;
+}
+
+/*
+ * Bring missing pages in from swap, to complete THP collapse.
+ * Only done if hpage_collapse_scan_pmd believes it is worthwhile.
+ *
+ * Called and returns without pte mapped or spinlocks held.
+ * Returns result: if not SCAN_SUCCEED, mmap_lock has been released.
+ */
+static int __collapse_huge_page_swapin(struct mm_struct *mm,
+				       struct vm_area_struct *vma,
+				       unsigned long haddr, pmd_t *pmd,
+				       int referenced)
+{
+	int swapped_in = 0;
+	vm_fault_t ret = 0;
+	unsigned long address, end = haddr + (HPAGE_PMD_NR * PAGE_SIZE);
+	int result;
+	pte_t *pte = NULL;
+	spinlock_t *ptl;
+
+	for (address = haddr; address < end; address += PAGE_SIZE) {
+		struct vm_fault vmf = {
+			.vma = vma,
+			.address = address,
+			.pgoff = linear_page_index(vma, address),
+			.flags = FAULT_FLAG_ALLOW_RETRY,
+			.pmd = pmd,
+		};
+
+		if (!pte++) {
+			pte = pte_offset_map_nolock(mm, pmd, address, &ptl);
+			if (!pte) {
+				mmap_read_unlock(mm);
+				result = SCAN_PMD_NULL;
+				goto out;
+			}
+		}
+
+		vmf.orig_pte = ptep_get_lockless(pte);
+		if (!is_swap_pte(vmf.orig_pte))
+			continue;
+
+		vmf.pte = pte;
+		vmf.ptl = ptl;
+		ret = do_swap_page(&vmf);
+		/* Which unmaps pte (after perhaps re-checking the entry) */
+		pte = NULL;
+
+		/*
+		 * do_swap_page returns VM_FAULT_RETRY with released mmap_lock.
+		 * Note we treat VM_FAULT_RETRY as VM_FAULT_ERROR here because
+		 * we do not retry here and swap entry will remain in pagetable
+		 * resulting in later failure.
+		 */
+		if (ret & VM_FAULT_RETRY) {
+			/* Likely, but not guaranteed, that page lock failed */
+			result = SCAN_PAGE_LOCK;
+			goto out;
+		}
+		if (ret & VM_FAULT_ERROR) {
+			mmap_read_unlock(mm);
+			result = SCAN_FAIL;
+			goto out;
+		}
+		swapped_in++;
+	}
+
+	if (pte)
+		pte_unmap(pte);
+
+	/* Drain LRU cache to remove extra pin on the swapped in pages */
+	if (swapped_in)
+		lru_add_drain();
+
+	result = SCAN_SUCCEED;
+out:
+	trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, result);
+	return result;
+}
+
+static int alloc_charge_hpage(struct page **hpage, struct mm_struct *mm,
+			      struct collapse_control *cc)
+{
+	gfp_t gfp = (cc->is_khugepaged ? alloc_hugepage_khugepaged_gfpmask() :
+		     GFP_TRANSHUGE);
+	int node = hpage_collapse_find_target_node(cc);
+	struct folio *folio;
+
+	if (!hpage_collapse_alloc_page(hpage, gfp, node, &cc->alloc_nmask))
+		return SCAN_ALLOC_HUGE_PAGE_FAIL;
+
+	folio = page_folio(*hpage);
+	if (unlikely(mem_cgroup_charge(folio, mm, gfp))) {
+		folio_put(folio);
+		*hpage = NULL;
+		return SCAN_CGROUP_CHARGE_FAIL;
+	}
+	count_memcg_page_event(*hpage, THP_COLLAPSE_ALLOC);
+
+	return SCAN_SUCCEED;
+}
+
+static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
+			      int referenced, int unmapped,
+			      struct collapse_control *cc)
+{
+	LIST_HEAD(compound_pagelist);
+	pmd_t *pmd, _pmd;
+	pte_t *pte;
+	pgtable_t pgtable;
+	struct page *hpage;
+	spinlock_t *pmd_ptl, *pte_ptl;
+	int result = SCAN_FAIL;
+	struct vm_area_struct *vma;
+	struct mmu_notifier_range range;
+
+	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
+
+	/*
+	 * 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);
+
+	result = alloc_charge_hpage(&hpage, mm, cc);
+	if (result != SCAN_SUCCEED)
+		goto out_nolock;
+
+	mmap_read_lock(mm);
+	result = hugepage_vma_revalidate(mm, address, true, &vma, cc);
+	if (result != SCAN_SUCCEED) {
+		mmap_read_unlock(mm);
+		goto out_nolock;
+	}
+
+	result = find_pmd_or_thp_or_none(mm, address, &pmd);
+	if (result != SCAN_SUCCEED) {
+		mmap_read_unlock(mm);
+		goto out_nolock;
+	}
+
+	if (unmapped) {
+		/*
+		 * __collapse_huge_page_swapin will return with mmap_lock
+		 * released when it fails. So we jump out_nolock directly in
+		 * that case.  Continuing to collapse causes inconsistency.
+		 */
+		result = __collapse_huge_page_swapin(mm, vma, address, pmd,
+						     referenced);
+		if (result != SCAN_SUCCEED)
+			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, true, &vma, cc);
+	if (result != SCAN_SUCCEED)
+		goto out_up_write;
+	/* check if the pmd is still valid */
+	result = check_pmd_still_valid(mm, address, pmd);
+	if (result != SCAN_SUCCEED)
+		goto out_up_write;
+
+	vma_start_write(vma);
+	anon_vma_lock_write(vma->anon_vma);
+
+	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,
+				address + HPAGE_PMD_SIZE);
+	mmu_notifier_invalidate_range_start(&range);
+
+	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();
+
+	pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
+	if (pte) {
+		result = __collapse_huge_page_isolate(vma, address, pte, cc,
+						      &compound_pagelist);
+		spin_unlock(pte_ptl);
+	} else {
+		result = SCAN_PMD_NULL;
+	}
+
+	if (unlikely(result != SCAN_SUCCEED)) {
+		if (pte)
+			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);
+		goto out_up_write;
+	}
+
+	/*
+	 * All pages are isolated and locked so anon_vma rmap
+	 * can't run anymore.
+	 */
+	anon_vma_unlock_write(vma->anon_vma);
+
+	result = __collapse_huge_page_copy(pte, hpage, pmd, _pmd,
+					   vma, address, pte_ptl,
+					   &compound_pagelist);
+	pte_unmap(pte);
+	if (unlikely(result != SCAN_SUCCEED))
+		goto out_up_write;
+
+	/*
+	 * spin_lock() below is not the equivalent of smp_wmb(), but
+	 * the smp_wmb() inside __SetPageUptodate() can be reused to
+	 * avoid the copy_huge_page writes to become visible after
+	 * the set_pmd_at() write.
+	 */
+	__SetPageUptodate(hpage);
+	pgtable = pmd_pgtable(_pmd);
+
+	_pmd = mk_huge_pmd(hpage, vma->vm_page_prot);
+	_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
+
+	spin_lock(pmd_ptl);
+	BUG_ON(!pmd_none(*pmd));
+	page_add_new_anon_rmap(hpage, vma, address);
+	lru_cache_add_inactive_or_unevictable(hpage, 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;
+
+	result = SCAN_SUCCEED;
+out_up_write:
+	mmap_write_unlock(mm);
+out_nolock:
+	if (hpage)
+		put_page(hpage);
+	trace_mm_collapse_huge_page(mm, result == SCAN_SUCCEED, result);
+	return result;
+}
+
+static int hpage_collapse_scan_pmd(struct mm_struct *mm,
+				   struct vm_area_struct *vma,
+				   unsigned long address, bool *mmap_locked,
+				   struct collapse_control *cc)
+{
+	pmd_t *pmd;
+	pte_t *pte, *_pte;
+	int result = SCAN_FAIL, 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);
+
+	result = find_pmd_or_thp_or_none(mm, address, &pmd);
+	if (result != SCAN_SUCCEED)
+		goto out;
+
+	memset(cc->node_load, 0, sizeof(cc->node_load));
+	nodes_clear(cc->alloc_nmask);
+	pte = pte_offset_map_lock(mm, pmd, address, &ptl);
+	if (!pte) {
+		result = SCAN_PMD_NULL;
+		goto out;
+	}
+
+	for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR;
+	     _pte++, _address += PAGE_SIZE) {
+		pte_t pteval = ptep_get(_pte);
+		if (is_swap_pte(pteval)) {
+			++unmapped;
+			if (!cc->is_khugepaged ||
+			    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_any(pteval)) {
+					result = SCAN_PTE_UFFD_WP;
+					goto out_unmap;
+				}
+				continue;
+			} else {
+				result = SCAN_EXCEED_SWAP_PTE;
+				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
+				goto out_unmap;
+			}
+		}
+		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
+			++none_or_zero;
+			if (!userfaultfd_armed(vma) &&
+			    (!cc->is_khugepaged ||
+			     none_or_zero <= khugepaged_max_ptes_none)) {
+				continue;
+			} else {
+				result = SCAN_EXCEED_NONE_PTE;
+				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
+				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 unknown
+			 * 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) || unlikely(is_zone_device_page(page))) {
+			result = SCAN_PAGE_NULL;
+			goto out_unmap;
+		}
+
+		if (page_mapcount(page) > 1) {
+			++shared;
+			if (cc->is_khugepaged &&
+			    shared > khugepaged_max_ptes_shared) {
+				result = SCAN_EXCEED_SHARED_PTE;
+				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
+				goto out_unmap;
+			}
+		}
+
+		page = compound_head(page);
+
+		/*
+		 * Record which node the original page is from and save this
+		 * information to cc->node_load[].
+		 * Khugepaged will allocate hugepage from the node has the max
+		 * hit record.
+		 */
+		node = page_to_nid(page);
+		if (hpage_collapse_scan_abort(node, cc)) {
+			result = SCAN_SCAN_ABORT;
+			goto out_unmap;
+		}
+		cc->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 may be racy:
+		 * it may see total_mapcount > refcount in some cases?
+		 * 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 collapse was initiated by khugepaged, check that there is
+		 * enough young pte to justify collapsing the page
+		 */
+		if (cc->is_khugepaged &&
+		    (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 (cc->is_khugepaged &&
+		   (!referenced ||
+		    (unmapped && referenced < HPAGE_PMD_NR / 2))) {
+		result = SCAN_LACK_REFERENCED_PAGE;
+	} else {
+		result = SCAN_SUCCEED;
+	}
+out_unmap:
+	pte_unmap_unlock(pte, ptl);
+	if (result == SCAN_SUCCEED) {
+		result = collapse_huge_page(mm, address, referenced,
+					    unmapped, cc);
+		/* collapse_huge_page will return with the mmap_lock released */
+		*mmap_locked = false;
+	}
+out:
+	trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,
+				     none_or_zero, result, unmapped);
+	return result;
+}
+
+static void collect_mm_slot(struct khugepaged_mm_slot *mm_slot)
+{
+	struct mm_slot *slot = &mm_slot->slot;
+	struct mm_struct *mm = slot->mm;
+
+	lockdep_assert_held(&khugepaged_mm_lock);
+
+	if (hpage_collapse_test_exit(mm)) {
+		/* free mm_slot */
+		hash_del(&slot->hash);
+		list_del(&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 */
+		mm_slot_free(mm_slot_cache, mm_slot);
+		mmdrop(mm);
+	}
+}
+
+#ifdef CONFIG_SHMEM
+/* hpage must be locked, and mmap_lock must be held */
+static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,
+			pmd_t *pmdp, struct page *hpage)
+{
+	struct vm_fault vmf = {
+		.vma = vma,
+		.address = addr,
+		.flags = 0,
+		.pmd = pmdp,
+	};
+
+	VM_BUG_ON(!PageTransHuge(hpage));
+	mmap_assert_locked(vma->vm_mm);
+
+	if (do_set_pmd(&vmf, hpage))
+		return SCAN_FAIL;
+
+	get_page(hpage);
+	return SCAN_SUCCEED;
+}
+
+/**
+ * collapse_pte_mapped_thp - Try to collapse a pte-mapped THP for mm at
+ * address haddr.
+ *
+ * @mm: process address space where collapse happens
+ * @addr: THP collapse address
+ * @install_pmd: If a huge PMD should be installed
+ *
+ * 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. Possibly install a huge PMD mapping the THP.
+ */
+int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
+			    bool install_pmd)
+{
+	struct mmu_notifier_range range;
+	bool notified = false;
+	unsigned long haddr = addr & HPAGE_PMD_MASK;
+	struct vm_area_struct *vma = vma_lookup(mm, haddr);
+	struct page *hpage;
+	pte_t *start_pte, *pte;
+	pmd_t *pmd, pgt_pmd;
+	spinlock_t *pml = NULL, *ptl;
+	int nr_ptes = 0, result = SCAN_FAIL;
+	int i;
+
+	mmap_assert_locked(mm);
+
+	/* First check VMA found, in case page tables are being torn down */
+	if (!vma || !vma->vm_file ||
+	    !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE))
+		return SCAN_VMA_CHECK;
+
+	/* Fast check before locking page if already PMD-mapped */
+	result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
+	if (result == SCAN_PMD_MAPPED)
+		return result;
+
+	/*
+	 * If we are here, we've succeeded in replacing all the native pages
+	 * in the page cache with a single hugepage. If a mm were to fault-in
+	 * this memory (mapped by a suitably aligned VMA), we'd get the hugepage
+	 * and map it by a PMD, regardless of sysfs THP settings. As such, let's
+	 * analogously elide sysfs THP settings here.
+	 */
+	if (!hugepage_vma_check(vma, vma->vm_flags, false, false, false))
+		return SCAN_VMA_CHECK;
+
+	/* Keep pmd pgtable for uffd-wp; see comment in retract_page_tables() */
+	if (userfaultfd_wp(vma))
+		return SCAN_PTE_UFFD_WP;
+
+	hpage = find_lock_page(vma->vm_file->f_mapping,
+			       linear_page_index(vma, haddr));
+	if (!hpage)
+		return SCAN_PAGE_NULL;
+
+	if (!PageHead(hpage)) {
+		result = SCAN_FAIL;
+		goto drop_hpage;
+	}
+
+	if (compound_order(hpage) != HPAGE_PMD_ORDER) {
+		result = SCAN_PAGE_COMPOUND;
+		goto drop_hpage;
+	}
+
+	result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
+	switch (result) {
+	case SCAN_SUCCEED:
+		break;
+	case SCAN_PMD_NONE:
+		/*
+		 * All pte entries have been removed and pmd cleared.
+		 * Skip all the pte checks and just update the pmd mapping.
+		 */
+		goto maybe_install_pmd;
+	default:
+		goto drop_hpage;
+	}
+
+	result = SCAN_FAIL;
+	start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl);
+	if (!start_pte)		/* mmap_lock + page lock should prevent this */
+		goto drop_hpage;
+
+	/* 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;
+		pte_t ptent = ptep_get(pte);
+
+		/* empty pte, skip */
+		if (pte_none(ptent))
+			continue;
+
+		/* page swapped out, abort */
+		if (!pte_present(ptent)) {
+			result = SCAN_PTE_NON_PRESENT;
+			goto abort;
+		}
+
+		page = vm_normal_page(vma, addr, ptent);
+		if (WARN_ON_ONCE(page && is_zone_device_page(page)))
+			page = NULL;
+		/*
+		 * 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;
+	}
+
+	pte_unmap_unlock(start_pte, ptl);
+	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm,
+				haddr, haddr + HPAGE_PMD_SIZE);
+	mmu_notifier_invalidate_range_start(&range);
+	notified = true;
+
+	/*
+	 * pmd_lock covers a wider range than ptl, and (if split from mm's
+	 * page_table_lock) ptl nests inside pml. The less time we hold pml,
+	 * the better; but userfaultfd's mfill_atomic_pte() on a private VMA
+	 * inserts a valid as-if-COWed PTE without even looking up page cache.
+	 * So page lock of hpage does not protect from it, so we must not drop
+	 * ptl before pgt_pmd is removed, so uffd private needs pml taken now.
+	 */
+	if (userfaultfd_armed(vma) && !(vma->vm_flags & VM_SHARED))
+		pml = pmd_lock(mm, pmd);
+
+	start_pte = pte_offset_map_nolock(mm, pmd, haddr, &ptl);
+	if (!start_pte)		/* mmap_lock + page lock should prevent this */
+		goto abort;
+	if (!pml)
+		spin_lock(ptl);
+	else if (ptl != pml)
+		spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
+
+	/* step 2: clear page table and adjust rmap */
+	for (i = 0, addr = haddr, pte = start_pte;
+	     i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {
+		struct page *page;
+		pte_t ptent = ptep_get(pte);
+
+		if (pte_none(ptent))
+			continue;
+		/*
+		 * We dropped ptl after the first scan, to do the mmu_notifier:
+		 * page lock stops more PTEs of the hpage being faulted in, but
+		 * does not stop write faults COWing anon copies from existing
+		 * PTEs; and does not stop those being swapped out or migrated.
+		 */
+		if (!pte_present(ptent)) {
+			result = SCAN_PTE_NON_PRESENT;
+			goto abort;
+		}
+		page = vm_normal_page(vma, addr, ptent);
+		if (hpage + i != page)
+			goto abort;
+
+		/*
+		 * Must clear entry, or a racing truncate may re-remove it.
+		 * TLB flush can be left until pmdp_collapse_flush() does it.
+		 * PTE dirty? Shmem page is already dirty; file is read-only.
+		 */
+		ptep_clear(mm, addr, pte);
+		page_remove_rmap(page, vma, false);
+		nr_ptes++;
+	}
+
+	pte_unmap(start_pte);
+	if (!pml)
+		spin_unlock(ptl);
+
+	/* step 3: set proper refcount and mm_counters. */
+	if (nr_ptes) {
+		page_ref_sub(hpage, nr_ptes);
+		add_mm_counter(mm, mm_counter_file(hpage), -nr_ptes);
+	}
+
+	/* step 4: remove empty page table */
+	if (!pml) {
+		pml = pmd_lock(mm, pmd);
+		if (ptl != pml)
+			spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
+	}
+	pgt_pmd = pmdp_collapse_flush(vma, haddr, pmd);
+	pmdp_get_lockless_sync();
+	if (ptl != pml)
+		spin_unlock(ptl);
+	spin_unlock(pml);
+
+	mmu_notifier_invalidate_range_end(&range);
+
+	mm_dec_nr_ptes(mm);
+	page_table_check_pte_clear_range(mm, haddr, pgt_pmd);
+	pte_free_defer(mm, pmd_pgtable(pgt_pmd));
+
+maybe_install_pmd:
+	/* step 5: install pmd entry */
+	result = install_pmd
+			? set_huge_pmd(vma, haddr, pmd, hpage)
+			: SCAN_SUCCEED;
+	goto drop_hpage;
+abort:
+	if (nr_ptes) {
+		flush_tlb_mm(mm);
+		page_ref_sub(hpage, nr_ptes);
+		add_mm_counter(mm, mm_counter_file(hpage), -nr_ptes);
+	}
+	if (start_pte)
+		pte_unmap_unlock(start_pte, ptl);
+	if (pml && pml != ptl)
+		spin_unlock(pml);
+	if (notified)
+		mmu_notifier_invalidate_range_end(&range);
+drop_hpage:
+	unlock_page(hpage);
+	put_page(hpage);
+	return result;
+}
+
+static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
+{
+	struct vm_area_struct *vma;
+
+	i_mmap_lock_read(mapping);
+	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
+		struct mmu_notifier_range range;
+		struct mm_struct *mm;
+		unsigned long addr;
+		pmd_t *pmd, pgt_pmd;
+		spinlock_t *pml;
+		spinlock_t *ptl;
+		bool skipped_uffd = false;
+
+		/*
+		 * Check vma->anon_vma to exclude MAP_PRIVATE mappings that
+		 * got written to. These VMAs are likely not worth removing
+		 * page tables from, as PMD-mapping is likely to be split later.
+		 */
+		if (READ_ONCE(vma->anon_vma))
+			continue;
+
+		addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
+		if (addr & ~HPAGE_PMD_MASK ||
+		    vma->vm_end < addr + HPAGE_PMD_SIZE)
+			continue;
+
+		mm = vma->vm_mm;
+		if (find_pmd_or_thp_or_none(mm, addr, &pmd) != SCAN_SUCCEED)
+			continue;
+
+		if (hpage_collapse_test_exit(mm))
+			continue;
+		/*
+		 * When a vma is registered with uffd-wp, we cannot recycle
+		 * the page table because there may be pte markers installed.
+		 * Other vmas can still have the same file mapped hugely, but
+		 * skip this one: it will always be mapped in small page size
+		 * for uffd-wp registered ranges.
+		 */
+		if (userfaultfd_wp(vma))
+			continue;
+
+		/* PTEs were notified when unmapped; but now for the PMD? */
+		mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm,
+					addr, addr + HPAGE_PMD_SIZE);
+		mmu_notifier_invalidate_range_start(&range);
+
+		pml = pmd_lock(mm, pmd);
+		ptl = pte_lockptr(mm, pmd);
+		if (ptl != pml)
+			spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
+
+		/*
+		 * Huge page lock is still held, so normally the page table
+		 * must remain empty; and we have already skipped anon_vma
+		 * and userfaultfd_wp() vmas.  But since the mmap_lock is not
+		 * held, it is still possible for a racing userfaultfd_ioctl()
+		 * to have inserted ptes or markers.  Now that we hold ptlock,
+		 * repeating the anon_vma check protects from one category,
+		 * and repeating the userfaultfd_wp() check from another.
+		 */
+		if (unlikely(vma->anon_vma || userfaultfd_wp(vma))) {
+			skipped_uffd = true;
+		} else {
+			pgt_pmd = pmdp_collapse_flush(vma, addr, pmd);
+			pmdp_get_lockless_sync();
+		}
+
+		if (ptl != pml)
+			spin_unlock(ptl);
+		spin_unlock(pml);
+
+		mmu_notifier_invalidate_range_end(&range);
+
+		if (!skipped_uffd) {
+			mm_dec_nr_ptes(mm);
+			page_table_check_pte_clear_range(mm, addr, pgt_pmd);
+			pte_free_defer(mm, pmd_pgtable(pgt_pmd));
+		}
+	}
+	i_mmap_unlock_read(mapping);
+}
+
+/**
+ * collapse_file - collapse filemap/tmpfs/shmem pages into huge one.
+ *
+ * @mm: process address space where collapse happens
+ * @addr: virtual collapse start address
+ * @file: file that collapse on
+ * @start: collapse start address
+ * @cc: collapse context and scratchpad
+ *
+ * Basic scheme is simple, details are more complex:
+ *  - allocate and lock a new huge page;
+ *  - scan page cache, locking old pages
+ *    + swap/gup in pages if necessary;
+ *  - copy data to new page
+ *  - handle shmem holes
+ *    + re-validate that holes weren't filled by someone else
+ *    + check for userfaultfd
+ *  - finalize updates to the page cache;
+ *  - if replacing succeeds:
+ *    + unlock huge page;
+ *    + free old pages;
+ *  - if replacing failed;
+ *    + unlock old pages
+ *    + unlock and free huge page;
+ */
+static int collapse_file(struct mm_struct *mm, unsigned long addr,
+			 struct file *file, pgoff_t start,
+			 struct collapse_control *cc)
+{
+	struct address_space *mapping = file->f_mapping;
+	struct page *hpage;
+	struct page *page;
+	struct page *tmp;
+	struct folio *folio;
+	pgoff_t index = 0, 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);
+	int nr = 0;
+
+	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
+	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
+
+	result = alloc_charge_hpage(&hpage, mm, cc);
+	if (result != SCAN_SUCCEED)
+		goto out;
+
+	__SetPageLocked(hpage);
+	if (is_shmem)
+		__SetPageSwapBacked(hpage);
+	hpage->index = start;
+	hpage->mapping = mapping;
+
+	/*
+	 * Ensure we have slots for all the pages in the range.  This is
+	 * almost certainly a no-op because most of the pages must be present
+	 */
+	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 rollback;
+		}
+	} while (1);
+
+	for (index = start; index < end; index++) {
+		xas_set(&xas, index);
+		page = xas_load(&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;
+					}
+				}
+				nr_none++;
+				continue;
+			}
+
+			if (xa_is_value(page) || !PageUptodate(page)) {
+				xas_unlock_irq(&xas);
+				/* swap in or instantiate fallocated page */
+				if (shmem_get_folio(mapping->host, index,
+						&folio, SGP_NOALLOC)) {
+					result = SCAN_FAIL;
+					goto xa_unlocked;
+				}
+				/* drain lru cache to help isolate_lru_page() */
+				lru_add_drain();
+				page = folio_file_page(folio, index);
+			} 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 lru cache 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.
+		 * This will be discovered on the first iteration.
+		 */
+		if (PageTransCompound(page)) {
+			struct page *head = compound_head(page);
+
+			result = compound_order(head) == HPAGE_PMD_ORDER &&
+					head->index == start
+					/* Maybe PMD-mapped */
+					? SCAN_PTE_MAPPED_HUGEPAGE
+					: SCAN_PAGE_COMPOUND;
+			goto out_unlock;
+		}
+
+		folio = page_folio(page);
+
+		if (folio_mapping(folio) != mapping) {
+			result = SCAN_TRUNCATED;
+			goto out_unlock;
+		}
+
+		if (!is_shmem && (folio_test_dirty(folio) ||
+				  folio_test_writeback(folio))) {
+			/*
+			 * 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 (!folio_isolate_lru(folio)) {
+			result = SCAN_DEL_PAGE_LRU;
+			goto out_unlock;
+		}
+
+		if (!filemap_release_folio(folio, GFP_KERNEL)) {
+			result = SCAN_PAGE_HAS_PRIVATE;
+			folio_putback_lru(folio);
+			goto out_unlock;
+		}
+
+		if (folio_mapped(folio))
+			try_to_unmap(folio,
+					TTU_IGNORE_MLOCK | TTU_BATCH_FLUSH);
+
+		xas_lock_irq(&xas);
+
+		VM_BUG_ON_PAGE(page != xa_load(xas.xa, index), page);
+
+		/*
+		 * We control three references to the page:
+		 *  - we hold a pin on it;
+		 *  - one reference from page cache;
+		 *  - one from isolate_lru_page;
+		 * If those are the only references, then any new usage of the
+		 * page will have to fetch it from the page cache. That requires
+		 * locking the page to handle truncate, so any new usage will be
+		 * blocked until we unlock page after collapse/during rollback.
+		 */
+		if (page_count(page) != 3) {
+			result = SCAN_PAGE_COUNT;
+			xas_unlock_irq(&xas);
+			putback_lru_page(page);
+			goto out_unlock;
+		}
+
+		/*
+		 * Accumulate the pages that are being collapsed.
+		 */
+		list_add_tail(&page->lru, &pagelist);
+		continue;
+out_unlock:
+		unlock_page(page);
+		put_page(page);
+		goto xa_unlocked;
+	}
+
+	if (!is_shmem) {
+		filemap_nr_thps_inc(mapping);
+		/*
+		 * Paired with smp_mb() in do_dentry_open() to ensure
+		 * i_writecount is up to date and the update to nr_thps is
+		 * visible. Ensures the page cache will be truncated if the
+		 * file is opened writable.
+		 */
+		smp_mb();
+		if (inode_is_open_for_write(mapping->host)) {
+			result = SCAN_FAIL;
+			filemap_nr_thps_dec(mapping);
+		}
+	}
+
+xa_locked:
+	xas_unlock_irq(&xas);
+xa_unlocked:
+
+	/*
+	 * If collapse is successful, flush must be done now before copying.
+	 * If collapse is unsuccessful, does flush actually need to be done?
+	 * Do it anyway, to clear the state.
+	 */
+	try_to_unmap_flush();
+
+	if (result == SCAN_SUCCEED && nr_none &&
+	    !shmem_charge(mapping->host, nr_none))
+		result = SCAN_FAIL;
+	if (result != SCAN_SUCCEED) {
+		nr_none = 0;
+		goto rollback;
+	}
+
+	/*
+	 * The old pages are locked, so they won't change anymore.
+	 */
+	index = start;
+	list_for_each_entry(page, &pagelist, lru) {
+		while (index < page->index) {
+			clear_highpage(hpage + (index % HPAGE_PMD_NR));
+			index++;
+		}
+		if (copy_mc_highpage(hpage + (page->index % HPAGE_PMD_NR), page) > 0) {
+			result = SCAN_COPY_MC;
+			goto rollback;
+		}
+		index++;
+	}
+	while (index < end) {
+		clear_highpage(hpage + (index % HPAGE_PMD_NR));
+		index++;
+	}
+
+	if (nr_none) {
+		struct vm_area_struct *vma;
+		int nr_none_check = 0;
+
+		i_mmap_lock_read(mapping);
+		xas_lock_irq(&xas);
+
+		xas_set(&xas, start);
+		for (index = start; index < end; index++) {
+			if (!xas_next(&xas)) {
+				xas_store(&xas, XA_RETRY_ENTRY);
+				if (xas_error(&xas)) {
+					result = SCAN_STORE_FAILED;
+					goto immap_locked;
+				}
+				nr_none_check++;
+			}
+		}
+
+		if (nr_none != nr_none_check) {
+			result = SCAN_PAGE_FILLED;
+			goto immap_locked;
+		}
+
+		/*
+		 * If userspace observed a missing page in a VMA with a MODE_MISSING
+		 * userfaultfd, then it might expect a UFFD_EVENT_PAGEFAULT for that
+		 * page. If so, we need to roll back to avoid suppressing such an
+		 * event. Since wp/minor userfaultfds don't give userspace any
+		 * guarantees that the kernel doesn't fill a missing page with a zero
+		 * page, so they don't matter here.
+		 *
+		 * Any userfaultfds registered after this point will not be able to
+		 * observe any missing pages due to the previously inserted retry
+		 * entries.
+		 */
+		vma_interval_tree_foreach(vma, &mapping->i_mmap, start, end) {
+			if (userfaultfd_missing(vma)) {
+				result = SCAN_EXCEED_NONE_PTE;
+				goto immap_locked;
+			}
+		}
+
+immap_locked:
+		i_mmap_unlock_read(mapping);
+		if (result != SCAN_SUCCEED) {
+			xas_set(&xas, start);
+			for (index = start; index < end; index++) {
+				if (xas_next(&xas) == XA_RETRY_ENTRY)
+					xas_store(&xas, NULL);
+			}
+
+			xas_unlock_irq(&xas);
+			goto rollback;
+		}
+	} else {
+		xas_lock_irq(&xas);
+	}
+
+	nr = thp_nr_pages(hpage);
+	if (is_shmem)
+		__mod_lruvec_page_state(hpage, NR_SHMEM_THPS, nr);
+	else
+		__mod_lruvec_page_state(hpage, NR_FILE_THPS, nr);
+
+	if (nr_none) {
+		__mod_lruvec_page_state(hpage, NR_FILE_PAGES, nr_none);
+		/* nr_none is always 0 for non-shmem. */
+		__mod_lruvec_page_state(hpage, NR_SHMEM, nr_none);
+	}
+
+	/*
+	 * Mark hpage as uptodate before inserting it into the page cache so
+	 * that it isn't mistaken for an fallocated but unwritten page.
+	 */
+	folio = page_folio(hpage);
+	folio_mark_uptodate(folio);
+	folio_ref_add(folio, HPAGE_PMD_NR - 1);
+
+	if (is_shmem)
+		folio_mark_dirty(folio);
+	folio_add_lru(folio);
+
+	/* Join all the small entries into a single multi-index entry. */
+	xas_set_order(&xas, start, HPAGE_PMD_ORDER);
+	xas_store(&xas, hpage);
+	WARN_ON_ONCE(xas_error(&xas));
+	xas_unlock_irq(&xas);
+
+	/*
+	 * Remove pte page tables, so we can re-fault the page as huge.
+	 * If MADV_COLLAPSE, adjust result to call collapse_pte_mapped_thp().
+	 */
+	retract_page_tables(mapping, start);
+	if (cc && !cc->is_khugepaged)
+		result = SCAN_PTE_MAPPED_HUGEPAGE;
+	unlock_page(hpage);
+
+	/*
+	 * The collapse has succeeded, so free the old pages.
+	 */
+	list_for_each_entry_safe(page, tmp, &pagelist, lru) {
+		list_del(&page->lru);
+		page->mapping = NULL;
+		ClearPageActive(page);
+		ClearPageUnevictable(page);
+		unlock_page(page);
+		folio_put_refs(page_folio(page), 3);
+	}
+
+	goto out;
+
+rollback:
+	/* Something went wrong: roll back page cache changes */
+	if (nr_none) {
+		xas_lock_irq(&xas);
+		mapping->nrpages -= nr_none;
+		xas_unlock_irq(&xas);
+		shmem_uncharge(mapping->host, nr_none);
+	}
+
+	list_for_each_entry_safe(page, tmp, &pagelist, lru) {
+		list_del(&page->lru);
+		unlock_page(page);
+		putback_lru_page(page);
+		put_page(page);
+	}
+	/*
+	 * Undo the updates of filemap_nr_thps_inc for non-SHMEM
+	 * file only. This undo is not needed unless failure is
+	 * due to SCAN_COPY_MC.
+	 */
+	if (!is_shmem && result == SCAN_COPY_MC) {
+		filemap_nr_thps_dec(mapping);
+		/*
+		 * Paired with smp_mb() in do_dentry_open() to
+		 * ensure the update to nr_thps is visible.
+		 */
+		smp_mb();
+	}
+
+	hpage->mapping = NULL;
+
+	unlock_page(hpage);
+	put_page(hpage);
+out:
+	VM_BUG_ON(!list_empty(&pagelist));
+	trace_mm_khugepaged_collapse_file(mm, hpage, index, is_shmem, addr, file, nr, result);
+	return result;
+}
+
+static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
+				    struct file *file, pgoff_t start,
+				    struct collapse_control *cc)
+{
+	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(cc->node_load, 0, sizeof(cc->node_load));
+	nodes_clear(cc->alloc_nmask);
+	rcu_read_lock();
+	xas_for_each(&xas, page, start + HPAGE_PMD_NR - 1) {
+		if (xas_retry(&xas, page))
+			continue;
+
+		if (xa_is_value(page)) {
+			++swap;
+			if (cc->is_khugepaged &&
+			    swap > khugepaged_max_ptes_swap) {
+				result = SCAN_EXCEED_SWAP_PTE;
+				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
+				break;
+			}
+			continue;
+		}
+
+		/*
+		 * TODO: khugepaged should compact smaller compound pages
+		 * into a PMD sized page
+		 */
+		if (PageTransCompound(page)) {
+			struct page *head = compound_head(page);
+
+			result = compound_order(head) == HPAGE_PMD_ORDER &&
+					head->index == start
+					/* Maybe PMD-mapped */
+					? SCAN_PTE_MAPPED_HUGEPAGE
+					: SCAN_PAGE_COMPOUND;
+			/*
+			 * For SCAN_PTE_MAPPED_HUGEPAGE, further processing
+			 * by the caller won't touch the page cache, and so
+			 * it's safe to skip LRU and refcount checks before
+			 * returning.
+			 */
+			break;
+		}
+
+		node = page_to_nid(page);
+		if (hpage_collapse_scan_abort(node, cc)) {
+			result = SCAN_SCAN_ABORT;
+			break;
+		}
+		cc->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 (cc->is_khugepaged &&
+		    present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
+			result = SCAN_EXCEED_NONE_PTE;
+			count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
+		} else {
+			result = collapse_file(mm, addr, file, start, cc);
+		}
+	}
+
+	trace_mm_khugepaged_scan_file(mm, page, file, present, swap, result);
+	return result;
+}
+#else
+static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
+				    struct file *file, pgoff_t start,
+				    struct collapse_control *cc)
+{
+	BUILD_BUG();
+}
+#endif
+
+static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
+					    struct collapse_control *cc)
+	__releases(&khugepaged_mm_lock)
+	__acquires(&khugepaged_mm_lock)
+{
+	struct vma_iterator vmi;
+	struct khugepaged_mm_slot *mm_slot;
+	struct mm_slot *slot;
+	struct mm_struct *mm;
+	struct vm_area_struct *vma;
+	int progress = 0;
+
+	VM_BUG_ON(!pages);
+	lockdep_assert_held(&khugepaged_mm_lock);
+	*result = SCAN_FAIL;
+
+	if (khugepaged_scan.mm_slot) {
+		mm_slot = khugepaged_scan.mm_slot;
+		slot = &mm_slot->slot;
+	} else {
+		slot = list_entry(khugepaged_scan.mm_head.next,
+				     struct mm_slot, mm_node);
+		mm_slot = mm_slot_entry(slot, struct khugepaged_mm_slot, slot);
+		khugepaged_scan.address = 0;
+		khugepaged_scan.mm_slot = mm_slot;
+	}
+	spin_unlock(&khugepaged_mm_lock);
+
+	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;
+
+	progress++;
+	if (unlikely(hpage_collapse_test_exit(mm)))
+		goto breakouterloop;
+
+	vma_iter_init(&vmi, mm, khugepaged_scan.address);
+	for_each_vma(vmi, vma) {
+		unsigned long hstart, hend;
+
+		cond_resched();
+		if (unlikely(hpage_collapse_test_exit(mm))) {
+			progress++;
+			break;
+		}
+		if (!hugepage_vma_check(vma, vma->vm_flags, false, false, true)) {
+skip:
+			progress++;
+			continue;
+		}
+		hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
+		hend = round_down(vma->vm_end, HPAGE_PMD_SIZE);
+		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);
+
+		while (khugepaged_scan.address < hend) {
+			bool mmap_locked = true;
+
+			cond_resched();
+			if (unlikely(hpage_collapse_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);
+				mmap_locked = false;
+				*result = hpage_collapse_scan_file(mm,
+					khugepaged_scan.address, file, pgoff, cc);
+				fput(file);
+				if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
+					mmap_read_lock(mm);
+					if (hpage_collapse_test_exit(mm))
+						goto breakouterloop;
+					*result = collapse_pte_mapped_thp(mm,
+						khugepaged_scan.address, false);
+					if (*result == SCAN_PMD_MAPPED)
+						*result = SCAN_SUCCEED;
+					mmap_read_unlock(mm);
+				}
+			} else {
+				*result = hpage_collapse_scan_pmd(mm, vma,
+					khugepaged_scan.address, &mmap_locked, cc);
+			}
+
+			if (*result == SCAN_SUCCEED)
+				++khugepaged_pages_collapsed;
+
+			/* move to next address */
+			khugepaged_scan.address += HPAGE_PMD_SIZE;
+			progress += HPAGE_PMD_NR;
+			if (!mmap_locked)
+				/*
+				 * We released mmap_lock so break loop.  Note
+				 * that we drop mmap_lock before all hugepage
+				 * allocations, so if allocation fails, we are
+				 * guaranteed to break here and report the
+				 * correct result back to caller.
+				 */
+				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 (hpage_collapse_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 (slot->mm_node.next != &khugepaged_scan.mm_head) {
+			slot = list_entry(slot->mm_node.next,
+					  struct mm_slot, mm_node);
+			khugepaged_scan.mm_slot =
+				mm_slot_entry(slot, struct khugepaged_mm_slot, slot);
+			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) &&
+		hugepage_flags_enabled();
+}
+
+static int khugepaged_wait_event(void)
+{
+	return !list_empty(&khugepaged_scan.mm_head) ||
+		kthread_should_stop();
+}
+
+static void khugepaged_do_scan(struct collapse_control *cc)
+{
+	unsigned int progress = 0, pass_through_head = 0;
+	unsigned int pages = READ_ONCE(khugepaged_pages_to_scan);
+	bool wait = true;
+	int result = SCAN_SUCCEED;
+
+	lru_add_drain_all();
+
+	while (true) {
+		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,
+							    &result, cc);
+		else
+			progress = pages;
+		spin_unlock(&khugepaged_mm_lock);
+
+		if (progress >= pages)
+			break;
+
+		if (result == SCAN_ALLOC_HUGE_PAGE_FAIL) {
+			/*
+			 * If fail to allocate the first time, try to sleep for
+			 * a while.  When hit again, cancel the scan.
+			 */
+			if (!wait)
+				break;
+			wait = false;
+			khugepaged_alloc_sleep();
+		}
+	}
+}
+
+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 (hugepage_flags_enabled())
+		wait_event_freezable(khugepaged_wait, khugepaged_wait_event());
+}
+
+static int khugepaged(void *none)
+{
+	struct khugepaged_mm_slot *mm_slot;
+
+	set_freezable();
+	set_user_nice(current, MAX_NICE);
+
+	while (!kthread_should_stop()) {
+		khugepaged_do_scan(&khugepaged_collapse_control);
+		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;
+
+	if (!hugepage_flags_enabled()) {
+		calculate_min_free_kbytes();
+		goto update_wmarks;
+	}
+
+	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;
+	}
+
+update_wmarks:
+	setup_per_zone_wmarks();
+}
+
+int start_stop_khugepaged(void)
+{
+	int err = 0;
+
+	mutex_lock(&khugepaged_mutex);
+	if (hugepage_flags_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);
+	} else if (khugepaged_thread) {
+		kthread_stop(khugepaged_thread);
+		khugepaged_thread = NULL;
+	}
+	set_recommended_min_free_kbytes();
+fail:
+	mutex_unlock(&khugepaged_mutex);
+	return err;
+}
+
+void khugepaged_min_free_kbytes_update(void)
+{
+	mutex_lock(&khugepaged_mutex);
+	if (hugepage_flags_enabled() && khugepaged_thread)
+		set_recommended_min_free_kbytes();
+	mutex_unlock(&khugepaged_mutex);
+}
+
+bool current_is_khugepaged(void)
+{
+	return kthread_func(current) == khugepaged;
+}
+
+static int madvise_collapse_errno(enum scan_result r)
+{
+	/*
+	 * MADV_COLLAPSE breaks from existing madvise(2) conventions to provide
+	 * actionable feedback to caller, so they may take an appropriate
+	 * fallback measure depending on the nature of the failure.
+	 */
+	switch (r) {
+	case SCAN_ALLOC_HUGE_PAGE_FAIL:
+		return -ENOMEM;
+	case SCAN_CGROUP_CHARGE_FAIL:
+	case SCAN_EXCEED_NONE_PTE:
+		return -EBUSY;
+	/* Resource temporary unavailable - trying again might succeed */
+	case SCAN_PAGE_COUNT:
+	case SCAN_PAGE_LOCK:
+	case SCAN_PAGE_LRU:
+	case SCAN_DEL_PAGE_LRU:
+	case SCAN_PAGE_FILLED:
+		return -EAGAIN;
+	/*
+	 * Other: Trying again likely not to succeed / error intrinsic to
+	 * specified memory range. khugepaged likely won't be able to collapse
+	 * either.
+	 */
+	default:
+		return -EINVAL;
+	}
+}
+
+int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
+		     unsigned long start, unsigned long end)
+{
+	struct collapse_control *cc;
+	struct mm_struct *mm = vma->vm_mm;
+	unsigned long hstart, hend, addr;
+	int thps = 0, last_fail = SCAN_FAIL;
+	bool mmap_locked = true;
+
+	BUG_ON(vma->vm_start > start);
+	BUG_ON(vma->vm_end < end);
+
+	*prev = vma;
+
+	if (!hugepage_vma_check(vma, vma->vm_flags, false, false, false))
+		return -EINVAL;
+
+	cc = kmalloc(sizeof(*cc), GFP_KERNEL);
+	if (!cc)
+		return -ENOMEM;
+	cc->is_khugepaged = false;
+
+	mmgrab(mm);
+	lru_add_drain_all();
+
+	hstart = (start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
+	hend = end & HPAGE_PMD_MASK;
+
+	for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
+		int result = SCAN_FAIL;
+
+		if (!mmap_locked) {
+			cond_resched();
+			mmap_read_lock(mm);
+			mmap_locked = true;
+			result = hugepage_vma_revalidate(mm, addr, false, &vma,
+							 cc);
+			if (result  != SCAN_SUCCEED) {
+				last_fail = result;
+				goto out_nolock;
+			}
+
+			hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
+		}
+		mmap_assert_locked(mm);
+		memset(cc->node_load, 0, sizeof(cc->node_load));
+		nodes_clear(cc->alloc_nmask);
+		if (IS_ENABLED(CONFIG_SHMEM) && vma->vm_file) {
+			struct file *file = get_file(vma->vm_file);
+			pgoff_t pgoff = linear_page_index(vma, addr);
+
+			mmap_read_unlock(mm);
+			mmap_locked = false;
+			result = hpage_collapse_scan_file(mm, addr, file, pgoff,
+							  cc);
+			fput(file);
+		} else {
+			result = hpage_collapse_scan_pmd(mm, vma, addr,
+							 &mmap_locked, cc);
+		}
+		if (!mmap_locked)
+			*prev = NULL;  /* Tell caller we dropped mmap_lock */
+
+handle_result:
+		switch (result) {
+		case SCAN_SUCCEED:
+		case SCAN_PMD_MAPPED:
+			++thps;
+			break;
+		case SCAN_PTE_MAPPED_HUGEPAGE:
+			BUG_ON(mmap_locked);
+			BUG_ON(*prev);
+			mmap_read_lock(mm);
+			result = collapse_pte_mapped_thp(mm, addr, true);
+			mmap_read_unlock(mm);
+			goto handle_result;
+		/* Whitelisted set of results where continuing OK */
+		case SCAN_PMD_NULL:
+		case SCAN_PTE_NON_PRESENT:
+		case SCAN_PTE_UFFD_WP:
+		case SCAN_PAGE_RO:
+		case SCAN_LACK_REFERENCED_PAGE:
+		case SCAN_PAGE_NULL:
+		case SCAN_PAGE_COUNT:
+		case SCAN_PAGE_LOCK:
+		case SCAN_PAGE_COMPOUND:
+		case SCAN_PAGE_LRU:
+		case SCAN_DEL_PAGE_LRU:
+			last_fail = result;
+			break;
+		default:
+			last_fail = result;
+			/* Other error, exit */
+			goto out_maybelock;
+		}
+	}
+
+out_maybelock:
+	/* Caller expects us to hold mmap_lock on return */
+	if (!mmap_locked)
+		mmap_read_lock(mm);
+out_nolock:
+	mmap_assert_locked(mm);
+	mmdrop(mm);
+	kfree(cc);
+
+	return thps == ((hend - hstart) >> HPAGE_PMD_SHIFT) ? 0
+			: madvise_collapse_errno(last_fail);
+}