scripts/objdump-func: Support multiple functions

[thirdparty/linux.git] / mm / khugepaged.c

// 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 <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,
};

#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 __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 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
 * @nr_pte_mapped_thp: number of pte mapped THP
 * @pte_mapped_thp: address array corresponding pte mapped THP
 */
struct khugepaged_mm_slot {
        struct mm_slot slot;

        /* 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 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;

        mm_slot = mm_slot_alloc(mm_slot_cache);
        if (!mm_slot)
                return;

        slot = &mm_slot->slot;

        /* __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))) {
                mm_slot_free(mm_slot_cache, mm_slot);
                return;
        }

        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 = *_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 = *_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;
                }
                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(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);
                                ptep_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);
                        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 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;
        }

        prep_transhuge_page(*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;
}

/*
 * See pmd_trans_unstable() for how the result may change out from
 * underneath us, even if we hold mmap_lock in read.
 */
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);

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
        /* See comments in pmd_none_or_trans_huge_or_clear_bad() */
        barrier();
#endif
        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.
 * Note that if false is returned, mmap_lock will be 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);

        for (address = haddr; address < end; address += PAGE_SIZE) {
                struct vm_fault vmf = {
                        .vma = vma,
                        .address = address,
                        .pgoff = linear_page_index(vma, haddr),
                        .flags = FAULT_FLAG_ALLOW_RETRY,
                        .pmd = pmd,
                };

                vmf.pte = pte_offset_map(pmd, address);
                vmf.orig_pte = *vmf.pte;
                if (!is_swap_pte(vmf.orig_pte)) {
                        pte_unmap(vmf.pte);
                        continue;
                }
                ret = do_swap_page(&vmf);

                /*
                 * 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) {
                        trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
                        /* Likely, but not guaranteed, that page lock failed */
                        return SCAN_PAGE_LOCK;
                }
                if (ret & VM_FAULT_ERROR) {
                        mmap_read_unlock(mm);
                        trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
                        return SCAN_FAIL;
                }
                swapped_in++;
        }

        /* 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 SCAN_SUCCEED;
}

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);

        if (!hpage_collapse_alloc_page(hpage, gfp, node, &cc->alloc_nmask))
                return SCAN_ALLOC_HUGE_PAGE_FAIL;
        if (unlikely(mem_cgroup_charge(page_folio(*hpage), mm, gfp)))
                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;

        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);

        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);
        result =  __collapse_huge_page_isolate(vma, address, pte, cc,
                                               &compound_pagelist);
        spin_unlock(pte_ptl);

        if (unlikely(result != SCAN_SUCCEED)) {
                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);

        __collapse_huge_page_copy(pte, hpage, vma, address, pte_ptl,
                                  &compound_pagelist);
        pte_unmap(pte);
        /*
         * 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) {
                mem_cgroup_uncharge(page_folio(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);
        for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR;
             _pte++, _address += PAGE_SIZE) {
                pte_t pteval = *_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(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
/*
 * Notify khugepaged that given addr of the mm is pte-mapped THP. Then
 * khugepaged should try to collapse the page table.
 *
 * Note that following race exists:
 * (1) khugepaged calls khugepaged_collapse_pte_mapped_thps() for mm_struct A,
 *     emptying the A's ->pte_mapped_thp[] array.
 * (2) MADV_COLLAPSE collapses some file extent with target mm_struct B, and
 *     retract_page_tables() finds a VMA in mm_struct A mapping the same extent
 *     (at virtual address X) and adds an entry (for X) into mm_struct A's
 *     ->pte-mapped_thp[] array.
 * (3) khugepaged calls khugepaged_collapse_scan_file() for mm_struct A at X,
 *     sees a pte-mapped THP (SCAN_PTE_MAPPED_HUGEPAGE) and adds an entry
 *     (for X) into mm_struct A's ->pte-mapped_thp[] array.
 * Thus, it's possible the same address is added multiple times for the same
 * mm_struct.  Should this happen, we'll simply attempt
 * collapse_pte_mapped_thp() multiple times for the same address, under the same
 * exclusive mmap_lock, and assuming the first call is successful, subsequent
 * attempts will return quickly (without grabbing any additional locks) when
 * a huge pmd is found in find_pmd_or_thp_or_none().  Since this is a cheap
 * check, and since this is a rare occurrence, the cost of preventing this
 * "multiple-add" is thought to be more expensive than just handling it, should
 * it occur.
 */
static bool khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
                                          unsigned long addr)
{
        struct khugepaged_mm_slot *mm_slot;
        struct mm_slot *slot;
        bool ret = false;

        VM_BUG_ON(addr & ~HPAGE_PMD_MASK);

        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 (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;
                ret = true;
        }
        spin_unlock(&khugepaged_mm_lock);
        return ret;
}

/* hpage must be locked, and mmap_lock must be held in write */
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_write_locked(vma->vm_mm);

        if (do_set_pmd(&vmf, hpage))
                return SCAN_FAIL;

        get_page(hpage);
        return SCAN_SUCCEED;
}

/*
 * A note about locking:
 * Trying to take the page table spinlocks would be useless here because those
 * are only used to synchronize:
 *
 *  - modifying terminal entries (ones that point to a data page, not to another
 *    page table)
 *  - installing *new* non-terminal entries
 *
 * Instead, we need roughly the same kind of protection as free_pgtables() or
 * mm_take_all_locks() (but only for a single VMA):
 * The mmap lock together with this VMA's rmap locks covers all paths towards
 * the page table entries we're messing with here, except for hardware page
 * table walks and lockless_pages_from_mm().
 */
static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
                                  unsigned long addr, pmd_t *pmdp)
{
        pmd_t pmd;
        struct mmu_notifier_range range;

        mmap_assert_write_locked(mm);
        if (vma->vm_file)
                lockdep_assert_held_write(&vma->vm_file->f_mapping->i_mmap_rwsem);
        /*
         * All anon_vmas attached to the VMA have the same root and are
         * therefore locked by the same lock.
         */
        if (vma->anon_vma)
                lockdep_assert_held_write(&vma->anon_vma->root->rwsem);

        mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, addr,
                                addr + HPAGE_PMD_SIZE);
        mmu_notifier_invalidate_range_start(&range);
        pmd = pmdp_collapse_flush(vma, addr, pmdp);
        tlb_remove_table_sync_one();
        mmu_notifier_invalidate_range_end(&range);
        mm_dec_nr_ptes(mm);
        page_table_check_pte_clear_range(mm, addr, pmd);
        pte_free(mm, pmd_pgtable(pmd));
}

/**
 * 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)
{
        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;
        spinlock_t *ptl;
        int count = 0, result = SCAN_FAIL;
        int i;

        mmap_assert_write_locked(mm);

        /* 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 (!vma || !vma->vm_file ||
            !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE))
                return SCAN_VMA_CHECK;

        /*
         * 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;
        }

        switch (result) {
        case SCAN_SUCCEED:
                break;
        case SCAN_PMD_NONE:
                /*
                 * In MADV_COLLAPSE path, possible race with khugepaged where
                 * all pte entries have been removed and pmd cleared.  If so,
                 * skip all the pte checks and just update the pmd mapping.
                 */
                goto maybe_install_pmd;
        default:
                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.
         * See collapse_and_free_pmd().
         */
        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);
        result = SCAN_FAIL;

        /* 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)) {
                        result = SCAN_PTE_NON_PRESENT;
                        goto abort;
                }

                page = vm_normal_page(vma, addr, *pte);
                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;
                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);
                if (WARN_ON_ONCE(page && is_zone_device_page(page)))
                        goto abort;
                page_remove_rmap(page, vma, 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: remove pte entries */
        /* we make no change to anon, but protect concurrent anon page lookup */
        if (vma->anon_vma)
                anon_vma_lock_write(vma->anon_vma);

        collapse_and_free_pmd(mm, vma, haddr, pmd);

        if (vma->anon_vma)
                anon_vma_unlock_write(vma->anon_vma);
        i_mmap_unlock_write(vma->vm_file->f_mapping);

maybe_install_pmd:
        /* step 5: install pmd entry */
        result = install_pmd
                        ? set_huge_pmd(vma, haddr, pmd, hpage)
                        : SCAN_SUCCEED;

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

abort:
        pte_unmap_unlock(start_pte, ptl);
        i_mmap_unlock_write(vma->vm_file->f_mapping);
        goto drop_hpage;
}

static void khugepaged_collapse_pte_mapped_thps(struct khugepaged_mm_slot *mm_slot)
{
        struct mm_slot *slot = &mm_slot->slot;
        struct mm_struct *mm = slot->mm;
        int i;

        if (likely(mm_slot->nr_pte_mapped_thp == 0))
                return;

        if (!mmap_write_trylock(mm))
                return;

        if (unlikely(hpage_collapse_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], false);

out:
        mm_slot->nr_pte_mapped_thp = 0;
        mmap_write_unlock(mm);
}

static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff,
                               struct mm_struct *target_mm,
                               unsigned long target_addr, struct page *hpage,
                               struct collapse_control *cc)
{
        struct vm_area_struct *vma;
        int target_result = SCAN_FAIL;

        i_mmap_lock_write(mapping);
        vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
                int result = SCAN_FAIL;
                struct mm_struct *mm = NULL;
                unsigned long addr = 0;
                pmd_t *pmd;
                bool is_target = false;

                /*
                 * 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.
                 *
                 * Note 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 (READ_ONCE(vma->anon_vma)) {
                        result = SCAN_PAGE_ANON;
                        goto next;
                }
                addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
                if (addr & ~HPAGE_PMD_MASK ||
                    vma->vm_end < addr + HPAGE_PMD_SIZE) {
                        result = SCAN_VMA_CHECK;
                        goto next;
                }
                mm = vma->vm_mm;
                is_target = mm == target_mm && addr == target_addr;
                result = find_pmd_or_thp_or_none(mm, addr, &pmd);
                if (result != SCAN_SUCCEED)
                        goto next;
                /*
                 * 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.
                 *
                 * Also, it's not MADV_COLLAPSE's job to collapse other
                 * mappings - let khugepaged take care of them later.
                 */
                result = SCAN_PTE_MAPPED_HUGEPAGE;
                if ((cc->is_khugepaged || is_target) &&
                    mmap_write_trylock(mm)) {
                        /*
                         * Re-check whether we have an ->anon_vma, because
                         * collapse_and_free_pmd() requires that either no
                         * ->anon_vma exists or the anon_vma is locked.
                         * We already checked ->anon_vma above, but that check
                         * is racy because ->anon_vma can be populated under the
                         * mmap lock in read mode.
                         */
                        if (vma->anon_vma) {
                                result = SCAN_PAGE_ANON;
                                goto unlock_next;
                        }
                        /*
                         * When a vma is registered with uffd-wp, we can't
                         * recycle the pmd pgtable because there can be pte
                         * markers installed.  Skip it only, so the rest mm/vma
                         * can still have the same file mapped hugely, however
                         * it'll always mapped in small page size for uffd-wp
                         * registered ranges.
                         */
                        if (hpage_collapse_test_exit(mm)) {
                                result = SCAN_ANY_PROCESS;
                                goto unlock_next;
                        }
                        if (userfaultfd_wp(vma)) {
                                result = SCAN_PTE_UFFD_WP;
                                goto unlock_next;
                        }
                        collapse_and_free_pmd(mm, vma, addr, pmd);
                        if (!cc->is_khugepaged && is_target)
                                result = set_huge_pmd(vma, addr, pmd, hpage);
                        else
                                result = SCAN_SUCCEED;

unlock_next:
                        mmap_write_unlock(mm);
                        goto next;
                }
                /*
                 * Calling context will handle target mm/addr. Otherwise, let
                 * khugepaged try again later.
                 */
                if (!is_target) {
                        khugepaged_add_pte_mapped_thp(mm, addr);
                        continue;
                }
next:
                if (is_target)
                        target_result = result;
        }
        i_mmap_unlock_write(mapping);
        return target_result;
}

/**
 * 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 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 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;
        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;

        /*
         * 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 out;
                }
        } while (1);

        __SetPageLocked(hpage);
        if (is_shmem)
                __SetPageSwapBacked(hpage);
        hpage->index = start;
        hpage->mapping = mapping;

        /*
         * At this point the hpage 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);
                struct folio *folio;

                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, hpage);
                                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;
                                }
                                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 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.
                 * 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 (folio_has_private(folio) &&
                    !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);
                xas_set(&xas, index);

                VM_BUG_ON_PAGE(page != xas_load(&xas), 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, hpage);
                continue;
out_unlock:
                unlock_page(page);
                put_page(page);
                goto xa_unlocked;
        }
        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);
                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;
                        __mod_lruvec_page_state(hpage, NR_FILE_THPS, -nr);
                        filemap_nr_thps_dec(mapping);
                        goto xa_locked;
                }
        }

        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);
        }

        /* Join all the small entries into a single multi-index entry */
        xas_set_order(&xas, start, HPAGE_PMD_ORDER);
        xas_store(&xas, hpage);
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) {
                struct page *page, *tmp;
                struct folio *folio;

                /*
                 * 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(hpage + (index % HPAGE_PMD_NR));
                                index++;
                        }
                        copy_highpage(hpage + (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(hpage + (index % HPAGE_PMD_NR));
                        index++;
                }

                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);

                /*
                 * Remove pte page tables, so we can re-fault the page as huge.
                 */
                result = retract_page_tables(mapping, start, mm, addr, hpage,
                                             cc);
                unlock_page(hpage);
                hpage = NULL;
        } else {
                struct page *page;

                /* Something went wrong: roll back page cache changes */
                xas_lock_irq(&xas);
                if (nr_none) {
                        mapping->nrpages -= nr_none;
                        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);

                hpage->mapping = NULL;
        }

        if (hpage)
                unlock_page(hpage);
out:
        VM_BUG_ON(!list_empty(&pagelist));
        if (hpage) {
                mem_cgroup_uncharge(page_folio(hpage));
                put_page(hpage);
        }

        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();
}

static void khugepaged_collapse_pte_mapped_thps(struct khugepaged_mm_slot *mm_slot)
{
}

static bool khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
                                          unsigned long addr)
{
        return false;
}
#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);
        khugepaged_collapse_pte_mapped_thps(mm_slot);

        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);
                                *result = hpage_collapse_scan_file(mm,
                                                                   khugepaged_scan.address,
                                                                   file, pgoff, cc);
                                mmap_locked = false;
                                fput(file);
                        } else {
                                *result = hpage_collapse_scan_pmd(mm, vma,
                                                                  khugepaged_scan.address,
                                                                  &mmap_locked,
                                                                  cc);
                        }
                        switch (*result) {
                        case SCAN_PTE_MAPPED_HUGEPAGE: {
                                pmd_t *pmd;

                                *result = find_pmd_or_thp_or_none(mm,
                                                                  khugepaged_scan.address,
                                                                  &pmd);
                                if (*result != SCAN_SUCCEED)
                                        break;
                                if (!khugepaged_add_pte_mapped_thp(mm,
                                                                   khugepaged_scan.address))
                                        break;
                        } fallthrough;
                        case SCAN_SUCCEED:
                                ++khugepaged_pages_collapsed;
                                break;
                        default:
                                break;
                        }

                        /* 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:
                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:
                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_write_lock(mm);
                        result = collapse_pte_mapped_thp(mm, addr, true);
                        mmap_write_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);
}