Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel...

[mirror_ubuntu-focal-kernel.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/swapops.h>
#include <linux/shmem_fs.h>

#include <asm/tlb.h>
#include <asm/pgalloc.h>
#include "internal.h"

enum scan_result {
        SCAN_FAIL,
        SCAN_SUCCEED,
        SCAN_PMD_NULL,
        SCAN_EXCEED_NONE_PTE,
        SCAN_PTE_NON_PRESENT,
        SCAN_PAGE_RO,
        SCAN_LACK_REFERENCED_PAGE,
        SCAN_PAGE_NULL,
        SCAN_SCAN_ABORT,
        SCAN_PAGE_COUNT,
        SCAN_PAGE_LRU,
        SCAN_PAGE_LOCK,
        SCAN_PAGE_ANON,
        SCAN_PAGE_COMPOUND,
        SCAN_ANY_PROCESS,
        SCAN_VMA_NULL,
        SCAN_VMA_CHECK,
        SCAN_ADDRESS_RANGE,
        SCAN_SWAP_CACHE_PAGE,
        SCAN_DEL_PAGE_LRU,
        SCAN_ALLOC_HUGE_PAGE_FAIL,
        SCAN_CGROUP_CHARGE_FAIL,
        SCAN_EXCEED_SWAP_PTE,
        SCAN_TRUNCATED,
        SCAN_PAGE_HAS_PRIVATE,
};

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

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

#define MM_SLOTS_HASH_BITS 10
static __read_mostly DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);

static struct kmem_cache *mm_slot_cache __read_mostly;

#define MAX_PTE_MAPPED_THP 8

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

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

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

static struct khugepaged_scan khugepaged_scan = {
        .mm_head = LIST_HEAD_INIT(khugepaged_scan.mm_head),
};

#ifdef CONFIG_SYSFS
static ssize_t scan_sleep_millisecs_show(struct kobject *kobj,
                                         struct kobj_attribute *attr,
                                         char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_scan_sleep_millisecs);
}

static ssize_t scan_sleep_millisecs_store(struct kobject *kobj,
                                          struct kobj_attribute *attr,
                                          const char *buf, size_t count)
{
        unsigned long msecs;
        int err;

        err = kstrtoul(buf, 10, &msecs);
        if (err || msecs > UINT_MAX)
                return -EINVAL;

        khugepaged_scan_sleep_millisecs = msecs;
        khugepaged_sleep_expire = 0;
        wake_up_interruptible(&khugepaged_wait);

        return count;
}
static struct kobj_attribute scan_sleep_millisecs_attr =
        __ATTR(scan_sleep_millisecs, 0644, scan_sleep_millisecs_show,
               scan_sleep_millisecs_store);

static ssize_t alloc_sleep_millisecs_show(struct kobject *kobj,
                                          struct kobj_attribute *attr,
                                          char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_alloc_sleep_millisecs);
}

static ssize_t alloc_sleep_millisecs_store(struct kobject *kobj,
                                           struct kobj_attribute *attr,
                                           const char *buf, size_t count)
{
        unsigned long msecs;
        int err;

        err = kstrtoul(buf, 10, &msecs);
        if (err || msecs > UINT_MAX)
                return -EINVAL;

        khugepaged_alloc_sleep_millisecs = msecs;
        khugepaged_sleep_expire = 0;
        wake_up_interruptible(&khugepaged_wait);

        return count;
}
static struct kobj_attribute alloc_sleep_millisecs_attr =
        __ATTR(alloc_sleep_millisecs, 0644, alloc_sleep_millisecs_show,
               alloc_sleep_millisecs_store);

static ssize_t pages_to_scan_show(struct kobject *kobj,
                                  struct kobj_attribute *attr,
                                  char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_pages_to_scan);
}
static ssize_t pages_to_scan_store(struct kobject *kobj,
                                   struct kobj_attribute *attr,
                                   const char *buf, size_t count)
{
        int err;
        unsigned long pages;

        err = kstrtoul(buf, 10, &pages);
        if (err || !pages || pages > UINT_MAX)
                return -EINVAL;

        khugepaged_pages_to_scan = pages;

        return count;
}
static struct kobj_attribute pages_to_scan_attr =
        __ATTR(pages_to_scan, 0644, pages_to_scan_show,
               pages_to_scan_store);

static ssize_t pages_collapsed_show(struct kobject *kobj,
                                    struct kobj_attribute *attr,
                                    char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_pages_collapsed);
}
static struct kobj_attribute pages_collapsed_attr =
        __ATTR_RO(pages_collapsed);

static ssize_t full_scans_show(struct kobject *kobj,
                               struct kobj_attribute *attr,
                               char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_full_scans);
}
static struct kobj_attribute full_scans_attr =
        __ATTR_RO(full_scans);

static ssize_t khugepaged_defrag_show(struct kobject *kobj,
                                      struct kobj_attribute *attr, char *buf)
{
        return single_hugepage_flag_show(kobj, attr, buf,
                                TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG);
}
static ssize_t khugepaged_defrag_store(struct kobject *kobj,
                                       struct kobj_attribute *attr,
                                       const char *buf, size_t count)
{
        return single_hugepage_flag_store(kobj, attr, buf, count,
                                 TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG);
}
static struct kobj_attribute khugepaged_defrag_attr =
        __ATTR(defrag, 0644, khugepaged_defrag_show,
               khugepaged_defrag_store);

/*
 * max_ptes_none controls if khugepaged should collapse hugepages over
 * any unmapped ptes in turn potentially increasing the memory
 * footprint of the vmas. When max_ptes_none is 0 khugepaged will not
 * reduce the available free memory in the system as it
 * runs. Increasing max_ptes_none will instead potentially reduce the
 * free memory in the system during the khugepaged scan.
 */
static ssize_t khugepaged_max_ptes_none_show(struct kobject *kobj,
                                             struct kobj_attribute *attr,
                                             char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_max_ptes_none);
}
static ssize_t khugepaged_max_ptes_none_store(struct kobject *kobj,
                                              struct kobj_attribute *attr,
                                              const char *buf, size_t count)
{
        int err;
        unsigned long max_ptes_none;

        err = kstrtoul(buf, 10, &max_ptes_none);
        if (err || max_ptes_none > HPAGE_PMD_NR-1)
                return -EINVAL;

        khugepaged_max_ptes_none = max_ptes_none;

        return count;
}
static struct kobj_attribute khugepaged_max_ptes_none_attr =
        __ATTR(max_ptes_none, 0644, khugepaged_max_ptes_none_show,
               khugepaged_max_ptes_none_store);

static ssize_t khugepaged_max_ptes_swap_show(struct kobject *kobj,
                                             struct kobj_attribute *attr,
                                             char *buf)
{
        return sprintf(buf, "%u\n", khugepaged_max_ptes_swap);
}

static ssize_t khugepaged_max_ptes_swap_store(struct kobject *kobj,
                                              struct kobj_attribute *attr,
                                              const char *buf, size_t count)
{
        int err;
        unsigned long max_ptes_swap;

        err  = kstrtoul(buf, 10, &max_ptes_swap);
        if (err || max_ptes_swap > HPAGE_PMD_NR-1)
                return -EINVAL;

        khugepaged_max_ptes_swap = max_ptes_swap;

        return count;
}

static struct kobj_attribute khugepaged_max_ptes_swap_attr =
        __ATTR(max_ptes_swap, 0644, khugepaged_max_ptes_swap_show,
               khugepaged_max_ptes_swap_store);

static struct attribute *khugepaged_attr[] = {
        &khugepaged_defrag_attr.attr,
        &khugepaged_max_ptes_none_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,
        &khugepaged_max_ptes_swap_attr.attr,
        NULL,
};

struct attribute_group khugepaged_attr_group = {
        .attrs = khugepaged_attr,
        .name = "khugepaged",
};
#endif /* CONFIG_SYSFS */

#define VM_NO_KHUGEPAGED (VM_SPECIAL | VM_HUGETLB)

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

        return 0;
}

int __init khugepaged_init(void)
{
        mm_slot_cache = kmem_cache_create("khugepaged_mm_slot",
                                          sizeof(struct mm_slot),
                                          __alignof__(struct mm_slot), 0, NULL);
        if (!mm_slot_cache)
                return -ENOMEM;

        khugepaged_pages_to_scan = HPAGE_PMD_NR * 8;
        khugepaged_max_ptes_none = HPAGE_PMD_NR - 1;
        khugepaged_max_ptes_swap = HPAGE_PMD_NR / 8;

        return 0;
}

void __init khugepaged_destroy(void)
{
        kmem_cache_destroy(mm_slot_cache);
}

static inline struct mm_slot *alloc_mm_slot(void)
{
        if (!mm_slot_cache)     /* initialization failed */
                return NULL;
        return kmem_cache_zalloc(mm_slot_cache, GFP_KERNEL);
}

static inline void free_mm_slot(struct mm_slot *mm_slot)
{
        kmem_cache_free(mm_slot_cache, mm_slot);
}

static struct mm_slot *get_mm_slot(struct mm_struct *mm)
{
        struct mm_slot *mm_slot;

        hash_for_each_possible(mm_slots_hash, mm_slot, hash, (unsigned long)mm)
                if (mm == mm_slot->mm)
                        return mm_slot;

        return NULL;
}

static void insert_to_mm_slots_hash(struct mm_struct *mm,
                                    struct mm_slot *mm_slot)
{
        mm_slot->mm = mm;
        hash_add(mm_slots_hash, &mm_slot->hash, (long)mm);
}

static inline int khugepaged_test_exit(struct mm_struct *mm)
{
        return atomic_read(&mm->mm_users) == 0;
}

static bool hugepage_vma_check(struct vm_area_struct *vma,
                               unsigned long vm_flags)
{
        if ((!(vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
            (vm_flags & VM_NOHUGEPAGE) ||
            test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
                return false;

        if (shmem_file(vma->vm_file) ||
            (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
             vma->vm_file &&
             (vm_flags & VM_DENYWRITE))) {
                if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
                        return false;
                return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
                                HPAGE_PMD_NR);
        }
        if (!vma->anon_vma || vma->vm_ops)
                return false;
        if (is_vma_temporary_stack(vma))
                return false;
        return !(vm_flags & VM_NO_KHUGEPAGED);
}

int __khugepaged_enter(struct mm_struct *mm)
{
        struct mm_slot *mm_slot;
        int wakeup;

        mm_slot = alloc_mm_slot();
        if (!mm_slot)
                return -ENOMEM;

        /* __khugepaged_exit() must not run from under us */
        VM_BUG_ON_MM(khugepaged_test_exit(mm), mm);
        if (unlikely(test_and_set_bit(MMF_VM_HUGEPAGE, &mm->flags))) {
                free_mm_slot(mm_slot);
                return 0;
        }

        spin_lock(&khugepaged_mm_lock);
        insert_to_mm_slots_hash(mm, mm_slot);
        /*
         * Insert just behind the scanning cursor, to let the area settle
         * down a little.
         */
        wakeup = list_empty(&khugepaged_scan.mm_head);
        list_add_tail(&mm_slot->mm_node, &khugepaged_scan.mm_head);
        spin_unlock(&khugepaged_mm_lock);

        mmgrab(mm);
        if (wakeup)
                wake_up_interruptible(&khugepaged_wait);

        return 0;
}

int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
                               unsigned long vm_flags)
{
        unsigned long hstart, hend;

        /*
         * khugepaged only supports read-only files for non-shmem files.
         * khugepaged does not yet work on special mappings. And
         * file-private shmem THP is not supported.
         */
        if (!hugepage_vma_check(vma, vm_flags))
                return 0;

        hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
        hend = vma->vm_end & HPAGE_PMD_MASK;
        if (hstart < hend)
                return khugepaged_enter(vma, vm_flags);
        return 0;
}

void __khugepaged_exit(struct mm_struct *mm)
{
        struct mm_slot *mm_slot;
        int free = 0;

        spin_lock(&khugepaged_mm_lock);
        mm_slot = get_mm_slot(mm);
        if (mm_slot && khugepaged_scan.mm_slot != mm_slot) {
                hash_del(&mm_slot->hash);
                list_del(&mm_slot->mm_node);
                free = 1;
        }
        spin_unlock(&khugepaged_mm_lock);

        if (free) {
                clear_bit(MMF_VM_HUGEPAGE, &mm->flags);
                free_mm_slot(mm_slot);
                mmdrop(mm);
        } else if (mm_slot) {
                /*
                 * This is required to serialize against
                 * khugepaged_test_exit() (which is guaranteed to run
                 * under mmap sem read mode). Stop here (after we
                 * return all pagetables will be destroyed) until
                 * khugepaged has finished working on the pagetables
                 * under the mmap_sem.
                 */
                down_write(&mm->mmap_sem);
                up_write(&mm->mmap_sem);
        }
}

static void release_pte_page(struct page *page)
{
        dec_node_page_state(page, NR_ISOLATED_ANON + page_is_file_cache(page));
        unlock_page(page);
        putback_lru_page(page);
}

static void release_pte_pages(pte_t *pte, pte_t *_pte)
{
        while (--_pte >= pte) {
                pte_t pteval = *_pte;
                if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)))
                        release_pte_page(pte_page(pteval));
        }
}

static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
                                        unsigned long address,
                                        pte_t *pte)
{
        struct page *page = NULL;
        pte_t *_pte;
        int none_or_zero = 0, result = 0, referenced = 0;
        bool writable = false;

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

                /* TODO: teach khugepaged to collapse THP mapped with pte */
                if (PageCompound(page)) {
                        result = SCAN_PAGE_COMPOUND;
                        goto out;
                }

                VM_BUG_ON_PAGE(!PageAnon(page), page);

                /*
                 * 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;
                }

                /*
                 * cannot use mapcount: can't collapse if there's a gup pin.
                 * The page must only be referenced by the scanned process
                 * and page swap cache.
                 */
                if (page_count(page) != 1 + PageSwapCache(page)) {
                        unlock_page(page);
                        result = SCAN_PAGE_COUNT;
                        goto out;
                }
                if (pte_write(pteval)) {
                        writable = true;
                } else {
                        if (PageSwapCache(page) &&
                            !reuse_swap_page(page, NULL)) {
                                unlock_page(page);
                                result = SCAN_SWAP_CACHE_PAGE;
                                goto out;
                        }
                        /*
                         * Page is not in the swap cache. It can be collapsed
                         * into a THP.
                         */
                }

                /*
                 * 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;
                }
                inc_node_page_state(page,
                                NR_ISOLATED_ANON + page_is_file_cache(page));
                VM_BUG_ON_PAGE(!PageLocked(page), page);
                VM_BUG_ON_PAGE(PageLRU(page), page);

                /* There should be enough young pte to collapse the page */
                if (pte_young(pteval) ||
                    page_is_young(page) || PageReferenced(page) ||
                    mmu_notifier_test_young(vma->vm_mm, address))
                        referenced++;
        }
        if (likely(writable)) {
                if (likely(referenced)) {
                        result = SCAN_SUCCEED;
                        trace_mm_collapse_huge_page_isolate(page, none_or_zero,
                                                            referenced, writable, result);
                        return 1;
                }
        } else {
                result = SCAN_PAGE_RO;
        }

out:
        release_pte_pages(pte, _pte);
        trace_mm_collapse_huge_page_isolate(page, none_or_zero,
                                            referenced, writable, result);
        return 0;
}

static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
                                      struct vm_area_struct *vma,
                                      unsigned long address,
                                      spinlock_t *ptl)
{
        pte_t *_pte;
        for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
                                _pte++, page++, address += PAGE_SIZE) {
                pte_t pteval = *_pte;
                struct page *src_page;

                if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
                        clear_user_highpage(page, address);
                        add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
                        if (is_zero_pfn(pte_pfn(pteval))) {
                                /*
                                 * ptl mostly unnecessary.
                                 */
                                spin_lock(ptl);
                                /*
                                 * paravirt calls inside pte_clear here are
                                 * superfluous.
                                 */
                                pte_clear(vma->vm_mm, address, _pte);
                                spin_unlock(ptl);
                        }
                } else {
                        src_page = pte_page(pteval);
                        copy_user_highpage(page, src_page, address, vma);
                        VM_BUG_ON_PAGE(page_mapcount(src_page) != 1, src_page);
                        release_pte_page(src_page);
                        /*
                         * ptl mostly unnecessary, but preempt has to
                         * be disabled to update the per-cpu stats
                         * inside page_remove_rmap().
                         */
                        spin_lock(ptl);
                        /*
                         * paravirt calls inside pte_clear here are
                         * superfluous.
                         */
                        pte_clear(vma->vm_mm, address, _pte);
                        page_remove_rmap(src_page, false);
                        spin_unlock(ptl);
                        free_page_and_swap_cache(src_page);
                }
        }
}

static void khugepaged_alloc_sleep(void)
{
        DEFINE_WAIT(wait);

        add_wait_queue(&khugepaged_wait, &wait);
        freezable_schedule_timeout_interruptible(
                msecs_to_jiffies(khugepaged_alloc_sleep_millisecs));
        remove_wait_queue(&khugepaged_wait, &wait);
}

static int khugepaged_node_load[MAX_NUMNODES];

static bool khugepaged_scan_abort(int nid)
{
        int i;

        /*
         * If node_reclaim_mode is disabled, then no extra effort is made to
         * allocate memory locally.
         */
        if (!node_reclaim_mode)
                return false;

        /* If there is a count for this node already, it must be acceptable */
        if (khugepaged_node_load[nid])
                return false;

        for (i = 0; i < MAX_NUMNODES; i++) {
                if (!khugepaged_node_load[i])
                        continue;
                if (node_distance(nid, i) > node_reclaim_distance)
                        return true;
        }
        return false;
}

/* Defrag for khugepaged will enter direct reclaim/compaction if necessary */
static inline gfp_t alloc_hugepage_khugepaged_gfpmask(void)
{
        return khugepaged_defrag() ? GFP_TRANSHUGE : GFP_TRANSHUGE_LIGHT;
}

#ifdef CONFIG_NUMA
static int khugepaged_find_target_node(void)
{
        static int last_khugepaged_target_node = NUMA_NO_NODE;
        int nid, target_node = 0, max_value = 0;

        /* find first node with max normal pages hit */
        for (nid = 0; nid < MAX_NUMNODES; nid++)
                if (khugepaged_node_load[nid] > max_value) {
                        max_value = khugepaged_node_load[nid];
                        target_node = nid;
                }

        /* do some balance if several nodes have the same hit record */
        if (target_node <= last_khugepaged_target_node)
                for (nid = last_khugepaged_target_node + 1; nid < MAX_NUMNODES;
                                nid++)
                        if (max_value == khugepaged_node_load[nid]) {
                                target_node = nid;
                                break;
                        }

        last_khugepaged_target_node = target_node;
        return target_node;
}

static bool khugepaged_prealloc_page(struct page **hpage, bool *wait)
{
        if (IS_ERR(*hpage)) {
                if (!*wait)
                        return false;

                *wait = false;
                *hpage = NULL;
                khugepaged_alloc_sleep();
        } else if (*hpage) {
                put_page(*hpage);
                *hpage = NULL;
        }

        return true;
}

static struct page *
khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)
{
        VM_BUG_ON_PAGE(*hpage, *hpage);

        *hpage = __alloc_pages_node(node, gfp, HPAGE_PMD_ORDER);
        if (unlikely(!*hpage)) {
                count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
                *hpage = ERR_PTR(-ENOMEM);
                return NULL;
        }

        prep_transhuge_page(*hpage);
        count_vm_event(THP_COLLAPSE_ALLOC);
        return *hpage;
}
#else
static int khugepaged_find_target_node(void)
{
        return 0;
}

static inline struct page *alloc_khugepaged_hugepage(void)
{
        struct page *page;

        page = alloc_pages(alloc_hugepage_khugepaged_gfpmask(),
                           HPAGE_PMD_ORDER);
        if (page)
                prep_transhuge_page(page);
        return page;
}

static struct page *khugepaged_alloc_hugepage(bool *wait)
{
        struct page *hpage;

        do {
                hpage = alloc_khugepaged_hugepage();
                if (!hpage) {
                        count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
                        if (!*wait)
                                return NULL;

                        *wait = false;
                        khugepaged_alloc_sleep();
                } else
                        count_vm_event(THP_COLLAPSE_ALLOC);
        } while (unlikely(!hpage) && likely(khugepaged_enabled()));

        return hpage;
}

static bool khugepaged_prealloc_page(struct page **hpage, bool *wait)
{
        if (!*hpage)
                *hpage = khugepaged_alloc_hugepage(wait);

        if (unlikely(!*hpage))
                return false;

        return true;
}

static struct page *
khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)
{
        VM_BUG_ON(!*hpage);

        return  *hpage;
}
#endif

/*
 * If mmap_sem temporarily dropped, revalidate vma
 * before taking mmap_sem.
 * Return 0 if succeeds, otherwise return none-zero
 * value (scan code).
 */

static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
                struct vm_area_struct **vmap)
{
        struct vm_area_struct *vma;
        unsigned long hstart, hend;

        if (unlikely(khugepaged_test_exit(mm)))
                return SCAN_ANY_PROCESS;

        *vmap = vma = find_vma(mm, address);
        if (!vma)
                return SCAN_VMA_NULL;

        hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
        hend = vma->vm_end & HPAGE_PMD_MASK;
        if (address < hstart || address + HPAGE_PMD_SIZE > hend)
                return SCAN_ADDRESS_RANGE;
        if (!hugepage_vma_check(vma, vma->vm_flags))
                return SCAN_VMA_CHECK;
        return 0;
}

/*
 * Bring missing pages in from swap, to complete THP collapse.
 * Only done if khugepaged_scan_pmd believes it is worthwhile.
 *
 * Called and returns without pte mapped or spinlocks held,
 * but with mmap_sem held to protect against vma changes.
 */

static bool __collapse_huge_page_swapin(struct mm_struct *mm,
                                        struct vm_area_struct *vma,
                                        unsigned long address, pmd_t *pmd,
                                        int referenced)
{
        int swapped_in = 0;
        vm_fault_t ret = 0;
        struct vm_fault vmf = {
                .vma = vma,
                .address = address,
                .flags = FAULT_FLAG_ALLOW_RETRY,
                .pmd = pmd,
                .pgoff = linear_page_index(vma, address),
        };

        /* we only decide to swapin, if there is enough young ptes */
        if (referenced < HPAGE_PMD_NR/2) {
                trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
                return false;
        }
        vmf.pte = pte_offset_map(pmd, address);
        for (; vmf.address < address + HPAGE_PMD_NR*PAGE_SIZE;
                        vmf.pte++, vmf.address += PAGE_SIZE) {
                vmf.orig_pte = *vmf.pte;
                if (!is_swap_pte(vmf.orig_pte))
                        continue;
                swapped_in++;
                ret = do_swap_page(&vmf);

                /* do_swap_page returns VM_FAULT_RETRY with released mmap_sem */
                if (ret & VM_FAULT_RETRY) {
                        down_read(&mm->mmap_sem);
                        if (hugepage_vma_revalidate(mm, address, &vmf.vma)) {
                                /* vma is no longer available, don't continue to swapin */
                                trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
                                return false;
                        }
                        /* check if the pmd is still valid */
                        if (mm_find_pmd(mm, address) != pmd) {
                                trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
                                return false;
                        }
                }
                if (ret & VM_FAULT_ERROR) {
                        trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
                        return false;
                }
                /* pte is unmapped now, we need to map it */
                vmf.pte = pte_offset_map(pmd, vmf.address);
        }
        vmf.pte--;
        pte_unmap(vmf.pte);
        trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 1);
        return true;
}

static void collapse_huge_page(struct mm_struct *mm,
                                   unsigned long address,
                                   struct page **hpage,
                                   int node, int referenced)
{
        pmd_t *pmd, _pmd;
        pte_t *pte;
        pgtable_t pgtable;
        struct page *new_page;
        spinlock_t *pmd_ptl, *pte_ptl;
        int isolated = 0, result = 0;
        struct mem_cgroup *memcg;
        struct vm_area_struct *vma;
        struct mmu_notifier_range range;
        gfp_t gfp;

        VM_BUG_ON(address & ~HPAGE_PMD_MASK);

        /* Only allocate from the target node */
        gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;

        /*
         * Before allocating the hugepage, release the mmap_sem read lock.
         * The allocation can take potentially a long time if it involves
         * sync compaction, and we do not need to hold the mmap_sem during
         * that. We will recheck the vma after taking it again in write mode.
         */
        up_read(&mm->mmap_sem);
        new_page = khugepaged_alloc_page(hpage, gfp, node);
        if (!new_page) {
                result = SCAN_ALLOC_HUGE_PAGE_FAIL;
                goto out_nolock;
        }

        if (unlikely(mem_cgroup_try_charge(new_page, mm, gfp, &memcg, true))) {
                result = SCAN_CGROUP_CHARGE_FAIL;
                goto out_nolock;
        }

        down_read(&mm->mmap_sem);
        result = hugepage_vma_revalidate(mm, address, &vma);
        if (result) {
                mem_cgroup_cancel_charge(new_page, memcg, true);
                up_read(&mm->mmap_sem);
                goto out_nolock;
        }

        pmd = mm_find_pmd(mm, address);
        if (!pmd) {
                result = SCAN_PMD_NULL;
                mem_cgroup_cancel_charge(new_page, memcg, true);
                up_read(&mm->mmap_sem);
                goto out_nolock;
        }

        /*
         * __collapse_huge_page_swapin always returns with mmap_sem locked.
         * If it fails, we release mmap_sem and jump out_nolock.
         * Continuing to collapse causes inconsistency.
         */
        if (!__collapse_huge_page_swapin(mm, vma, address, pmd, referenced)) {
                mem_cgroup_cancel_charge(new_page, memcg, true);
                up_read(&mm->mmap_sem);
                goto out_nolock;
        }

        up_read(&mm->mmap_sem);
        /*
         * 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.
         */
        down_write(&mm->mmap_sem);
        result = SCAN_ANY_PROCESS;
        if (!mmget_still_valid(mm))
                goto out;
        result = hugepage_vma_revalidate(mm, address, &vma);
        if (result)
                goto out;
        /* check if the pmd is still valid */
        if (mm_find_pmd(mm, address) != pmd)
                goto out;

        anon_vma_lock_write(vma->anon_vma);

        mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, NULL, mm,
                                address, address + HPAGE_PMD_SIZE);
        mmu_notifier_invalidate_range_start(&range);

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

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

        spin_lock(pte_ptl);
        isolated = __collapse_huge_page_isolate(vma, address, pte);
        spin_unlock(pte_ptl);

        if (unlikely(!isolated)) {
                pte_unmap(pte);
                spin_lock(pmd_ptl);
                BUG_ON(!pmd_none(*pmd));
                /*
                 * We can only use set_pmd_at when establishing
                 * hugepmds and never for establishing regular pmds that
                 * points to regular pagetables. Use pmd_populate for that
                 */
                pmd_populate(mm, pmd, pmd_pgtable(_pmd));
                spin_unlock(pmd_ptl);
                anon_vma_unlock_write(vma->anon_vma);
                result = SCAN_FAIL;
                goto out;
        }

        /*
         * All pages are isolated and locked so anon_vma rmap
         * can't run anymore.
         */
        anon_vma_unlock_write(vma->anon_vma);

        __collapse_huge_page_copy(pte, new_page, vma, address, pte_ptl);
        pte_unmap(pte);
        __SetPageUptodate(new_page);
        pgtable = pmd_pgtable(_pmd);

        _pmd = mk_huge_pmd(new_page, vma->vm_page_prot);
        _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);

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

        spin_lock(pmd_ptl);
        BUG_ON(!pmd_none(*pmd));
        page_add_new_anon_rmap(new_page, vma, address, true);
        mem_cgroup_commit_charge(new_page, memcg, false, true);
        count_memcg_events(memcg, THP_COLLAPSE_ALLOC, 1);
        lru_cache_add_active_or_unevictable(new_page, vma);
        pgtable_trans_huge_deposit(mm, pmd, pgtable);
        set_pmd_at(mm, address, pmd, _pmd);
        update_mmu_cache_pmd(vma, address, pmd);
        spin_unlock(pmd_ptl);

        *hpage = NULL;

        khugepaged_pages_collapsed++;
        result = SCAN_SUCCEED;
out_up_write:
        up_write(&mm->mmap_sem);
out_nolock:
        trace_mm_collapse_huge_page(mm, isolated, result);
        return;
out:
        mem_cgroup_cancel_charge(new_page, memcg, true);
        goto out_up_write;
}

static int khugepaged_scan_pmd(struct mm_struct *mm,
                               struct vm_area_struct *vma,
                               unsigned long address,
                               struct page **hpage)
{
        pmd_t *pmd;
        pte_t *pte, *_pte;
        int ret = 0, none_or_zero = 0, result = 0, referenced = 0;
        struct page *page = NULL;
        unsigned long _address;
        spinlock_t *ptl;
        int node = NUMA_NO_NODE, unmapped = 0;
        bool writable = false;

        VM_BUG_ON(address & ~HPAGE_PMD_MASK);

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

        memset(khugepaged_node_load, 0, sizeof(khugepaged_node_load));
        pte = pte_offset_map_lock(mm, pmd, address, &ptl);
        for (_address = address, _pte = pte; _pte < pte+HPAGE_PMD_NR;
             _pte++, _address += PAGE_SIZE) {
                pte_t pteval = *_pte;
                if (is_swap_pte(pteval)) {
                        if (++unmapped <= khugepaged_max_ptes_swap) {
                                continue;
                        } else {
                                result = SCAN_EXCEED_SWAP_PTE;
                                goto out_unmap;
                        }
                }
                if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
                        if (!userfaultfd_armed(vma) &&
                            ++none_or_zero <= khugepaged_max_ptes_none) {
                                continue;
                        } else {
                                result = SCAN_EXCEED_NONE_PTE;
                                goto out_unmap;
                        }
                }
                if (!pte_present(pteval)) {
                        result = SCAN_PTE_NON_PRESENT;
                        goto out_unmap;
                }
                if (pte_write(pteval))
                        writable = true;

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

                /* TODO: teach khugepaged to collapse THP mapped with pte */
                if (PageCompound(page)) {
                        result = SCAN_PAGE_COMPOUND;
                        goto out_unmap;
                }

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

                /*
                 * cannot use mapcount: can't collapse if there's a gup pin.
                 * The page must only be referenced by the scanned process
                 * and page swap cache.
                 */
                if (page_count(page) != 1 + PageSwapCache(page)) {
                        result = SCAN_PAGE_COUNT;
                        goto out_unmap;
                }
                if (pte_young(pteval) ||
                    page_is_young(page) || PageReferenced(page) ||
                    mmu_notifier_test_young(vma->vm_mm, address))
                        referenced++;
        }
        if (writable) {
                if (referenced) {
                        result = SCAN_SUCCEED;
                        ret = 1;
                } else {
                        result = SCAN_LACK_REFERENCED_PAGE;
                }
        } else {
                result = SCAN_PAGE_RO;
        }
out_unmap:
        pte_unmap_unlock(pte, ptl);
        if (ret) {
                node = khugepaged_find_target_node();
                /* collapse_huge_page will return with the mmap_sem released */
                collapse_huge_page(mm, address, hpage, node, referenced);
        }
out:
        trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,
                                     none_or_zero, result, unmapped);
        return ret;
}

static void collect_mm_slot(struct mm_slot *mm_slot)
{
        struct mm_struct *mm = mm_slot->mm;

        lockdep_assert_held(&khugepaged_mm_lock);

        if (khugepaged_test_exit(mm)) {
                /* free mm_slot */
                hash_del(&mm_slot->hash);
                list_del(&mm_slot->mm_node);

                /*
                 * Not strictly needed because the mm exited already.
                 *
                 * clear_bit(MMF_VM_HUGEPAGE, &mm->flags);
                 */

                /* khugepaged_mm_lock actually not necessary for the below */
                free_mm_slot(mm_slot);
                mmdrop(mm);
        }
}

#if defined(CONFIG_SHMEM) && defined(CONFIG_TRANSPARENT_HUGE_PAGECACHE)
/*
 * Notify khugepaged that given addr of the mm is pte-mapped THP. Then
 * khugepaged should try to collapse the page table.
 */
static int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
                                         unsigned long addr)
{
        struct mm_slot *mm_slot;

        VM_BUG_ON(addr & ~HPAGE_PMD_MASK);

        spin_lock(&khugepaged_mm_lock);
        mm_slot = get_mm_slot(mm);
        if (likely(mm_slot && mm_slot->nr_pte_mapped_thp < MAX_PTE_MAPPED_THP))
                mm_slot->pte_mapped_thp[mm_slot->nr_pte_mapped_thp++] = addr;
        spin_unlock(&khugepaged_mm_lock);
        return 0;
}

/**
 * Try to collapse a pte-mapped THP for mm at address haddr.
 *
 * This function checks whether all the PTEs in the PMD are pointing to the
 * right THP. If so, retract the page table so the THP can refault in with
 * as pmd-mapped.
 */
void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
{
        unsigned long haddr = addr & HPAGE_PMD_MASK;
        struct vm_area_struct *vma = find_vma(mm, haddr);
        struct page *hpage = NULL;
        pte_t *start_pte, *pte;
        pmd_t *pmd, _pmd;
        spinlock_t *ptl;
        int count = 0;
        int i;

        if (!vma || !vma->vm_file ||
            vma->vm_start > haddr || vma->vm_end < haddr + HPAGE_PMD_SIZE)
                return;

        /*
         * This vm_flags may not have VM_HUGEPAGE if the page was not
         * collapsed by this mm. But we can still collapse if the page is
         * the valid THP. Add extra VM_HUGEPAGE so hugepage_vma_check()
         * will not fail the vma for missing VM_HUGEPAGE
         */
        if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE))
                return;

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

        start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl);

        /* step 1: check all mapped PTEs are to the right huge page */
        for (i = 0, addr = haddr, pte = start_pte;
             i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {
                struct page *page;

                /* empty pte, skip */
                if (pte_none(*pte))
                        continue;

                /* page swapped out, abort */
                if (!pte_present(*pte))
                        goto abort;

                page = vm_normal_page(vma, addr, *pte);

                if (!page || !PageCompound(page))
                        goto abort;

                if (!hpage) {
                        hpage = compound_head(page);
                        /*
                         * The mapping of the THP should not change.
                         *
                         * Note that uprobe, debugger, or MAP_PRIVATE may
                         * change the page table, but the new page will
                         * not pass PageCompound() check.
                         */
                        if (WARN_ON(hpage->mapping != vma->vm_file->f_mapping))
                                goto abort;
                }

                /*
                 * Confirm the page maps to the correct subpage.
                 *
                 * Note that uprobe, debugger, or MAP_PRIVATE may change
                 * the page table, but the new page will not pass
                 * PageCompound() check.
                 */
                if (WARN_ON(hpage + i != page))
                        goto abort;
                count++;
        }

        /* step 2: adjust rmap */
        for (i = 0, addr = haddr, pte = start_pte;
             i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {
                struct page *page;

                if (pte_none(*pte))
                        continue;
                page = vm_normal_page(vma, addr, *pte);
                page_remove_rmap(page, false);
        }

        pte_unmap_unlock(start_pte, ptl);

        /* step 3: set proper refcount and mm_counters. */
        if (hpage) {
                page_ref_sub(hpage, count);
                add_mm_counter(vma->vm_mm, mm_counter_file(hpage), -count);
        }

        /* step 4: collapse pmd */
        ptl = pmd_lock(vma->vm_mm, pmd);
        _pmd = pmdp_collapse_flush(vma, addr, pmd);
        spin_unlock(ptl);
        mm_dec_nr_ptes(mm);
        pte_free(mm, pmd_pgtable(_pmd));
        return;

abort:
        pte_unmap_unlock(start_pte, ptl);
}

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

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

        if (!down_write_trylock(&mm->mmap_sem))
                return -EBUSY;

        if (unlikely(khugepaged_test_exit(mm)))
                goto out;

        for (i = 0; i < mm_slot->nr_pte_mapped_thp; i++)
                collapse_pte_mapped_thp(mm, mm_slot->pte_mapped_thp[i]);

out:
        mm_slot->nr_pte_mapped_thp = 0;
        up_write(&mm->mmap_sem);
        return 0;
}

static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
{
        struct vm_area_struct *vma;
        unsigned long addr;
        pmd_t *pmd, _pmd;

        i_mmap_lock_write(mapping);
        vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
                /*
                 * Check vma->anon_vma to exclude MAP_PRIVATE mappings that
                 * got written to. These VMAs are likely not worth investing
                 * down_write(mmap_sem) as PMD-mapping is likely to be split
                 * later.
                 *
                 * Not that vma->anon_vma check is racy: it can be set up after
                 * the check but before we took mmap_sem by the fault path.
                 * But page lock would prevent establishing any new ptes of the
                 * page, so we are safe.
                 *
                 * An alternative would be drop the check, but check that page
                 * table is clear before calling pmdp_collapse_flush() under
                 * ptl. It has higher chance to recover THP for the VMA, but
                 * has higher cost too.
                 */
                if (vma->anon_vma)
                        continue;
                addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
                if (addr & ~HPAGE_PMD_MASK)
                        continue;
                if (vma->vm_end < addr + HPAGE_PMD_SIZE)
                        continue;
                pmd = mm_find_pmd(vma->vm_mm, addr);
                if (!pmd)
                        continue;
                /*
                 * We need exclusive mmap_sem to retract page table.
                 *
                 * We use trylock due to lock inversion: we need to acquire
                 * mmap_sem while holding page lock. Fault path does it in
                 * reverse order. Trylock is a way to avoid deadlock.
                 */
                if (down_write_trylock(&vma->vm_mm->mmap_sem)) {
                        spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);
                        /* assume page table is clear */
                        _pmd = pmdp_collapse_flush(vma, addr, pmd);
                        spin_unlock(ptl);
                        up_write(&vma->vm_mm->mmap_sem);
                        mm_dec_nr_ptes(vma->vm_mm);
                        pte_free(vma->vm_mm, pmd_pgtable(_pmd));
                } else {
                        /* Try again later */
                        khugepaged_add_pte_mapped_thp(vma->vm_mm, addr);
                }
        }
        i_mmap_unlock_write(mapping);
}

/**
 * collapse_file - collapse filemap/tmpfs/shmem pages into huge one.
 *
 * Basic scheme is simple, details are more complex:
 *  - allocate and lock a new huge page;
 *  - scan page cache replacing old pages with the new one
 *    + swap/gup in pages if necessary;
 *    + fill in gaps;
 *    + keep old pages around in case rollback is required;
 *  - if replacing succeeds:
 *    + copy data over;
 *    + free old pages;
 *    + unlock huge page;
 *  - if replacing failed;
 *    + put all pages back and unfreeze them;
 *    + restore gaps in the page cache;
 *    + unlock and free huge page;
 */
static void collapse_file(struct mm_struct *mm,
                struct file *file, pgoff_t start,
                struct page **hpage, int node)
{
        struct address_space *mapping = file->f_mapping;
        gfp_t gfp;
        struct page *new_page;
        struct mem_cgroup *memcg;
        pgoff_t index, end = start + HPAGE_PMD_NR;
        LIST_HEAD(pagelist);
        XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
        int nr_none = 0, result = SCAN_SUCCEED;
        bool is_shmem = shmem_file(file);

        VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
        VM_BUG_ON(start & (HPAGE_PMD_NR - 1));

        /* Only allocate from the target node */
        gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;

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

        if (unlikely(mem_cgroup_try_charge(new_page, mm, gfp, &memcg, true))) {
                result = SCAN_CGROUP_CHARGE_FAIL;
                goto out;
        }

        /* This will be less messy when we use multi-index entries */
        do {
                xas_lock_irq(&xas);
                xas_create_range(&xas);
                if (!xas_error(&xas))
                        break;
                xas_unlock_irq(&xas);
                if (!xas_nomem(&xas, GFP_KERNEL)) {
                        mem_cgroup_cancel_charge(new_page, memcg, true);
                        result = SCAN_FAIL;
                        goto out;
                }
        } while (1);

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

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

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

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

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

                /*
                 * The page must be locked, so we can drop the i_pages lock
                 * without racing with truncate.
                 */
                VM_BUG_ON_PAGE(!PageLocked(page), page);

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

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

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

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

                if (isolate_lru_page(page)) {
                        result = SCAN_DEL_PAGE_LRU;
                        goto out_unlock;
                }

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

                if (page_mapped(page))
                        unmap_mapping_pages(mapping, index, 1, false);

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

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

                /*
                 * The page is expected to have page_count() == 3:
                 *  - we hold a pin on it;
                 *  - one reference from page cache;
                 *  - one from isolate_lru_page;
                 */
                if (!page_ref_freeze(page, 3)) {
                        result = SCAN_PAGE_COUNT;
                        xas_unlock_irq(&xas);
                        putback_lru_page(page);
                        goto out_unlock;
                }

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

                /* Finally, replace with the new page. */
                xas_store(&xas, new_page);
                continue;
out_unlock:
                unlock_page(page);
                put_page(page);
                goto xa_unlocked;
        }

        if (is_shmem)
                __inc_node_page_state(new_page, NR_SHMEM_THPS);
        else {
                __inc_node_page_state(new_page, NR_FILE_THPS);
                filemap_nr_thps_inc(mapping);
        }

        if (nr_none) {
                struct zone *zone = page_zone(new_page);

                __mod_node_page_state(zone->zone_pgdat, NR_FILE_PAGES, nr_none);
                if (is_shmem)
                        __mod_node_page_state(zone->zone_pgdat,
                                              NR_SHMEM, nr_none);
        }

xa_locked:
        xas_unlock_irq(&xas);
xa_unlocked:

        if (result == SCAN_SUCCEED) {
                struct page *page, *tmp;

                /*
                 * Replacing old pages with new one has succeeded, now we
                 * need to copy the content and free the old pages.
                 */
                index = start;
                list_for_each_entry_safe(page, tmp, &pagelist, lru) {
                        while (index < page->index) {
                                clear_highpage(new_page + (index % HPAGE_PMD_NR));
                                index++;
                        }
                        copy_highpage(new_page + (page->index % HPAGE_PMD_NR),
                                        page);
                        list_del(&page->lru);
                        page->mapping = NULL;
                        page_ref_unfreeze(page, 1);
                        ClearPageActive(page);
                        ClearPageUnevictable(page);
                        unlock_page(page);
                        put_page(page);
                        index++;
                }
                while (index < end) {
                        clear_highpage(new_page + (index % HPAGE_PMD_NR));
                        index++;
                }

                SetPageUptodate(new_page);
                page_ref_add(new_page, HPAGE_PMD_NR - 1);
                mem_cgroup_commit_charge(new_page, memcg, false, true);

                if (is_shmem) {
                        set_page_dirty(new_page);
                        lru_cache_add_anon(new_page);
                } else {
                        lru_cache_add_file(new_page);
                }
                count_memcg_events(memcg, THP_COLLAPSE_ALLOC, 1);

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

                khugepaged_pages_collapsed++;
        } else {
                struct page *page;

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

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

                xas_set(&xas, start);
                xas_for_each(&xas, page, end - 1) {
                        page = list_first_entry_or_null(&pagelist,
                                        struct page, lru);
                        if (!page || xas.xa_index < page->index) {
                                if (!nr_none)
                                        break;
                                nr_none--;
                                /* Put holes back where they were */
                                xas_store(&xas, NULL);
                                continue;
                        }

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

                        /* Unfreeze the page. */
                        list_del(&page->lru);
                        page_ref_unfreeze(page, 2);
                        xas_store(&xas, page);
                        xas_pause(&xas);
                        xas_unlock_irq(&xas);
                        unlock_page(page);
                        putback_lru_page(page);
                        xas_lock_irq(&xas);
                }
                VM_BUG_ON(nr_none);
                xas_unlock_irq(&xas);

                mem_cgroup_cancel_charge(new_page, memcg, true);
                new_page->mapping = NULL;
        }

        unlock_page(new_page);
out:
        VM_BUG_ON(!list_empty(&pagelist));
        /* TODO: tracepoints */
}

static void khugepaged_scan_file(struct mm_struct *mm,
                struct file *file, pgoff_t start, struct page **hpage)
{
        struct page *page = NULL;
        struct address_space *mapping = file->f_mapping;
        XA_STATE(xas, &mapping->i_pages, start);
        int present, swap;
        int node = NUMA_NO_NODE;
        int result = SCAN_SUCCEED;

        present = 0;
        swap = 0;
        memset(khugepaged_node_load, 0, sizeof(khugepaged_node_load));
        rcu_read_lock();
        xas_for_each(&xas, page, start + HPAGE_PMD_NR - 1) {
                if (xas_retry(&xas, page))
                        continue;

                if (xa_is_value(page)) {
                        if (++swap > khugepaged_max_ptes_swap) {
                                result = SCAN_EXCEED_SWAP_PTE;
                                break;
                        }
                        continue;
                }

                if (PageTransCompound(page)) {
                        result = SCAN_PAGE_COMPOUND;
                        break;
                }

                node = page_to_nid(page);
                if (khugepaged_scan_abort(node)) {
                        result = SCAN_SCAN_ABORT;
                        break;
                }
                khugepaged_node_load[node]++;

                if (!PageLRU(page)) {
                        result = SCAN_PAGE_LRU;
                        break;
                }

                if (page_count(page) !=
                    1 + page_mapcount(page) + page_has_private(page)) {
                        result = SCAN_PAGE_COUNT;
                        break;
                }

                /*
                 * We probably should check if the page is referenced here, but
                 * nobody would transfer pte_young() to PageReferenced() for us.
                 * And rmap walk here is just too costly...
                 */

                present++;

                if (need_resched()) {
                        xas_pause(&xas);
                        cond_resched_rcu();
                }
        }
        rcu_read_unlock();

        if (result == SCAN_SUCCEED) {
                if (present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
                        result = SCAN_EXCEED_NONE_PTE;
                } else {
                        node = khugepaged_find_target_node();
                        collapse_file(mm, file, start, hpage, node);
                }
        }

        /* TODO: tracepoints */
}
#else
static void khugepaged_scan_file(struct mm_struct *mm,
                struct file *file, pgoff_t start, struct page **hpage)
{
        BUILD_BUG();
}

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

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

        VM_BUG_ON(!pages);
        lockdep_assert_held(&khugepaged_mm_lock);

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

        mm = mm_slot->mm;
        /*
         * Don't wait for semaphore (to avoid long wait times).  Just move to
         * the next mm on the list.
         */
        vma = NULL;
        if (unlikely(!down_read_trylock(&mm->mmap_sem)))
                goto breakouterloop_mmap_sem;
        if (likely(!khugepaged_test_exit(mm)))
                vma = find_vma(mm, khugepaged_scan.address);

        progress++;
        for (; vma; vma = vma->vm_next) {
                unsigned long hstart, hend;

                cond_resched();
                if (unlikely(khugepaged_test_exit(mm))) {
                        progress++;
                        break;
                }
                if (!hugepage_vma_check(vma, vma->vm_flags)) {
skip:
                        progress++;
                        continue;
                }
                hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
                hend = vma->vm_end & HPAGE_PMD_MASK;
                if (hstart >= hend)
                        goto skip;
                if (khugepaged_scan.address > hend)
                        goto skip;
                if (khugepaged_scan.address < hstart)
                        khugepaged_scan.address = hstart;
                VM_BUG_ON(khugepaged_scan.address & ~HPAGE_PMD_MASK);

                while (khugepaged_scan.address < hend) {
                        int ret;
                        cond_resched();
                        if (unlikely(khugepaged_test_exit(mm)))
                                goto breakouterloop;

                        VM_BUG_ON(khugepaged_scan.address < hstart ||
                                  khugepaged_scan.address + HPAGE_PMD_SIZE >
                                  hend);
                        if (IS_ENABLED(CONFIG_SHMEM) && vma->vm_file) {
                                struct file *file;
                                pgoff_t pgoff = linear_page_index(vma,
                                                khugepaged_scan.address);

                                if (shmem_file(vma->vm_file)
                                    && !shmem_huge_enabled(vma))
                                        goto skip;
                                file = get_file(vma->vm_file);
                                up_read(&mm->mmap_sem);
                                ret = 1;
                                khugepaged_scan_file(mm, file, pgoff, hpage);
                                fput(file);
                        } else {
                                ret = khugepaged_scan_pmd(mm, vma,
                                                khugepaged_scan.address,
                                                hpage);
                        }
                        /* move to next address */
                        khugepaged_scan.address += HPAGE_PMD_SIZE;
                        progress += HPAGE_PMD_NR;
                        if (ret)
                                /* we released mmap_sem so break loop */
                                goto breakouterloop_mmap_sem;
                        if (progress >= pages)
                                goto breakouterloop;
                }
        }
breakouterloop:
        up_read(&mm->mmap_sem); /* exit_mmap will destroy ptes after this */
breakouterloop_mmap_sem:

        spin_lock(&khugepaged_mm_lock);
        VM_BUG_ON(khugepaged_scan.mm_slot != mm_slot);
        /*
         * Release the current mm_slot if this mm is about to die, or
         * if we scanned all vmas of this mm.
         */
        if (khugepaged_test_exit(mm) || !vma) {
                /*
                 * Make sure that if mm_users is reaching zero while
                 * khugepaged runs here, khugepaged_exit will find
                 * mm_slot not pointing to the exiting mm.
                 */
                if (mm_slot->mm_node.next != &khugepaged_scan.mm_head) {
                        khugepaged_scan.mm_slot = list_entry(
                                mm_slot->mm_node.next,
                                struct mm_slot, mm_node);
                        khugepaged_scan.address = 0;
                } else {
                        khugepaged_scan.mm_slot = NULL;
                        khugepaged_full_scans++;
                }

                collect_mm_slot(mm_slot);
        }

        return progress;
}

static int khugepaged_has_work(void)
{
        return !list_empty(&khugepaged_scan.mm_head) &&
                khugepaged_enabled();
}

static int khugepaged_wait_event(void)
{
        return !list_empty(&khugepaged_scan.mm_head) ||
                kthread_should_stop();
}

static void khugepaged_do_scan(void)
{
        struct page *hpage = NULL;
        unsigned int progress = 0, pass_through_head = 0;
        unsigned int pages = khugepaged_pages_to_scan;
        bool wait = true;

        barrier(); /* write khugepaged_pages_to_scan to local stack */

        while (progress < pages) {
                if (!khugepaged_prealloc_page(&hpage, &wait))
                        break;

                cond_resched();

                if (unlikely(kthread_should_stop() || try_to_freeze()))
                        break;

                spin_lock(&khugepaged_mm_lock);
                if (!khugepaged_scan.mm_slot)
                        pass_through_head++;
                if (khugepaged_has_work() &&
                    pass_through_head < 2)
                        progress += khugepaged_scan_mm_slot(pages - progress,
                                                            &hpage);
                else
                        progress = pages;
                spin_unlock(&khugepaged_mm_lock);
        }

        if (!IS_ERR_OR_NULL(hpage))
                put_page(hpage);
}

static bool khugepaged_should_wakeup(void)
{
        return kthread_should_stop() ||
               time_after_eq(jiffies, khugepaged_sleep_expire);
}

static void khugepaged_wait_work(void)
{
        if (khugepaged_has_work()) {
                const unsigned long scan_sleep_jiffies =
                        msecs_to_jiffies(khugepaged_scan_sleep_millisecs);

                if (!scan_sleep_jiffies)
                        return;

                khugepaged_sleep_expire = jiffies + scan_sleep_jiffies;
                wait_event_freezable_timeout(khugepaged_wait,
                                             khugepaged_should_wakeup(),
                                             scan_sleep_jiffies);
                return;
        }

        if (khugepaged_enabled())
                wait_event_freezable(khugepaged_wait, khugepaged_wait_event());
}

static int khugepaged(void *none)
{
        struct mm_slot *mm_slot;

        set_freezable();
        set_user_nice(current, MAX_NICE);

        while (!kthread_should_stop()) {
                khugepaged_do_scan();
                khugepaged_wait_work();
        }

        spin_lock(&khugepaged_mm_lock);
        mm_slot = khugepaged_scan.mm_slot;
        khugepaged_scan.mm_slot = NULL;
        if (mm_slot)
                collect_mm_slot(mm_slot);
        spin_unlock(&khugepaged_mm_lock);
        return 0;
}

static void set_recommended_min_free_kbytes(void)
{
        struct zone *zone;
        int nr_zones = 0;
        unsigned long recommended_min;

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

                nr_zones++;
        }

        /* Ensure 2 pageblocks are free to assist fragmentation avoidance */
        recommended_min = pageblock_nr_pages * nr_zones * 2;

        /*
         * Make sure that on average at least two pageblocks are almost free
         * of another type, one for a migratetype to fall back to and a
         * second to avoid subsequent fallbacks of other types There are 3
         * MIGRATE_TYPES we care about.
         */
        recommended_min += pageblock_nr_pages * nr_zones *
                           MIGRATE_PCPTYPES * MIGRATE_PCPTYPES;

        /* don't ever allow to reserve more than 5% of the lowmem */
        recommended_min = min(recommended_min,
                              (unsigned long) nr_free_buffer_pages() / 20);
        recommended_min <<= (PAGE_SHIFT-10);

        if (recommended_min > min_free_kbytes) {
                if (user_min_free_kbytes >= 0)
                        pr_info("raising min_free_kbytes from %d to %lu to help transparent hugepage allocations\n",
                                min_free_kbytes, recommended_min);

                min_free_kbytes = recommended_min;
        }
        setup_per_zone_wmarks();
}

int start_stop_khugepaged(void)
{
        static struct task_struct *khugepaged_thread __read_mostly;
        static DEFINE_MUTEX(khugepaged_mutex);
        int err = 0;

        mutex_lock(&khugepaged_mutex);
        if (khugepaged_enabled()) {
                if (!khugepaged_thread)
                        khugepaged_thread = kthread_run(khugepaged, NULL,
                                                        "khugepaged");
                if (IS_ERR(khugepaged_thread)) {
                        pr_err("khugepaged: kthread_run(khugepaged) failed\n");
                        err = PTR_ERR(khugepaged_thread);
                        khugepaged_thread = NULL;
                        goto fail;
                }

                if (!list_empty(&khugepaged_scan.mm_head))
                        wake_up_interruptible(&khugepaged_wait);

                set_recommended_min_free_kbytes();
        } else if (khugepaged_thread) {
                kthread_stop(khugepaged_thread);
                khugepaged_thread = NULL;
        }
fail:
        mutex_unlock(&khugepaged_mutex);
        return err;
}