huge_memory.c

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/*
 *  Copyright (C) 2009  Red Hat, Inc.
 *
 *  This work is licensed under the terms of the GNU GPL, version 2. See
 *  the COPYING file in the top-level directory.
 */

#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/highmem.h>
#include <linux/hugetlb.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/pagemap.h>
#include <asm/tlb.h>
#include <asm/pgalloc.h>
#include "internal.h"

/*
 * By default transparent hugepage support is enabled for all mappings
 * and khugepaged scans all mappings. Defrag is only invoked by
 * khugepaged hugepage allocations and by page faults inside
 * MADV_HUGEPAGE regions to avoid the risk of slowing down short lived
 * allocations.
 */
unsigned long transparent_hugepage_flags __read_mostly =
#ifdef CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS
    (1<<TRANSPARENT_HUGEPAGE_FLAG)|
#endif
#ifdef CONFIG_TRANSPARENT_HUGEPAGE_MADVISE
    (1<<TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG)|
#endif
    (1<<TRANSPARENT_HUGEPAGE_DEFRAG_FLAG)|
    (1<<TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG);

/* default scan 8*512 pte (or vmas) every 30 second */
static unsigned int khugepaged_pages_to_scan __read_mostly = HPAGE_PMD_NR*8;
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 struct task_struct *khugepaged_thread __read_mostly;
static DEFINE_MUTEX(khugepaged_mutex);
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 = HPAGE_PMD_NR-1;

static int khugepaged(void *none);
static int mm_slots_hash_init(void);
static int khugepaged_slab_init(void);
static void khugepaged_slab_free(void);

#define MM_SLOTS_HASH_HEADS 1024
static struct hlist_head *mm_slots_hash __read_mostly;
static struct kmem_cache *mm_slot_cache __read_mostly;

struct mm_slot {
    struct hlist_node hash;
    struct list_head mm_node;
    struct mm_struct *mm;
};

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),
};


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

    if (!khugepaged_enabled())
        return 0;

    for_each_populated_zone(zone)
        nr_zones++;

    /* Make sure at least 2 hugepages are free for MIGRATE_RESERVE */
    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)
        min_free_kbytes = recommended_min;
    setup_per_zone_wmarks();
    return 0;
}
late_initcall(set_recommended_min_free_kbytes);

static int start_khugepaged(void)
{
    int err = 0;
    if (khugepaged_enabled()) {
        if (!khugepaged_thread)
            khugepaged_thread = kthread_run(khugepaged, NULL,
                            "khugepaged");
        if (unlikely(IS_ERR(khugepaged_thread))) {
            printk(KERN_ERR
                   "khugepaged: kthread_run(khugepaged) failed\n");
            err = PTR_ERR(khugepaged_thread);
            khugepaged_thread = NULL;
        }

        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;
    }

    return err;
}

#ifdef CONFIG_SYSFS

static ssize_t double_flag_show(struct kobject *kobj,
                struct kobj_attribute *attr, char *buf,
                enum transparent_hugepage_flag enabled,
                enum transparent_hugepage_flag req_madv)
{
    if (test_bit(enabled, &transparent_hugepage_flags)) {
        VM_BUG_ON(test_bit(req_madv, &transparent_hugepage_flags));
        return sprintf(buf, "[always] madvise never\n");
    } else if (test_bit(req_madv, &transparent_hugepage_flags))
        return sprintf(buf, "always [madvise] never\n");
    else
        return sprintf(buf, "always madvise [never]\n");
}
static ssize_t double_flag_store(struct kobject *kobj,
                 struct kobj_attribute *attr,
                 const char *buf, size_t count,
                 enum transparent_hugepage_flag enabled,
                 enum transparent_hugepage_flag req_madv)
{
    if (!memcmp("always", buf,
            min(sizeof("always")-1, count))) {
        set_bit(enabled, &transparent_hugepage_flags);
        clear_bit(req_madv, &transparent_hugepage_flags);
    } else if (!memcmp("madvise", buf,
               min(sizeof("madvise")-1, count))) {
        clear_bit(enabled, &transparent_hugepage_flags);
        set_bit(req_madv, &transparent_hugepage_flags);
    } else if (!memcmp("never", buf,
               min(sizeof("never")-1, count))) {
        clear_bit(enabled, &transparent_hugepage_flags);
        clear_bit(req_madv, &transparent_hugepage_flags);
    } else
        return -EINVAL;

    return count;
}

static ssize_t enabled_show(struct kobject *kobj,
                struct kobj_attribute *attr, char *buf)
{
    return double_flag_show(kobj, attr, buf,
                TRANSPARENT_HUGEPAGE_FLAG,
                TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG);
}
static ssize_t enabled_store(struct kobject *kobj,
                 struct kobj_attribute *attr,
                 const char *buf, size_t count)
{
    ssize_t ret;

    ret = double_flag_store(kobj, attr, buf, count,
                TRANSPARENT_HUGEPAGE_FLAG,
                TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG);

    if (ret > 0) {
        int err;

        mutex_lock(&khugepaged_mutex);
        err = start_khugepaged();
        mutex_unlock(&khugepaged_mutex);

        if (err)
            ret = err;
    }

    return ret;
}
static struct kobj_attribute enabled_attr =
    __ATTR(enabled, 0644, enabled_show, enabled_store);

static ssize_t single_flag_show(struct kobject *kobj,
                struct kobj_attribute *attr, char *buf,
                enum transparent_hugepage_flag flag)
{
    return sprintf(buf, "%d\n",
               !!test_bit(flag, &transparent_hugepage_flags));
}

static ssize_t single_flag_store(struct kobject *kobj,
                 struct kobj_attribute *attr,
                 const char *buf, size_t count,
                 enum transparent_hugepage_flag flag)
{
    unsigned long value;
    int ret;

    ret = kstrtoul(buf, 10, &value);
    if (ret < 0)
        return ret;
    if (value > 1)
        return -EINVAL;

    if (value)
        set_bit(flag, &transparent_hugepage_flags);
    else
        clear_bit(flag, &transparent_hugepage_flags);

    return count;
}

/*
 * Currently defrag only disables __GFP_NOWAIT for allocation. A blind
 * __GFP_REPEAT is too aggressive, it's never worth swapping tons of
 * memory just to allocate one more hugepage.
 */
static ssize_t defrag_show(struct kobject *kobj,
               struct kobj_attribute *attr, char *buf)
{
    return double_flag_show(kobj, attr, buf,
                TRANSPARENT_HUGEPAGE_DEFRAG_FLAG,
                TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG);
}
static ssize_t defrag_store(struct kobject *kobj,
                struct kobj_attribute *attr,
                const char *buf, size_t count)
{
    return double_flag_store(kobj, attr, buf, count,
                 TRANSPARENT_HUGEPAGE_DEFRAG_FLAG,
                 TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG);
}
static struct kobj_attribute defrag_attr =
    __ATTR(defrag, 0644, defrag_show, defrag_store);

#ifdef CONFIG_DEBUG_VM
static ssize_t debug_cow_show(struct kobject *kobj,
                struct kobj_attribute *attr, char *buf)
{
    return single_flag_show(kobj, attr, buf,
                TRANSPARENT_HUGEPAGE_DEBUG_COW_FLAG);
}
static ssize_t debug_cow_store(struct kobject *kobj,
                   struct kobj_attribute *attr,
                   const char *buf, size_t count)
{
    return single_flag_store(kobj, attr, buf, count,
                 TRANSPARENT_HUGEPAGE_DEBUG_COW_FLAG);
}
static struct kobj_attribute debug_cow_attr =
    __ATTR(debug_cow, 0644, debug_cow_show, debug_cow_store);
#endif /* CONFIG_DEBUG_VM */

static struct attribute *hugepage_attr[] = {
    &enabled_attr.attr,
    &defrag_attr.attr,
#ifdef CONFIG_DEBUG_VM
    &debug_cow_attr.attr,
#endif
    NULL,
};

static struct attribute_group hugepage_attr_group = {
    .attrs = hugepage_attr,
};

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

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

    khugepaged_alloc_sleep_millisecs = msecs;
    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 = strict_strtoul(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_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_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 = strict_strtoul(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 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,
    NULL,
};

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

static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
{
    int err;

    *hugepage_kobj = kobject_create_and_add("transparent_hugepage", mm_kobj);
    if (unlikely(!*hugepage_kobj)) {
        printk(KERN_ERR "hugepage: failed kobject create\n");
        return -ENOMEM;
    }

    err = sysfs_create_group(*hugepage_kobj, &hugepage_attr_group);
    if (err) {
        printk(KERN_ERR "hugepage: failed register hugeage group\n");
        goto delete_obj;
    }

    err = sysfs_create_group(*hugepage_kobj, &khugepaged_attr_group);
    if (err) {
        printk(KERN_ERR "hugepage: failed register hugeage group\n");
        goto remove_hp_group;
    }

    return 0;

remove_hp_group:
    sysfs_remove_group(*hugepage_kobj, &hugepage_attr_group);
delete_obj:
    kobject_put(*hugepage_kobj);
    return err;
}

static void __init hugepage_exit_sysfs(struct kobject *hugepage_kobj)
{
    sysfs_remove_group(hugepage_kobj, &khugepaged_attr_group);
    sysfs_remove_group(hugepage_kobj, &hugepage_attr_group);
    kobject_put(hugepage_kobj);
}
#else
static inline int hugepage_init_sysfs(struct kobject **hugepage_kobj)
{
    return 0;
}

static inline void hugepage_exit_sysfs(struct kobject *hugepage_kobj)
{
}
#endif /* CONFIG_SYSFS */

static int __init hugepage_init(void)
{
    int err;
    struct kobject *hugepage_kobj;

    if (!has_transparent_hugepage()) {
        transparent_hugepage_flags = 0;
        return -EINVAL;
    }

    err = hugepage_init_sysfs(&hugepage_kobj);
    if (err)
        return err;

    err = khugepaged_slab_init();
    if (err)
        goto out;

    err = mm_slots_hash_init();
    if (err) {
        khugepaged_slab_free();
        goto out;
    }

    /*
     * By default disable transparent hugepages on smaller systems,
     * where the extra memory used could hurt more than TLB overhead
     * is likely to save.  The admin can still enable it through /sys.
     */
    if (totalram_pages < (512 << (20 - PAGE_SHIFT)))
        transparent_hugepage_flags = 0;

    start_khugepaged();

    return 0;
out:
    hugepage_exit_sysfs(hugepage_kobj);
    return err;
}
module_init(hugepage_init)

static int __init setup_transparent_hugepage(char *str)
{
    int ret = 0;
    if (!str)
        goto out;
    if (!strcmp(str, "always")) {
        set_bit(TRANSPARENT_HUGEPAGE_FLAG,
            &transparent_hugepage_flags);
        clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
              &transparent_hugepage_flags);
        ret = 1;
    } else if (!strcmp(str, "madvise")) {
        clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
              &transparent_hugepage_flags);
        set_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
            &transparent_hugepage_flags);
        ret = 1;
    } else if (!strcmp(str, "never")) {
        clear_bit(TRANSPARENT_HUGEPAGE_FLAG,
              &transparent_hugepage_flags);
        clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
              &transparent_hugepage_flags);
        ret = 1;
    }
out:
    if (!ret)
        printk(KERN_WARNING
               "transparent_hugepage= cannot parse, ignored\n");
    return ret;
}
__setup("transparent_hugepage=", setup_transparent_hugepage);

static inline pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
{
    if (likely(vma->vm_flags & VM_WRITE))
        pmd = pmd_mkwrite(pmd);
    return pmd;
}

static int __do_huge_pmd_anonymous_page(struct mm_struct *mm,
                    struct vm_area_struct *vma,
                    unsigned long haddr, pmd_t *pmd,
                    struct page *page)
{
    pgtable_t pgtable;

    VM_BUG_ON(!PageCompound(page));
    pgtable = pte_alloc_one(mm, haddr);
    if (unlikely(!pgtable))
        return VM_FAULT_OOM;

    clear_huge_page(page, haddr, HPAGE_PMD_NR);
    __SetPageUptodate(page);

    spin_lock(&mm->page_table_lock);
    if (unlikely(!pmd_none(*pmd))) {
        spin_unlock(&mm->page_table_lock);
        mem_cgroup_uncharge_page(page);
        put_page(page);
        pte_free(mm, pgtable);
    } else {
        pmd_t entry;
        entry = mk_pmd(page, vma->vm_page_prot);
        entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
        entry = pmd_mkhuge(entry);
        /*
         * The spinlocking to take the lru_lock inside
         * page_add_new_anon_rmap() acts as a full memory
         * barrier to be sure clear_huge_page writes become
         * visible after the set_pmd_at() write.
         */
        page_add_new_anon_rmap(page, vma, haddr);
        set_pmd_at(mm, haddr, pmd, entry);
        pgtable_trans_huge_deposit(mm, pgtable);
        add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR);
        mm->nr_ptes++;
        spin_unlock(&mm->page_table_lock);
    }

    return 0;
}

static inline gfp_t alloc_hugepage_gfpmask(int defrag, gfp_t extra_gfp)
{
    return (GFP_TRANSHUGE & ~(defrag ? 0 : __GFP_WAIT)) | extra_gfp;
}

static inline struct page *alloc_hugepage_vma(int defrag,
                          struct vm_area_struct *vma,
                          unsigned long haddr, int nd,
                          gfp_t extra_gfp)
{
    return alloc_pages_vma(alloc_hugepage_gfpmask(defrag, extra_gfp),
                   HPAGE_PMD_ORDER, vma, haddr, nd);
}

#ifndef CONFIG_NUMA
static inline struct page *alloc_hugepage(int defrag)
{
    return alloc_pages(alloc_hugepage_gfpmask(defrag, 0),
               HPAGE_PMD_ORDER);
}
#endif

int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
                   unsigned long address, pmd_t *pmd,
                   unsigned int flags)
{
    struct page *page;
    unsigned long haddr = address & HPAGE_PMD_MASK;
    pte_t *pte;

    if (haddr >= vma->vm_start && haddr + HPAGE_PMD_SIZE <= vma->vm_end) {
        if (unlikely(anon_vma_prepare(vma)))
            return VM_FAULT_OOM;
        if (unlikely(khugepaged_enter(vma)))
            return VM_FAULT_OOM;
        page = alloc_hugepage_vma(transparent_hugepage_defrag(vma),
                      vma, haddr, numa_node_id(), 0);
        if (unlikely(!page)) {
            count_vm_event(THP_FAULT_FALLBACK);
            goto out;
        }
        count_vm_event(THP_FAULT_ALLOC);
        if (unlikely(mem_cgroup_newpage_charge(page, mm, GFP_KERNEL))) {
            put_page(page);
            goto out;
        }
        if (unlikely(__do_huge_pmd_anonymous_page(mm, vma, haddr, pmd,
                              page))) {
            mem_cgroup_uncharge_page(page);
            put_page(page);
            goto out;
        }

        return 0;
    }
out:
    /*
     * Use __pte_alloc instead of pte_alloc_map, because we can't
     * run pte_offset_map on the pmd, if an huge pmd could
     * materialize from under us from a different thread.
     */
    if (unlikely(__pte_alloc(mm, vma, pmd, address)))
        return VM_FAULT_OOM;
    /* if an huge pmd materialized from under us just retry later */
    if (unlikely(pmd_trans_huge(*pmd)))
        return 0;
    /*
     * A regular pmd is established and it can't morph into a huge pmd
     * from under us anymore at this point because we hold the mmap_sem
     * read mode and khugepaged takes it in write mode. So now it's
     * safe to run pte_offset_map().
     */
    pte = pte_offset_map(pmd, address);
    return handle_pte_fault(mm, vma, address, pte, pmd, flags);
}

int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
          pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
          struct vm_area_struct *vma)
{
    struct page *src_page;
    pmd_t pmd;
    pgtable_t pgtable;
    int ret;

    ret = -ENOMEM;
    pgtable = pte_alloc_one(dst_mm, addr);
    if (unlikely(!pgtable))
        goto out;

    spin_lock(&dst_mm->page_table_lock);
    spin_lock_nested(&src_mm->page_table_lock, SINGLE_DEPTH_NESTING);

    ret = -EAGAIN;
    pmd = *src_pmd;
    if (unlikely(!pmd_trans_huge(pmd))) {
        pte_free(dst_mm, pgtable);
        goto out_unlock;
    }
    if (unlikely(pmd_trans_splitting(pmd))) {
        /* split huge page running from under us */
        spin_unlock(&src_mm->page_table_lock);
        spin_unlock(&dst_mm->page_table_lock);
        pte_free(dst_mm, pgtable);

        wait_split_huge_page(vma->anon_vma, src_pmd); /* src_vma */
        goto out;
    }
    src_page = pmd_page(pmd);
    VM_BUG_ON(!PageHead(src_page));
    get_page(src_page);
    page_dup_rmap(src_page);
    add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);

    pmdp_set_wrprotect(src_mm, addr, src_pmd);
    pmd = pmd_mkold(pmd_wrprotect(pmd));
    set_pmd_at(dst_mm, addr, dst_pmd, pmd);
    pgtable_trans_huge_deposit(dst_mm, pgtable);
    dst_mm->nr_ptes++;

    ret = 0;
out_unlock:
    spin_unlock(&src_mm->page_table_lock);
    spin_unlock(&dst_mm->page_table_lock);
out:
    return ret;
}

static int do_huge_pmd_wp_page_fallback(struct mm_struct *mm,
                    struct vm_area_struct *vma,
                    unsigned long address,
                    pmd_t *pmd, pmd_t orig_pmd,
                    struct page *page,
                    unsigned long haddr)
{
    pgtable_t pgtable;
    pmd_t _pmd;
    int ret = 0, i;
    struct page **pages;
    unsigned long mmun_start;   /* For mmu_notifiers */
    unsigned long mmun_end;     /* For mmu_notifiers */

    pages = kmalloc(sizeof(struct page *) * HPAGE_PMD_NR,
            GFP_KERNEL);
    if (unlikely(!pages)) {
        ret |= VM_FAULT_OOM;
        goto out;
    }

    for (i = 0; i < HPAGE_PMD_NR; i++) {
        pages[i] = alloc_page_vma_node(GFP_HIGHUSER_MOVABLE |
                           __GFP_OTHER_NODE,
                           vma, address, page_to_nid(page));
        if (unlikely(!pages[i] ||
                 mem_cgroup_newpage_charge(pages[i], mm,
                               GFP_KERNEL))) {
            if (pages[i])
                put_page(pages[i]);
            mem_cgroup_uncharge_start();
            while (--i >= 0) {
                mem_cgroup_uncharge_page(pages[i]);
                put_page(pages[i]);
            }
            mem_cgroup_uncharge_end();
            kfree(pages);
            ret |= VM_FAULT_OOM;
            goto out;
        }
    }

    for (i = 0; i < HPAGE_PMD_NR; i++) {
        copy_user_highpage(pages[i], page + i,
                   haddr + PAGE_SIZE * i, vma);
        __SetPageUptodate(pages[i]);
        cond_resched();
    }

    mmun_start = haddr;
    mmun_end   = haddr + HPAGE_PMD_SIZE;
    mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);

    spin_lock(&mm->page_table_lock);
    if (unlikely(!pmd_same(*pmd, orig_pmd)))
        goto out_free_pages;
    VM_BUG_ON(!PageHead(page));

    pmdp_clear_flush(vma, haddr, pmd);
    /* leave pmd empty until pte is filled */

    pgtable = pgtable_trans_huge_withdraw(mm);
    pmd_populate(mm, &_pmd, pgtable);

    for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
        pte_t *pte, entry;
        entry = mk_pte(pages[i], vma->vm_page_prot);
        entry = maybe_mkwrite(pte_mkdirty(entry), vma);
        page_add_new_anon_rmap(pages[i], vma, haddr);
        pte = pte_offset_map(&_pmd, haddr);
        VM_BUG_ON(!pte_none(*pte));
        set_pte_at(mm, haddr, pte, entry);
        pte_unmap(pte);
    }
    kfree(pages);

    smp_wmb(); /* make pte visible before pmd */
    pmd_populate(mm, pmd, pgtable);
    page_remove_rmap(page);
    spin_unlock(&mm->page_table_lock);

    mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);

    ret |= VM_FAULT_WRITE;
    put_page(page);

out:
    return ret;

out_free_pages:
    spin_unlock(&mm->page_table_lock);
    mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
    mem_cgroup_uncharge_start();
    for (i = 0; i < HPAGE_PMD_NR; i++) {
        mem_cgroup_uncharge_page(pages[i]);
        put_page(pages[i]);
    }
    mem_cgroup_uncharge_end();
    kfree(pages);
    goto out;
}

int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
            unsigned long address, pmd_t *pmd, pmd_t orig_pmd)
{
    int ret = 0;
    struct page *page, *new_page;
    unsigned long haddr;
    unsigned long mmun_start;   /* For mmu_notifiers */
    unsigned long mmun_end;     /* For mmu_notifiers */

    VM_BUG_ON(!vma->anon_vma);
    spin_lock(&mm->page_table_lock);
    if (unlikely(!pmd_same(*pmd, orig_pmd)))
        goto out_unlock;

    page = pmd_page(orig_pmd);
    VM_BUG_ON(!PageCompound(page) || !PageHead(page));
    haddr = address & HPAGE_PMD_MASK;
    if (page_mapcount(page) == 1) {
        pmd_t entry;
        entry = pmd_mkyoung(orig_pmd);
        entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
        if (pmdp_set_access_flags(vma, haddr, pmd, entry,  1))
            update_mmu_cache_pmd(vma, address, pmd);
        ret |= VM_FAULT_WRITE;
        goto out_unlock;
    }
    get_page(page);
    spin_unlock(&mm->page_table_lock);

    if (transparent_hugepage_enabled(vma) &&
        !transparent_hugepage_debug_cow())
        new_page = alloc_hugepage_vma(transparent_hugepage_defrag(vma),
                          vma, haddr, numa_node_id(), 0);
    else
        new_page = NULL;

    if (unlikely(!new_page)) {
        count_vm_event(THP_FAULT_FALLBACK);
        ret = do_huge_pmd_wp_page_fallback(mm, vma, address,
                           pmd, orig_pmd, page, haddr);
        if (ret & VM_FAULT_OOM)
            split_huge_page(page);
        put_page(page);
        goto out;
    }
    count_vm_event(THP_FAULT_ALLOC);

    if (unlikely(mem_cgroup_newpage_charge(new_page, mm, GFP_KERNEL))) {
        put_page(new_page);
        split_huge_page(page);
        put_page(page);
        ret |= VM_FAULT_OOM;
        goto out;
    }

    copy_user_huge_page(new_page, page, haddr, vma, HPAGE_PMD_NR);
    __SetPageUptodate(new_page);

    mmun_start = haddr;
    mmun_end   = haddr + HPAGE_PMD_SIZE;
    mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);

    spin_lock(&mm->page_table_lock);
    put_page(page);
    if (unlikely(!pmd_same(*pmd, orig_pmd))) {
        spin_unlock(&mm->page_table_lock);
        mem_cgroup_uncharge_page(new_page);
        put_page(new_page);
        goto out_mn;
    } else {
        pmd_t entry;
        VM_BUG_ON(!PageHead(page));
        entry = mk_pmd(new_page, vma->vm_page_prot);
        entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
        entry = pmd_mkhuge(entry);
        pmdp_clear_flush(vma, haddr, pmd);
        page_add_new_anon_rmap(new_page, vma, haddr);
        set_pmd_at(mm, haddr, pmd, entry);
        update_mmu_cache_pmd(vma, address, pmd);
        page_remove_rmap(page);
        put_page(page);
        ret |= VM_FAULT_WRITE;
    }
    spin_unlock(&mm->page_table_lock);
out_mn:
    mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
out:
    return ret;
out_unlock:
    spin_unlock(&mm->page_table_lock);
    return ret;
}

struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
                   unsigned long addr,
                   pmd_t *pmd,
                   unsigned int flags)
{
    struct mm_struct *mm = vma->vm_mm;
    struct page *page = NULL;

    assert_spin_locked(&mm->page_table_lock);

    if (flags & FOLL_WRITE && !pmd_write(*pmd))
        goto out;

    page = pmd_page(*pmd);
    VM_BUG_ON(!PageHead(page));
    if (flags & FOLL_TOUCH) {
        pmd_t _pmd;
        /*
         * We should set the dirty bit only for FOLL_WRITE but
         * for now the dirty bit in the pmd is meaningless.
         * And if the dirty bit will become meaningful and
         * we'll only set it with FOLL_WRITE, an atomic
         * set_bit will be required on the pmd to set the
         * young bit, instead of the current set_pmd_at.
         */
        _pmd = pmd_mkyoung(pmd_mkdirty(*pmd));
        set_pmd_at(mm, addr & HPAGE_PMD_MASK, pmd, _pmd);
    }
    if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
        if (page->mapping && trylock_page(page)) {
            lru_add_drain();
            if (page->mapping)
                mlock_vma_page(page);
            unlock_page(page);
        }
    }
    page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
    VM_BUG_ON(!PageCompound(page));
    if (flags & FOLL_GET)
        get_page_foll(page);

out:
    return page;
}

int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
         pmd_t *pmd, unsigned long addr)
{
    int ret = 0;

    if (__pmd_trans_huge_lock(pmd, vma) == 1) {
        struct page *page;
        pgtable_t pgtable;
        pmd_t orig_pmd;
        pgtable = pgtable_trans_huge_withdraw(tlb->mm);
        orig_pmd = pmdp_get_and_clear(tlb->mm, addr, pmd);
        page = pmd_page(orig_pmd);
        tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
        page_remove_rmap(page);
        VM_BUG_ON(page_mapcount(page) < 0);
        add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
        VM_BUG_ON(!PageHead(page));
        tlb->mm->nr_ptes--;
        spin_unlock(&tlb->mm->page_table_lock);
        tlb_remove_page(tlb, page);
        pte_free(tlb->mm, pgtable);
        ret = 1;
    }
    return ret;
}

int mincore_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
        unsigned long addr, unsigned long end,
        unsigned char *vec)
{
    int ret = 0;

    if (__pmd_trans_huge_lock(pmd, vma) == 1) {
        /*
         * All logical pages in the range are present
         * if backed by a huge page.
         */
        spin_unlock(&vma->vm_mm->page_table_lock);
        memset(vec, 1, (end - addr) >> PAGE_SHIFT);
        ret = 1;
    }

    return ret;
}

int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
          unsigned long old_addr,
          unsigned long new_addr, unsigned long old_end,
          pmd_t *old_pmd, pmd_t *new_pmd)
{
    int ret = 0;
    pmd_t pmd;

    struct mm_struct *mm = vma->vm_mm;

    if ((old_addr & ~HPAGE_PMD_MASK) ||
        (new_addr & ~HPAGE_PMD_MASK) ||
        old_end - old_addr < HPAGE_PMD_SIZE ||
        (new_vma->vm_flags & VM_NOHUGEPAGE))
        goto out;

    /*
     * The destination pmd shouldn't be established, free_pgtables()
     * should have release it.
     */
    if (WARN_ON(!pmd_none(*new_pmd))) {
        VM_BUG_ON(pmd_trans_huge(*new_pmd));
        goto out;
    }

    ret = __pmd_trans_huge_lock(old_pmd, vma);
    if (ret == 1) {
        pmd = pmdp_get_and_clear(mm, old_addr, old_pmd);
        VM_BUG_ON(!pmd_none(*new_pmd));
        set_pmd_at(mm, new_addr, new_pmd, pmd);
        spin_unlock(&mm->page_table_lock);
    }
out:
    return ret;
}

int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
        unsigned long addr, pgprot_t newprot)
{
    struct mm_struct *mm = vma->vm_mm;
    int ret = 0;

    if (__pmd_trans_huge_lock(pmd, vma) == 1) {
        pmd_t entry;
        entry = pmdp_get_and_clear(mm, addr, pmd);
        entry = pmd_modify(entry, newprot);
        set_pmd_at(mm, addr, pmd, entry);
        spin_unlock(&vma->vm_mm->page_table_lock);
        ret = 1;
    }

    return ret;
}

/*
 * Returns 1 if a given pmd maps a stable (not under splitting) thp.
 * Returns -1 if it maps a thp under splitting. Returns 0 otherwise.
 *
 * Note that if it returns 1, this routine returns without unlocking page
 * table locks. So callers must unlock them.
 */
int __pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma)
{
    spin_lock(&vma->vm_mm->page_table_lock);
    if (likely(pmd_trans_huge(*pmd))) {
        if (unlikely(pmd_trans_splitting(*pmd))) {
            spin_unlock(&vma->vm_mm->page_table_lock);
            wait_split_huge_page(vma->anon_vma, pmd);
            return -1;
        } else {
            /* Thp mapped by 'pmd' is stable, so we can
             * handle it as it is. */
            return 1;
        }
    }
    spin_unlock(&vma->vm_mm->page_table_lock);
    return 0;
}

pmd_t *page_check_address_pmd(struct page *page,
                  struct mm_struct *mm,
                  unsigned long address,
                  enum page_check_address_pmd_flag flag)
{
    pgd_t *pgd;
    pud_t *pud;
    pmd_t *pmd, *ret = NULL;

    if (address & ~HPAGE_PMD_MASK)
        goto out;

    pgd = pgd_offset(mm, address);
    if (!pgd_present(*pgd))
        goto out;

    pud = pud_offset(pgd, address);
    if (!pud_present(*pud))
        goto out;

    pmd = pmd_offset(pud, address);
    if (pmd_none(*pmd))
        goto out;
    if (pmd_page(*pmd) != page)
        goto out;
    /*
     * split_vma() may create temporary aliased mappings. There is
     * no risk as long as all huge pmd are found and have their
     * splitting bit set before __split_huge_page_refcount
     * runs. Finding the same huge pmd more than once during the
     * same rmap walk is not a problem.
     */
    if (flag == PAGE_CHECK_ADDRESS_PMD_NOTSPLITTING_FLAG &&
        pmd_trans_splitting(*pmd))
        goto out;
    if (pmd_trans_huge(*pmd)) {
        VM_BUG_ON(flag == PAGE_CHECK_ADDRESS_PMD_SPLITTING_FLAG &&
              !pmd_trans_splitting(*pmd));
        ret = pmd;
    }
out:
    return ret;
}

static int __split_huge_page_splitting(struct page *page,
                       struct vm_area_struct *vma,
                       unsigned long address)
{
    struct mm_struct *mm = vma->vm_mm;
    pmd_t *pmd;
    int ret = 0;
    /* For mmu_notifiers */
    const unsigned long mmun_start = address;
    const unsigned long mmun_end   = address + HPAGE_PMD_SIZE;

    mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
    spin_lock(&mm->page_table_lock);
    pmd = page_check_address_pmd(page, mm, address,
                     PAGE_CHECK_ADDRESS_PMD_NOTSPLITTING_FLAG);
    if (pmd) {
        /*
         * We can't temporarily set the pmd to null in order
         * to split it, the pmd must remain marked huge at all
         * times or the VM won't take the pmd_trans_huge paths
         * and it won't wait on the anon_vma->root->mutex to
         * serialize against split_huge_page*.
         */
        pmdp_splitting_flush(vma, address, pmd);
        ret = 1;
    }
    spin_unlock(&mm->page_table_lock);
    mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);

    return ret;
}

static void __split_huge_page_refcount(struct page *page)
{
    int i;
    struct zone *zone = page_zone(page);
    struct lruvec *lruvec;
    int tail_count = 0;

    /* prevent PageLRU to go away from under us, and freeze lru stats */
    spin_lock_irq(&zone->lru_lock);
    lruvec = mem_cgroup_page_lruvec(page, zone);

    compound_lock(page);
    /* complete memcg works before add pages to LRU */
    mem_cgroup_split_huge_fixup(page);

    for (i = HPAGE_PMD_NR - 1; i >= 1; i--) {
        struct page *page_tail = page + i;

        /* tail_page->_mapcount cannot change */
        BUG_ON(page_mapcount(page_tail) < 0);
        tail_count += page_mapcount(page_tail);
        /* check for overflow */
        BUG_ON(tail_count < 0);
        BUG_ON(atomic_read(&page_tail->_count) != 0);
        /*
         * tail_page->_count is zero and not changing from
         * under us. But get_page_unless_zero() may be running
         * from under us on the tail_page. If we used
         * atomic_set() below instead of atomic_add(), we
         * would then run atomic_set() concurrently with
         * get_page_unless_zero(), and atomic_set() is
         * implemented in C not using locked ops. spin_unlock
         * on x86 sometime uses locked ops because of PPro
         * errata 66, 92, so unless somebody can guarantee
         * atomic_set() here would be safe on all archs (and
         * not only on x86), it's safer to use atomic_add().
         */
        atomic_add(page_mapcount(page) + page_mapcount(page_tail) + 1,
               &page_tail->_count);

        /* after clearing PageTail the gup refcount can be released */
        smp_mb();

        /*
         * retain hwpoison flag of the poisoned tail page:
         *   fix for the unsuitable process killed on Guest Machine(KVM)
         *   by the memory-failure.
         */
        page_tail->flags &= ~PAGE_FLAGS_CHECK_AT_PREP | __PG_HWPOISON;
        page_tail->flags |= (page->flags &
                     ((1L << PG_referenced) |
                      (1L << PG_swapbacked) |
                      (1L << PG_mlocked) |
                      (1L << PG_uptodate)));
        page_tail->flags |= (1L << PG_dirty);

        /* clear PageTail before overwriting first_page */
        smp_wmb();

        /*
         * __split_huge_page_splitting() already set the
         * splitting bit in all pmd that could map this
         * hugepage, that will ensure no CPU can alter the
         * mapcount on the head page. The mapcount is only
         * accounted in the head page and it has to be
         * transferred to all tail pages in the below code. So
         * for this code to be safe, the split the mapcount
         * can't change. But that doesn't mean userland can't
         * keep changing and reading the page contents while
         * we transfer the mapcount, so the pmd splitting
         * status is achieved setting a reserved bit in the
         * pmd, not by clearing the present bit.
        */
        page_tail->_mapcount = page->_mapcount;

        BUG_ON(page_tail->mapping);
        page_tail->mapping = page->mapping;

        page_tail->index = page->index + i;

        BUG_ON(!PageAnon(page_tail));
        BUG_ON(!PageUptodate(page_tail));
        BUG_ON(!PageDirty(page_tail));
        BUG_ON(!PageSwapBacked(page_tail));

        lru_add_page_tail(page, page_tail, lruvec);
    }
    atomic_sub(tail_count, &page->_count);
    BUG_ON(atomic_read(&page->_count) <= 0);

    __mod_zone_page_state(zone, NR_ANON_TRANSPARENT_HUGEPAGES, -1);
    __mod_zone_page_state(zone, NR_ANON_PAGES, HPAGE_PMD_NR);

    ClearPageCompound(page);
    compound_unlock(page);
    spin_unlock_irq(&zone->lru_lock);

    for (i = 1; i < HPAGE_PMD_NR; i++) {
        struct page *page_tail = page + i;
        BUG_ON(page_count(page_tail) <= 0);
        /*
         * Tail pages may be freed if there wasn't any mapping
         * like if add_to_swap() is running on a lru page that
         * had its mapping zapped. And freeing these pages
         * requires taking the lru_lock so we do the put_page
         * of the tail pages after the split is complete.
         */
        put_page(page_tail);
    }

    /*
     * Only the head page (now become a regular page) is required
     * to be pinned by the caller.
     */
    BUG_ON(page_count(page) <= 0);
}

static int __split_huge_page_map(struct page *page,
                 struct vm_area_struct *vma,
                 unsigned long address)
{
    struct mm_struct *mm = vma->vm_mm;
    pmd_t *pmd, _pmd;
    int ret = 0, i;
    pgtable_t pgtable;
    unsigned long haddr;

    spin_lock(&mm->page_table_lock);
    pmd = page_check_address_pmd(page, mm, address,
                     PAGE_CHECK_ADDRESS_PMD_SPLITTING_FLAG);
    if (pmd) {
        pgtable = pgtable_trans_huge_withdraw(mm);
        pmd_populate(mm, &_pmd, pgtable);

        haddr = address;
        for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
            pte_t *pte, entry;
            BUG_ON(PageCompound(page+i));
            entry = mk_pte(page + i, vma->vm_page_prot);
            entry = maybe_mkwrite(pte_mkdirty(entry), vma);
            if (!pmd_write(*pmd))
                entry = pte_wrprotect(entry);
            else
                BUG_ON(page_mapcount(page) != 1);
            if (!pmd_young(*pmd))
                entry = pte_mkold(entry);
            pte = pte_offset_map(&_pmd, haddr);
            BUG_ON(!pte_none(*pte));
            set_pte_at(mm, haddr, pte, entry);
            pte_unmap(pte);
        }

        smp_wmb(); /* make pte visible before pmd */
        /*
         * Up to this point the pmd is present and huge and
         * userland has the whole access to the hugepage
         * during the split (which happens in place). If we
         * overwrite the pmd with the not-huge version
         * pointing to the pte here (which of course we could
         * if all CPUs were bug free), userland could trigger
         * a small page size TLB miss on the small sized TLB
         * while the hugepage TLB entry is still established
         * in the huge TLB. Some CPU doesn't like that. See
         * http://support.amd.com/us/Processor_TechDocs/41322.pdf,
         * Erratum 383 on page 93. Intel should be safe but is
         * also warns that it's only safe if the permission
         * and cache attributes of the two entries loaded in
         * the two TLB is identical (which should be the case
         * here). But it is generally safer to never allow
         * small and huge TLB entries for the same virtual
         * address to be loaded simultaneously. So instead of
         * doing "pmd_populate(); flush_tlb_range();" we first
         * mark the current pmd notpresent (atomically because
         * here the pmd_trans_huge and pmd_trans_splitting
         * must remain set at all times on the pmd until the
         * split is complete for this pmd), then we flush the
         * SMP TLB and finally we write the non-huge version
         * of the pmd entry with pmd_populate.
         */
        pmdp_invalidate(vma, address, pmd);
        pmd_populate(mm, pmd, pgtable);
        ret = 1;
    }
    spin_unlock(&mm->page_table_lock);

    return ret;
}

/* must be called with anon_vma->root->mutex hold */
static void __split_huge_page(struct page *page,
                  struct anon_vma *anon_vma)
{
    int mapcount, mapcount2;
    pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
    struct anon_vma_chain *avc;

    BUG_ON(!PageHead(page));
    BUG_ON(PageTail(page));

    mapcount = 0;
    anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
        struct vm_area_struct *vma = avc->vma;
        unsigned long addr = vma_address(page, vma);
        BUG_ON(is_vma_temporary_stack(vma));
        mapcount += __split_huge_page_splitting(page, vma, addr);
    }
    /*
     * It is critical that new vmas are added to the tail of the
     * anon_vma list. This guarantes that if copy_huge_pmd() runs
     * and establishes a child pmd before
     * __split_huge_page_splitting() freezes the parent pmd (so if
     * we fail to prevent copy_huge_pmd() from running until the
     * whole __split_huge_page() is complete), we will still see
     * the newly established pmd of the child later during the
     * walk, to be able to set it as pmd_trans_splitting too.
     */
    if (mapcount != page_mapcount(page))
        printk(KERN_ERR "mapcount %d page_mapcount %d\n",
               mapcount, page_mapcount(page));
    BUG_ON(mapcount != page_mapcount(page));

    __split_huge_page_refcount(page);

    mapcount2 = 0;
    anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
        struct vm_area_struct *vma = avc->vma;
        unsigned long addr = vma_address(page, vma);
        BUG_ON(is_vma_temporary_stack(vma));
        mapcount2 += __split_huge_page_map(page, vma, addr);
    }
    if (mapcount != mapcount2)
        printk(KERN_ERR "mapcount %d mapcount2 %d page_mapcount %d\n",
               mapcount, mapcount2, page_mapcount(page));
    BUG_ON(mapcount != mapcount2);
}

int split_huge_page(struct page *page)
{
    struct anon_vma *anon_vma;
    int ret = 1;

    BUG_ON(!PageAnon(page));
    anon_vma = page_lock_anon_vma(page);
    if (!anon_vma)
        goto out;
    ret = 0;
    if (!PageCompound(page))
        goto out_unlock;

    BUG_ON(!PageSwapBacked(page));
    __split_huge_page(page, anon_vma);
    count_vm_event(THP_SPLIT);

    BUG_ON(PageCompound(page));
out_unlock:
    page_unlock_anon_vma(anon_vma);
out:
    return ret;
}

#define VM_NO_THP (VM_SPECIAL|VM_MIXEDMAP|VM_HUGETLB|VM_SHARED|VM_MAYSHARE)

int hugepage_madvise(struct vm_area_struct *vma,
             unsigned long *vm_flags, int advice)
{
    struct mm_struct *mm = vma->vm_mm;

    switch (advice) {
    case MADV_HUGEPAGE:
        /*
         * Be somewhat over-protective like KSM for now!
         */
        if (*vm_flags & (VM_HUGEPAGE | VM_NO_THP))
            return -EINVAL;
        if (mm->def_flags & VM_NOHUGEPAGE)
            return -EINVAL;
        *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 (unlikely(khugepaged_enter_vma_merge(vma)))
            return -ENOMEM;
        break;
    case MADV_NOHUGEPAGE:
        /*
         * Be somewhat over-protective like KSM for now!
         */
        if (*vm_flags & (VM_NOHUGEPAGE | VM_NO_THP))
            return -EINVAL;
        *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;
}

static int __init khugepaged_slab_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;

    return 0;
}

static void __init khugepaged_slab_free(void)
{
    kmem_cache_destroy(mm_slot_cache);
    mm_slot_cache = NULL;
}

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 int __init mm_slots_hash_init(void)
{
    mm_slots_hash = kzalloc(MM_SLOTS_HASH_HEADS * sizeof(struct hlist_head),
                GFP_KERNEL);
    if (!mm_slots_hash)
        return -ENOMEM;
    return 0;
}

#if 0
static void __init mm_slots_hash_free(void)
{
    kfree(mm_slots_hash);
    mm_slots_hash = NULL;
}
#endif

static struct mm_slot *get_mm_slot(struct mm_struct *mm)
{
    struct mm_slot *mm_slot;
    struct hlist_head *bucket;
    struct hlist_node *node;

    bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct))
                % MM_SLOTS_HASH_HEADS];
    hlist_for_each_entry(mm_slot, node, bucket, hash) {
        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)
{
    struct hlist_head *bucket;

    bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct))
                % MM_SLOTS_HASH_HEADS];
    mm_slot->mm = mm;
    hlist_add_head(&mm_slot->hash, bucket);
}

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

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(khugepaged_test_exit(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);

    atomic_inc(&mm->mm_count);
    if (wakeup)
        wake_up_interruptible(&khugepaged_wait);

    return 0;
}

int khugepaged_enter_vma_merge(struct vm_area_struct *vma)
{
    unsigned long hstart, hend;
    if (!vma->anon_vma)
        /*
         * Not yet faulted in so we will register later in the
         * page fault if needed.
         */
        return 0;
    if (vma->vm_ops)
        /* khugepaged not yet working on file or special mappings */
        return 0;
    VM_BUG_ON(vma->vm_flags & VM_NO_THP);
    hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
    hend = vma->vm_end & HPAGE_PMD_MASK;
    if (hstart < hend)
        return khugepaged_enter(vma);
    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) {
        hlist_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)
{
    /* 0 stands for page_is_file_cache(page) == false */
    dec_zone_page_state(page, NR_ISOLATED_ANON + 0);
    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))
            release_pte_page(pte_page(pteval));
    }
}

static void release_all_pte_pages(pte_t *pte)
{
    release_pte_pages(pte, pte + HPAGE_PMD_NR);
}

static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
                    unsigned long address,
                    pte_t *pte)
{
    struct page *page;
    pte_t *_pte;
    int referenced = 0, isolated = 0, none = 0;
    for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
         _pte++, address += PAGE_SIZE) {
        pte_t pteval = *_pte;
        if (pte_none(pteval)) {
            if (++none <= khugepaged_max_ptes_none)
                continue;
            else {
                release_pte_pages(pte, _pte);
                goto out;
            }
        }
        if (!pte_present(pteval) || !pte_write(pteval)) {
            release_pte_pages(pte, _pte);
            goto out;
        }
        page = vm_normal_page(vma, address, pteval);
        if (unlikely(!page)) {
            release_pte_pages(pte, _pte);
            goto out;
        }
        VM_BUG_ON(PageCompound(page));
        BUG_ON(!PageAnon(page));
        VM_BUG_ON(!PageSwapBacked(page));

        /* cannot use mapcount: can't collapse if there's a gup pin */
        if (page_count(page) != 1) {
            release_pte_pages(pte, _pte);
            goto out;
        }
        /*
         * 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)) {
            release_pte_pages(pte, _pte);
            goto out;
        }
        /*
         * Isolate the page to avoid collapsing an hugepage
         * currently in use by the VM.
         */
        if (isolate_lru_page(page)) {
            unlock_page(page);
            release_pte_pages(pte, _pte);
            goto out;
        }
        /* 0 stands for page_is_file_cache(page) == false */
        inc_zone_page_state(page, NR_ISOLATED_ANON + 0);
        VM_BUG_ON(!PageLocked(page));
        VM_BUG_ON(PageLRU(page));

        /* If there is no mapped pte young don't collapse the page */
        if (pte_young(pteval) || PageReferenced(page) ||
            mmu_notifier_test_young(vma->vm_mm, address))
            referenced = 1;
    }
    if (unlikely(!referenced))
        release_all_pte_pages(pte);
    else
        isolated = 1;
out:
    return isolated;
}

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++) {
        pte_t pteval = *_pte;
        struct page *src_page;

        if (pte_none(pteval)) {
            clear_user_highpage(page, address);
            add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
        } else {
            src_page = pte_page(pteval);
            copy_user_highpage(page, src_page, address, vma);
            VM_BUG_ON(page_mapcount(src_page) != 1);
            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);
            spin_unlock(ptl);
            free_page_and_swap_cache(src_page);
        }

        address += PAGE_SIZE;
        page++;
    }
}

static void khugepaged_alloc_sleep(void)
{
    wait_event_freezable_timeout(khugepaged_wait, false,
            msecs_to_jiffies(khugepaged_alloc_sleep_millisecs));
}

#ifdef CONFIG_NUMA
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, struct mm_struct *mm,
               struct vm_area_struct *vma, unsigned long address,
               int node)
{
    VM_BUG_ON(*hpage);
    /*
     * Allocate the page while the vma is still valid and under
     * the mmap_sem read mode so there is no memory allocation
     * later when we take the mmap_sem in write mode. This is more
     * friendly behavior (OTOH it may actually hide bugs) to
     * filesystems in userland with daemons allocating memory in
     * the userland I/O paths.  Allocating memory with the
     * mmap_sem in read mode is good idea also to allow greater
     * scalability.
     */
    *hpage  = alloc_hugepage_vma(khugepaged_defrag(), vma, address,
                      node, __GFP_OTHER_NODE);

    /*
     * After allocating the hugepage, release the mmap_sem read lock in
     * preparation for taking it in write mode.
     */
    up_read(&mm->mmap_sem);
    if (unlikely(!*hpage)) {
        count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
        *hpage = ERR_PTR(-ENOMEM);
        return NULL;
    }

    count_vm_event(THP_COLLAPSE_ALLOC);
    return *hpage;
}
#else
static struct page *khugepaged_alloc_hugepage(bool *wait)
{
    struct page *hpage;

    do {
        hpage = alloc_hugepage(khugepaged_defrag());
        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, struct mm_struct *mm,
               struct vm_area_struct *vma, unsigned long address,
               int node)
{
    up_read(&mm->mmap_sem);
    VM_BUG_ON(!*hpage);
    return  *hpage;
}
#endif

static void collapse_huge_page(struct mm_struct *mm,
                   unsigned long address,
                   struct page **hpage,
                   struct vm_area_struct *vma,
                   int node)
{
    pgd_t *pgd;
    pud_t *pud;
    pmd_t *pmd, _pmd;
    pte_t *pte;
    pgtable_t pgtable;
    struct page *new_page;
    spinlock_t *ptl;
    int isolated;
    unsigned long hstart, hend;
    unsigned long mmun_start;   /* For mmu_notifiers */
    unsigned long mmun_end;     /* For mmu_notifiers */

    VM_BUG_ON(address & ~HPAGE_PMD_MASK);

    /* release the mmap_sem read lock. */
    new_page = khugepaged_alloc_page(hpage, mm, vma, address, node);
    if (!new_page)
        return;

    if (unlikely(mem_cgroup_newpage_charge(new_page, mm, GFP_KERNEL)))
        return;

    /*
     * Prevent all access to pagetables with the exception of
     * gup_fast later hanlded by the ptep_clear_flush and the VM
     * handled by the anon_vma lock + PG_lock.
     */
    down_write(&mm->mmap_sem);
    if (unlikely(khugepaged_test_exit(mm)))
        goto out;

    vma = find_vma(mm, address);
    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)
        goto out;

    if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
        (vma->vm_flags & VM_NOHUGEPAGE))
        goto out;

    if (!vma->anon_vma || vma->vm_ops)
        goto out;
    if (is_vma_temporary_stack(vma))
        goto out;
    VM_BUG_ON(vma->vm_flags & VM_NO_THP);

    pgd = pgd_offset(mm, address);
    if (!pgd_present(*pgd))
        goto out;

    pud = pud_offset(pgd, address);
    if (!pud_present(*pud))
        goto out;

    pmd = pmd_offset(pud, address);
    /* pmd can't go away or become huge under us */
    if (!pmd_present(*pmd) || pmd_trans_huge(*pmd))
        goto out;

    anon_vma_lock(vma->anon_vma);

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

    mmun_start = address;
    mmun_end   = address + HPAGE_PMD_SIZE;
    mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
    spin_lock(&mm->page_table_lock); /* 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_clear_flush(vma, address, pmd);
    spin_unlock(&mm->page_table_lock);
    mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);

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

    if (unlikely(!isolated)) {
        pte_unmap(pte);
        spin_lock(&mm->page_table_lock);
        BUG_ON(!pmd_none(*pmd));
        set_pmd_at(mm, address, pmd, _pmd);
        spin_unlock(&mm->page_table_lock);
        anon_vma_unlock(vma->anon_vma);
        goto out;
    }

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

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

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

    /*
     * 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(&mm->page_table_lock);
    BUG_ON(!pmd_none(*pmd));
    page_add_new_anon_rmap(new_page, vma, address);
    set_pmd_at(mm, address, pmd, _pmd);
    update_mmu_cache_pmd(vma, address, pmd);
    pgtable_trans_huge_deposit(mm, pgtable);
    spin_unlock(&mm->page_table_lock);

    *hpage = NULL;

    khugepaged_pages_collapsed++;
out_up_write:
    up_write(&mm->mmap_sem);
    return;

out:
    mem_cgroup_uncharge_page(new_page);
    goto out_up_write;
}

static int khugepaged_scan_pmd(struct mm_struct *mm,
                   struct vm_area_struct *vma,
                   unsigned long address,
                   struct page **hpage)
{
    pgd_t *pgd;
    pud_t *pud;
    pmd_t *pmd;
    pte_t *pte, *_pte;
    int ret = 0, referenced = 0, none = 0;
    struct page *page;
    unsigned long _address;
    spinlock_t *ptl;
    int node = -1;

    VM_BUG_ON(address & ~HPAGE_PMD_MASK);

    pgd = pgd_offset(mm, address);
    if (!pgd_present(*pgd))
        goto out;

    pud = pud_offset(pgd, address);
    if (!pud_present(*pud))
        goto out;

    pmd = pmd_offset(pud, address);
    if (!pmd_present(*pmd) || pmd_trans_huge(*pmd))
        goto out;

    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 (pte_none(pteval)) {
            if (++none <= khugepaged_max_ptes_none)
                continue;
            else
                goto out_unmap;
        }
        if (!pte_present(pteval) || !pte_write(pteval))
            goto out_unmap;
        page = vm_normal_page(vma, _address, pteval);
        if (unlikely(!page))
            goto out_unmap;
        /*
         * Chose the node of the first page. This could
         * be more sophisticated and look at more pages,
         * but isn't for now.
         */
        if (node == -1)
            node = page_to_nid(page);
        VM_BUG_ON(PageCompound(page));
        if (!PageLRU(page) || PageLocked(page) || !PageAnon(page))
            goto out_unmap;
        /* cannot use mapcount: can't collapse if there's a gup pin */
        if (page_count(page) != 1)
            goto out_unmap;
        if (pte_young(pteval) || PageReferenced(page) ||
            mmu_notifier_test_young(vma->vm_mm, address))
            referenced = 1;
    }
    if (referenced)
        ret = 1;
out_unmap:
    pte_unmap_unlock(pte, ptl);
    if (ret)
        /* collapse_huge_page will return with the mmap_sem released */
        collapse_huge_page(mm, address, hpage, vma, node);
out:
    return ret;
}

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

    VM_BUG_ON(NR_CPUS != 1 && !spin_is_locked(&khugepaged_mm_lock));

    if (khugepaged_test_exit(mm)) {
        /* free mm_slot */
        hlist_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);
    }
}

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);
    VM_BUG_ON(NR_CPUS != 1 && !spin_is_locked(&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);

    mm = mm_slot->mm;
    down_read(&mm->mmap_sem);
    if (unlikely(khugepaged_test_exit(mm)))
        vma = NULL;
    else
        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 ((!(vma->vm_flags & VM_HUGEPAGE) &&
             !khugepaged_always()) ||
            (vma->vm_flags & VM_NOHUGEPAGE)) {
        skip:
            progress++;
            continue;
        }
        if (!vma->anon_vma || vma->vm_ops)
            goto skip;
        if (is_vma_temporary_stack(vma))
            goto skip;
        VM_BUG_ON(vma->vm_flags & VM_NO_THP);

        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);
            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() || freezing(current)))
            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 void khugepaged_wait_work(void)
{
    try_to_freeze();

    if (khugepaged_has_work()) {
        if (!khugepaged_scan_sleep_millisecs)
            return;

        wait_event_freezable_timeout(khugepaged_wait,
                         kthread_should_stop(),
            msecs_to_jiffies(khugepaged_scan_sleep_millisecs));
        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, 19);

    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;
}

void __split_huge_page_pmd(struct mm_struct *mm, pmd_t *pmd)
{
    struct page *page;

    spin_lock(&mm->page_table_lock);
    if (unlikely(!pmd_trans_huge(*pmd))) {
        spin_unlock(&mm->page_table_lock);
        return;
    }
    page = pmd_page(*pmd);
    VM_BUG_ON(!page_count(page));
    get_page(page);
    spin_unlock(&mm->page_table_lock);

    split_huge_page(page);

    put_page(page);
    BUG_ON(pmd_trans_huge(*pmd));
}

static void split_huge_page_address(struct mm_struct *mm,
                    unsigned long address)
{
    pgd_t *pgd;
    pud_t *pud;
    pmd_t *pmd;

    VM_BUG_ON(!(address & ~HPAGE_PMD_MASK));

    pgd = pgd_offset(mm, address);
    if (!pgd_present(*pgd))
        return;

    pud = pud_offset(pgd, address);
    if (!pud_present(*pud))
        return;

    pmd = pmd_offset(pud, address);
    if (!pmd_present(*pmd))
        return;
    /*
     * Caller holds the mmap_sem write mode, so a huge pmd cannot
     * materialize from under us.
     */
    split_huge_page_pmd(mm, pmd);
}

void __vma_adjust_trans_huge(struct vm_area_struct *vma,
                 unsigned long start,
                 unsigned long end,
                 long adjust_next)
{
    /*
     * If the new start address isn't hpage aligned and it could
     * previously contain an hugepage: check if we need to split
     * an huge pmd.
     */
    if (start & ~HPAGE_PMD_MASK &&
        (start & HPAGE_PMD_MASK) >= vma->vm_start &&
        (start & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= vma->vm_end)
        split_huge_page_address(vma->vm_mm, start);

    /*
     * If the new end address isn't hpage aligned and it could
     * previously contain an hugepage: check if we need to split
     * an huge pmd.
     */
    if (end & ~HPAGE_PMD_MASK &&
        (end & HPAGE_PMD_MASK) >= vma->vm_start &&
        (end & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= vma->vm_end)
        split_huge_page_address(vma->vm_mm, end);

    /*
     * If we're also updating the vma->vm_next->vm_start, if the new
     * vm_next->vm_start isn't page aligned and it could previously
     * contain an hugepage: check if we need to split an huge pmd.
     */
    if (adjust_next > 0) {
        struct vm_area_struct *next = vma->vm_next;
        unsigned long nstart = next->vm_start;
        nstart += adjust_next << PAGE_SHIFT;
        if (nstart & ~HPAGE_PMD_MASK &&
            (nstart & HPAGE_PMD_MASK) >= next->vm_start &&
            (nstart & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= next->vm_end)
            split_huge_page_address(next->vm_mm, nstart);
    }
}