mm: workingset: move shadow entry tracking to radix tree exceptional tracking

[mirror_ubuntu-artful-kernel.git] / mm / huge_memory.c

/*
 *  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.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#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/shrinker.h>
#include <linux/mm_inline.h>
#include <linux/swapops.h>
#include <linux/dax.h>
#include <linux/khugepaged.h>
#include <linux/freezer.h>
#include <linux/pfn_t.h>
#include <linux/mman.h>
#include <linux/memremap.h>
#include <linux/pagemap.h>
#include <linux/debugfs.h>
#include <linux/migrate.h>
#include <linux/hashtable.h>
#include <linux/userfaultfd_k.h>
#include <linux/page_idle.h>
#include <linux/shmem_fs.h>

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

/*
 * By default transparent hugepage support is disabled in order that avoid
 * to risk increase the memory footprint of applications without a guaranteed
 * benefit. When transparent hugepage support is enabled, is for all mappings,
 * and khugepaged scans all mappings.
 * Defrag is invoked by khugepaged hugepage allocations and by page faults
 * for all hugepage 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_REQ_MADV_FLAG)|
        (1<<TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG)|
        (1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG);

static struct shrinker deferred_split_shrinker;

static atomic_t huge_zero_refcount;
struct page *huge_zero_page __read_mostly;

static struct page *get_huge_zero_page(void)
{
        struct page *zero_page;
retry:
        if (likely(atomic_inc_not_zero(&huge_zero_refcount)))
                return READ_ONCE(huge_zero_page);

        zero_page = alloc_pages((GFP_TRANSHUGE | __GFP_ZERO) & ~__GFP_MOVABLE,
                        HPAGE_PMD_ORDER);
        if (!zero_page) {
                count_vm_event(THP_ZERO_PAGE_ALLOC_FAILED);
                return NULL;
        }
        count_vm_event(THP_ZERO_PAGE_ALLOC);
        preempt_disable();
        if (cmpxchg(&huge_zero_page, NULL, zero_page)) {
                preempt_enable();
                __free_pages(zero_page, compound_order(zero_page));
                goto retry;
        }

        /* We take additional reference here. It will be put back by shrinker */
        atomic_set(&huge_zero_refcount, 2);
        preempt_enable();
        return READ_ONCE(huge_zero_page);
}

static void put_huge_zero_page(void)
{
        /*
         * Counter should never go to zero here. Only shrinker can put
         * last reference.
         */
        BUG_ON(atomic_dec_and_test(&huge_zero_refcount));
}

struct page *mm_get_huge_zero_page(struct mm_struct *mm)
{
        if (test_bit(MMF_HUGE_ZERO_PAGE, &mm->flags))
                return READ_ONCE(huge_zero_page);

        if (!get_huge_zero_page())
                return NULL;

        if (test_and_set_bit(MMF_HUGE_ZERO_PAGE, &mm->flags))
                put_huge_zero_page();

        return READ_ONCE(huge_zero_page);
}

void mm_put_huge_zero_page(struct mm_struct *mm)
{
        if (test_bit(MMF_HUGE_ZERO_PAGE, &mm->flags))
                put_huge_zero_page();
}

static unsigned long shrink_huge_zero_page_count(struct shrinker *shrink,
                                        struct shrink_control *sc)
{
        /* we can free zero page only if last reference remains */
        return atomic_read(&huge_zero_refcount) == 1 ? HPAGE_PMD_NR : 0;
}

static unsigned long shrink_huge_zero_page_scan(struct shrinker *shrink,
                                       struct shrink_control *sc)
{
        if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) == 1) {
                struct page *zero_page = xchg(&huge_zero_page, NULL);
                BUG_ON(zero_page == NULL);
                __free_pages(zero_page, compound_order(zero_page));
                return HPAGE_PMD_NR;
        }

        return 0;
}

static struct shrinker huge_zero_page_shrinker = {
        .count_objects = shrink_huge_zero_page_count,
        .scan_objects = shrink_huge_zero_page_scan,
        .seeks = DEFAULT_SEEKS,
};

#ifdef CONFIG_SYSFS

static ssize_t triple_flag_store(struct kobject *kobj,
                                 struct kobj_attribute *attr,
                                 const char *buf, size_t count,
                                 enum transparent_hugepage_flag enabled,
                                 enum transparent_hugepage_flag deferred,
                                 enum transparent_hugepage_flag req_madv)
{
        if (!memcmp("defer", buf,
                    min(sizeof("defer")-1, count))) {
                if (enabled == deferred)
                        return -EINVAL;
                clear_bit(enabled, &transparent_hugepage_flags);
                clear_bit(req_madv, &transparent_hugepage_flags);
                set_bit(deferred, &transparent_hugepage_flags);
        } else if (!memcmp("always", buf,
                    min(sizeof("always")-1, count))) {
                clear_bit(deferred, &transparent_hugepage_flags);
                clear_bit(req_madv, &transparent_hugepage_flags);
                set_bit(enabled, &transparent_hugepage_flags);
        } else if (!memcmp("madvise", buf,
                           min(sizeof("madvise")-1, count))) {
                clear_bit(enabled, &transparent_hugepage_flags);
                clear_bit(deferred, &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);
                clear_bit(deferred, &transparent_hugepage_flags);
        } else
                return -EINVAL;

        return count;
}

static ssize_t enabled_show(struct kobject *kobj,
                            struct kobj_attribute *attr, char *buf)
{
        if (test_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags))
                return sprintf(buf, "[always] madvise never\n");
        else if (test_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags))
                return sprintf(buf, "always [madvise] never\n");
        else
                return sprintf(buf, "always madvise [never]\n");
}

static ssize_t enabled_store(struct kobject *kobj,
                             struct kobj_attribute *attr,
                             const char *buf, size_t count)
{
        ssize_t ret;

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

        if (ret > 0) {
                int err = start_stop_khugepaged();
                if (err)
                        ret = err;
        }

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

ssize_t single_hugepage_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));
}

ssize_t single_hugepage_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)
{
        if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG, &transparent_hugepage_flags))
                return sprintf(buf, "[always] defer madvise never\n");
        if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG, &transparent_hugepage_flags))
                return sprintf(buf, "always [defer] madvise never\n");
        else if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG, &transparent_hugepage_flags))
                return sprintf(buf, "always defer [madvise] never\n");
        else
                return sprintf(buf, "always defer madvise [never]\n");

}
static ssize_t defrag_store(struct kobject *kobj,
                            struct kobj_attribute *attr,
                            const char *buf, size_t count)
{
        return triple_flag_store(kobj, attr, buf, count,
                                 TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG,
                                 TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG,
                                 TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG);
}
static struct kobj_attribute defrag_attr =
        __ATTR(defrag, 0644, defrag_show, defrag_store);

static ssize_t use_zero_page_show(struct kobject *kobj,
                struct kobj_attribute *attr, char *buf)
{
        return single_hugepage_flag_show(kobj, attr, buf,
                                TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG);
}
static ssize_t use_zero_page_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_USE_ZERO_PAGE_FLAG);
}
static struct kobj_attribute use_zero_page_attr =
        __ATTR(use_zero_page, 0644, use_zero_page_show, use_zero_page_store);
#ifdef CONFIG_DEBUG_VM
static ssize_t debug_cow_show(struct kobject *kobj,
                                struct kobj_attribute *attr, char *buf)
{
        return single_hugepage_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_hugepage_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,
        &use_zero_page_attr.attr,
#if defined(CONFIG_SHMEM) && defined(CONFIG_TRANSPARENT_HUGE_PAGECACHE)
        &shmem_enabled_attr.attr,
#endif
#ifdef CONFIG_DEBUG_VM
        &debug_cow_attr.attr,
#endif
        NULL,
};

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

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)) {
                pr_err("failed to create transparent hugepage kobject\n");
                return -ENOMEM;
        }

        err = sysfs_create_group(*hugepage_kobj, &hugepage_attr_group);
        if (err) {
                pr_err("failed to register transparent hugepage group\n");
                goto delete_obj;
        }

        err = sysfs_create_group(*hugepage_kobj, &khugepaged_attr_group);
        if (err) {
                pr_err("failed to register transparent hugepage 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;
        }

        /*
         * hugepages can't be allocated by the buddy allocator
         */
        MAYBE_BUILD_BUG_ON(HPAGE_PMD_ORDER >= MAX_ORDER);
        /*
         * we use page->mapping and page->index in second tail page
         * as list_head: assuming THP order >= 2
         */
        MAYBE_BUILD_BUG_ON(HPAGE_PMD_ORDER < 2);

        err = hugepage_init_sysfs(&hugepage_kobj);
        if (err)
                goto err_sysfs;

        err = khugepaged_init();
        if (err)
                goto err_slab;

        err = register_shrinker(&huge_zero_page_shrinker);
        if (err)
                goto err_hzp_shrinker;
        err = register_shrinker(&deferred_split_shrinker);
        if (err)
                goto err_split_shrinker;

        /*
         * 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;
                return 0;
        }

        err = start_stop_khugepaged();
        if (err)
                goto err_khugepaged;

        return 0;
err_khugepaged:
        unregister_shrinker(&deferred_split_shrinker);
err_split_shrinker:
        unregister_shrinker(&huge_zero_page_shrinker);
err_hzp_shrinker:
        khugepaged_destroy();
err_slab:
        hugepage_exit_sysfs(hugepage_kobj);
err_sysfs:
        return err;
}
subsys_initcall(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)
                pr_warn("transparent_hugepage= cannot parse, ignored\n");
        return ret;
}
__setup("transparent_hugepage=", setup_transparent_hugepage);

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 inline struct list_head *page_deferred_list(struct page *page)
{
        /*
         * ->lru in the tail pages is occupied by compound_head.
         * Let's use ->mapping + ->index in the second tail page as list_head.
         */
        return (struct list_head *)&page[2].mapping;
}

void prep_transhuge_page(struct page *page)
{
        /*
         * we use page->mapping and page->indexlru in second tail page
         * as list_head: assuming THP order >= 2
         */

        INIT_LIST_HEAD(page_deferred_list(page));
        set_compound_page_dtor(page, TRANSHUGE_PAGE_DTOR);
}

unsigned long __thp_get_unmapped_area(struct file *filp, unsigned long len,
                loff_t off, unsigned long flags, unsigned long size)
{
        unsigned long addr;
        loff_t off_end = off + len;
        loff_t off_align = round_up(off, size);
        unsigned long len_pad;

        if (off_end <= off_align || (off_end - off_align) < size)
                return 0;

        len_pad = len + size;
        if (len_pad < len || (off + len_pad) < off)
                return 0;

        addr = current->mm->get_unmapped_area(filp, 0, len_pad,
                                              off >> PAGE_SHIFT, flags);
        if (IS_ERR_VALUE(addr))
                return 0;

        addr += (off - addr) & (size - 1);
        return addr;
}

unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
                unsigned long len, unsigned long pgoff, unsigned long flags)
{
        loff_t off = (loff_t)pgoff << PAGE_SHIFT;

        if (addr)
                goto out;
        if (!IS_DAX(filp->f_mapping->host) || !IS_ENABLED(CONFIG_FS_DAX_PMD))
                goto out;

        addr = __thp_get_unmapped_area(filp, len, off, flags, PMD_SIZE);
        if (addr)
                return addr;

 out:
        return current->mm->get_unmapped_area(filp, addr, len, pgoff, flags);
}
EXPORT_SYMBOL_GPL(thp_get_unmapped_area);

static int __do_huge_pmd_anonymous_page(struct fault_env *fe, struct page *page,
                gfp_t gfp)
{
        struct vm_area_struct *vma = fe->vma;
        struct mem_cgroup *memcg;
        pgtable_t pgtable;
        unsigned long haddr = fe->address & HPAGE_PMD_MASK;

        VM_BUG_ON_PAGE(!PageCompound(page), page);

        if (mem_cgroup_try_charge(page, vma->vm_mm, gfp, &memcg, true)) {
                put_page(page);
                count_vm_event(THP_FAULT_FALLBACK);
                return VM_FAULT_FALLBACK;
        }

        pgtable = pte_alloc_one(vma->vm_mm, haddr);
        if (unlikely(!pgtable)) {
                mem_cgroup_cancel_charge(page, memcg, true);
                put_page(page);
                return VM_FAULT_OOM;
        }

        clear_huge_page(page, haddr, HPAGE_PMD_NR);
        /*
         * The memory barrier inside __SetPageUptodate makes sure that
         * clear_huge_page writes become visible before the set_pmd_at()
         * write.
         */
        __SetPageUptodate(page);

        fe->ptl = pmd_lock(vma->vm_mm, fe->pmd);
        if (unlikely(!pmd_none(*fe->pmd))) {
                spin_unlock(fe->ptl);
                mem_cgroup_cancel_charge(page, memcg, true);
                put_page(page);
                pte_free(vma->vm_mm, pgtable);
        } else {
                pmd_t entry;

                /* Deliver the page fault to userland */
                if (userfaultfd_missing(vma)) {
                        int ret;

                        spin_unlock(fe->ptl);
                        mem_cgroup_cancel_charge(page, memcg, true);
                        put_page(page);
                        pte_free(vma->vm_mm, pgtable);
                        ret = handle_userfault(fe, VM_UFFD_MISSING);
                        VM_BUG_ON(ret & VM_FAULT_FALLBACK);
                        return ret;
                }

                entry = mk_huge_pmd(page, vma->vm_page_prot);
                entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
                page_add_new_anon_rmap(page, vma, haddr, true);
                mem_cgroup_commit_charge(page, memcg, false, true);
                lru_cache_add_active_or_unevictable(page, vma);
                pgtable_trans_huge_deposit(vma->vm_mm, fe->pmd, pgtable);
                set_pmd_at(vma->vm_mm, haddr, fe->pmd, entry);
                add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
                atomic_long_inc(&vma->vm_mm->nr_ptes);
                spin_unlock(fe->ptl);
                count_vm_event(THP_FAULT_ALLOC);
        }

        return 0;
}

/*
 * If THP defrag is set to always then directly reclaim/compact as necessary
 * If set to defer then do only background reclaim/compact and defer to khugepaged
 * If set to madvise and the VMA is flagged then directly reclaim/compact
 * When direct reclaim/compact is allowed, don't retry except for flagged VMA's
 */
static inline gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma)
{
        bool vma_madvised = !!(vma->vm_flags & VM_HUGEPAGE);

        if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG,
                                &transparent_hugepage_flags) && vma_madvised)
                return GFP_TRANSHUGE;
        else if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG,
                                                &transparent_hugepage_flags))
                return GFP_TRANSHUGE_LIGHT | __GFP_KSWAPD_RECLAIM;
        else if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG,
                                                &transparent_hugepage_flags))
                return GFP_TRANSHUGE | (vma_madvised ? 0 : __GFP_NORETRY);

        return GFP_TRANSHUGE_LIGHT;
}

/* Caller must hold page table lock. */
static bool set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
                struct vm_area_struct *vma, unsigned long haddr, pmd_t *pmd,
                struct page *zero_page)
{
        pmd_t entry;
        if (!pmd_none(*pmd))
                return false;
        entry = mk_pmd(zero_page, vma->vm_page_prot);
        entry = pmd_mkhuge(entry);
        if (pgtable)
                pgtable_trans_huge_deposit(mm, pmd, pgtable);
        set_pmd_at(mm, haddr, pmd, entry);
        atomic_long_inc(&mm->nr_ptes);
        return true;
}

int do_huge_pmd_anonymous_page(struct fault_env *fe)
{
        struct vm_area_struct *vma = fe->vma;
        gfp_t gfp;
        struct page *page;
        unsigned long haddr = fe->address & HPAGE_PMD_MASK;

        if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
                return VM_FAULT_FALLBACK;
        if (unlikely(anon_vma_prepare(vma)))
                return VM_FAULT_OOM;
        if (unlikely(khugepaged_enter(vma, vma->vm_flags)))
                return VM_FAULT_OOM;
        if (!(fe->flags & FAULT_FLAG_WRITE) &&
                        !mm_forbids_zeropage(vma->vm_mm) &&
                        transparent_hugepage_use_zero_page()) {
                pgtable_t pgtable;
                struct page *zero_page;
                bool set;
                int ret;
                pgtable = pte_alloc_one(vma->vm_mm, haddr);
                if (unlikely(!pgtable))
                        return VM_FAULT_OOM;
                zero_page = mm_get_huge_zero_page(vma->vm_mm);
                if (unlikely(!zero_page)) {
                        pte_free(vma->vm_mm, pgtable);
                        count_vm_event(THP_FAULT_FALLBACK);
                        return VM_FAULT_FALLBACK;
                }
                fe->ptl = pmd_lock(vma->vm_mm, fe->pmd);
                ret = 0;
                set = false;
                if (pmd_none(*fe->pmd)) {
                        if (userfaultfd_missing(vma)) {
                                spin_unlock(fe->ptl);
                                ret = handle_userfault(fe, VM_UFFD_MISSING);
                                VM_BUG_ON(ret & VM_FAULT_FALLBACK);
                        } else {
                                set_huge_zero_page(pgtable, vma->vm_mm, vma,
                                                   haddr, fe->pmd, zero_page);
                                spin_unlock(fe->ptl);
                                set = true;
                        }
                } else
                        spin_unlock(fe->ptl);
                if (!set)
                        pte_free(vma->vm_mm, pgtable);
                return ret;
        }
        gfp = alloc_hugepage_direct_gfpmask(vma);
        page = alloc_hugepage_vma(gfp, vma, haddr, HPAGE_PMD_ORDER);
        if (unlikely(!page)) {
                count_vm_event(THP_FAULT_FALLBACK);
                return VM_FAULT_FALLBACK;
        }
        prep_transhuge_page(page);
        return __do_huge_pmd_anonymous_page(fe, page, gfp);
}

static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
                pmd_t *pmd, pfn_t pfn, pgprot_t prot, bool write)
{
        struct mm_struct *mm = vma->vm_mm;
        pmd_t entry;
        spinlock_t *ptl;

        ptl = pmd_lock(mm, pmd);
        entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
        if (pfn_t_devmap(pfn))
                entry = pmd_mkdevmap(entry);
        if (write) {
                entry = pmd_mkyoung(pmd_mkdirty(entry));
                entry = maybe_pmd_mkwrite(entry, vma);
        }
        set_pmd_at(mm, addr, pmd, entry);
        update_mmu_cache_pmd(vma, addr, pmd);
        spin_unlock(ptl);
}

int vmf_insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
                        pmd_t *pmd, pfn_t pfn, bool write)
{
        pgprot_t pgprot = vma->vm_page_prot;
        /*
         * If we had pmd_special, we could avoid all these restrictions,
         * but we need to be consistent with PTEs and architectures that
         * can't support a 'special' bit.
         */
        BUG_ON(!(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)));
        BUG_ON((vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)) ==
                                                (VM_PFNMAP|VM_MIXEDMAP));
        BUG_ON((vma->vm_flags & VM_PFNMAP) && is_cow_mapping(vma->vm_flags));
        BUG_ON(!pfn_t_devmap(pfn));

        if (addr < vma->vm_start || addr >= vma->vm_end)
                return VM_FAULT_SIGBUS;

        track_pfn_insert(vma, &pgprot, pfn);

        insert_pfn_pmd(vma, addr, pmd, pfn, pgprot, write);
        return VM_FAULT_NOPAGE;
}
EXPORT_SYMBOL_GPL(vmf_insert_pfn_pmd);

static void touch_pmd(struct vm_area_struct *vma, unsigned long addr,
                pmd_t *pmd)
{
        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));
        if (pmdp_set_access_flags(vma, addr & HPAGE_PMD_MASK,
                                pmd, _pmd,  1))
                update_mmu_cache_pmd(vma, addr, pmd);
}

struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
                pmd_t *pmd, int flags)
{
        unsigned long pfn = pmd_pfn(*pmd);
        struct mm_struct *mm = vma->vm_mm;
        struct dev_pagemap *pgmap;
        struct page *page;

        assert_spin_locked(pmd_lockptr(mm, pmd));

        if (flags & FOLL_WRITE && !pmd_write(*pmd))
                return NULL;

        if (pmd_present(*pmd) && pmd_devmap(*pmd))
                /* pass */;
        else
                return NULL;

        if (flags & FOLL_TOUCH)
                touch_pmd(vma, addr, pmd);

        /*
         * device mapped pages can only be returned if the
         * caller will manage the page reference count.
         */
        if (!(flags & FOLL_GET))
                return ERR_PTR(-EEXIST);

        pfn += (addr & ~PMD_MASK) >> PAGE_SHIFT;
        pgmap = get_dev_pagemap(pfn, NULL);
        if (!pgmap)
                return ERR_PTR(-EFAULT);
        page = pfn_to_page(pfn);
        get_page(page);
        put_dev_pagemap(pgmap);

        return page;
}

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)
{
        spinlock_t *dst_ptl, *src_ptl;
        struct page *src_page;
        pmd_t pmd;
        pgtable_t pgtable = NULL;
        int ret = -ENOMEM;

        /* Skip if can be re-fill on fault */
        if (!vma_is_anonymous(vma))
                return 0;

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

        dst_ptl = pmd_lock(dst_mm, dst_pmd);
        src_ptl = pmd_lockptr(src_mm, src_pmd);
        spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);

        ret = -EAGAIN;
        pmd = *src_pmd;
        if (unlikely(!pmd_trans_huge(pmd))) {
                pte_free(dst_mm, pgtable);
                goto out_unlock;
        }
        /*
         * When page table lock is held, the huge zero pmd should not be
         * under splitting since we don't split the page itself, only pmd to
         * a page table.
         */
        if (is_huge_zero_pmd(pmd)) {
                struct page *zero_page;
                /*
                 * get_huge_zero_page() will never allocate a new page here,
                 * since we already have a zero page to copy. It just takes a
                 * reference.
                 */
                zero_page = mm_get_huge_zero_page(dst_mm);
                set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd,
                                zero_page);
                ret = 0;
                goto out_unlock;
        }

        src_page = pmd_page(pmd);
        VM_BUG_ON_PAGE(!PageHead(src_page), src_page);
        get_page(src_page);
        page_dup_rmap(src_page, true);
        add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
        atomic_long_inc(&dst_mm->nr_ptes);
        pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);

        pmdp_set_wrprotect(src_mm, addr, src_pmd);
        pmd = pmd_mkold(pmd_wrprotect(pmd));
        set_pmd_at(dst_mm, addr, dst_pmd, pmd);

        ret = 0;
out_unlock:
        spin_unlock(src_ptl);
        spin_unlock(dst_ptl);
out:
        return ret;
}

void huge_pmd_set_accessed(struct fault_env *fe, pmd_t orig_pmd)
{
        pmd_t entry;
        unsigned long haddr;

        fe->ptl = pmd_lock(fe->vma->vm_mm, fe->pmd);
        if (unlikely(!pmd_same(*fe->pmd, orig_pmd)))
                goto unlock;

        entry = pmd_mkyoung(orig_pmd);
        haddr = fe->address & HPAGE_PMD_MASK;
        if (pmdp_set_access_flags(fe->vma, haddr, fe->pmd, entry,
                                fe->flags & FAULT_FLAG_WRITE))
                update_mmu_cache_pmd(fe->vma, fe->address, fe->pmd);

unlock:
        spin_unlock(fe->ptl);
}

static int do_huge_pmd_wp_page_fallback(struct fault_env *fe, pmd_t orig_pmd,
                struct page *page)
{
        struct vm_area_struct *vma = fe->vma;
        unsigned long haddr = fe->address & HPAGE_PMD_MASK;
        struct mem_cgroup *memcg;
        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,
                                               fe->address, page_to_nid(page));
                if (unlikely(!pages[i] ||
                             mem_cgroup_try_charge(pages[i], vma->vm_mm,
                                     GFP_KERNEL, &memcg, false))) {
                        if (pages[i])
                                put_page(pages[i]);
                        while (--i >= 0) {
                                memcg = (void *)page_private(pages[i]);
                                set_page_private(pages[i], 0);
                                mem_cgroup_cancel_charge(pages[i], memcg,
                                                false);
                                put_page(pages[i]);
                        }
                        kfree(pages);
                        ret |= VM_FAULT_OOM;
                        goto out;
                }
                set_page_private(pages[i], (unsigned long)memcg);
        }

        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(vma->vm_mm, mmun_start, mmun_end);

        fe->ptl = pmd_lock(vma->vm_mm, fe->pmd);
        if (unlikely(!pmd_same(*fe->pmd, orig_pmd)))
                goto out_free_pages;
        VM_BUG_ON_PAGE(!PageHead(page), page);

        pmdp_huge_clear_flush_notify(vma, haddr, fe->pmd);
        /* leave pmd empty until pte is filled */

        pgtable = pgtable_trans_huge_withdraw(vma->vm_mm, fe->pmd);
        pmd_populate(vma->vm_mm, &_pmd, pgtable);

        for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
                pte_t entry;
                entry = mk_pte(pages[i], vma->vm_page_prot);
                entry = maybe_mkwrite(pte_mkdirty(entry), vma);
                memcg = (void *)page_private(pages[i]);
                set_page_private(pages[i], 0);
                page_add_new_anon_rmap(pages[i], fe->vma, haddr, false);
                mem_cgroup_commit_charge(pages[i], memcg, false, false);
                lru_cache_add_active_or_unevictable(pages[i], vma);
                fe->pte = pte_offset_map(&_pmd, haddr);
                VM_BUG_ON(!pte_none(*fe->pte));
                set_pte_at(vma->vm_mm, haddr, fe->pte, entry);
                pte_unmap(fe->pte);
        }
        kfree(pages);

        smp_wmb(); /* make pte visible before pmd */
        pmd_populate(vma->vm_mm, fe->pmd, pgtable);
        page_remove_rmap(page, true);
        spin_unlock(fe->ptl);

        mmu_notifier_invalidate_range_end(vma->vm_mm, mmun_start, mmun_end);

        ret |= VM_FAULT_WRITE;
        put_page(page);

out:
        return ret;

out_free_pages:
        spin_unlock(fe->ptl);
        mmu_notifier_invalidate_range_end(vma->vm_mm, mmun_start, mmun_end);
        for (i = 0; i < HPAGE_PMD_NR; i++) {
                memcg = (void *)page_private(pages[i]);
                set_page_private(pages[i], 0);
                mem_cgroup_cancel_charge(pages[i], memcg, false);
                put_page(pages[i]);
        }
        kfree(pages);
        goto out;
}

int do_huge_pmd_wp_page(struct fault_env *fe, pmd_t orig_pmd)
{
        struct vm_area_struct *vma = fe->vma;
        struct page *page = NULL, *new_page;
        struct mem_cgroup *memcg;
        unsigned long haddr = fe->address & HPAGE_PMD_MASK;
        unsigned long mmun_start;       /* For mmu_notifiers */
        unsigned long mmun_end;         /* For mmu_notifiers */
        gfp_t huge_gfp;                 /* for allocation and charge */
        int ret = 0;

        fe->ptl = pmd_lockptr(vma->vm_mm, fe->pmd);
        VM_BUG_ON_VMA(!vma->anon_vma, vma);
        if (is_huge_zero_pmd(orig_pmd))
                goto alloc;
        spin_lock(fe->ptl);
        if (unlikely(!pmd_same(*fe->pmd, orig_pmd)))
                goto out_unlock;

        page = pmd_page(orig_pmd);
        VM_BUG_ON_PAGE(!PageCompound(page) || !PageHead(page), page);
        /*
         * We can only reuse the page if nobody else maps the huge page or it's
         * part.
         */
        if (page_trans_huge_mapcount(page, NULL) == 1) {
                pmd_t entry;
                entry = pmd_mkyoung(orig_pmd);
                entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
                if (pmdp_set_access_flags(vma, haddr, fe->pmd, entry,  1))
                        update_mmu_cache_pmd(vma, fe->address, fe->pmd);
                ret |= VM_FAULT_WRITE;
                goto out_unlock;
        }
        get_page(page);
        spin_unlock(fe->ptl);
alloc:
        if (transparent_hugepage_enabled(vma) &&
            !transparent_hugepage_debug_cow()) {
                huge_gfp = alloc_hugepage_direct_gfpmask(vma);
                new_page = alloc_hugepage_vma(huge_gfp, vma, haddr, HPAGE_PMD_ORDER);
        } else
                new_page = NULL;

        if (likely(new_page)) {
                prep_transhuge_page(new_page);
        } else {
                if (!page) {
                        split_huge_pmd(vma, fe->pmd, fe->address);
                        ret |= VM_FAULT_FALLBACK;
                } else {
                        ret = do_huge_pmd_wp_page_fallback(fe, orig_pmd, page);
                        if (ret & VM_FAULT_OOM) {
                                split_huge_pmd(vma, fe->pmd, fe->address);
                                ret |= VM_FAULT_FALLBACK;
                        }
                        put_page(page);
                }
                count_vm_event(THP_FAULT_FALLBACK);
                goto out;
        }

        if (unlikely(mem_cgroup_try_charge(new_page, vma->vm_mm,
                                        huge_gfp, &memcg, true))) {
                put_page(new_page);
                split_huge_pmd(vma, fe->pmd, fe->address);
                if (page)
                        put_page(page);
                ret |= VM_FAULT_FALLBACK;
                count_vm_event(THP_FAULT_FALLBACK);
                goto out;
        }

        count_vm_event(THP_FAULT_ALLOC);

        if (!page)
                clear_huge_page(new_page, haddr, HPAGE_PMD_NR);
        else
                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(vma->vm_mm, mmun_start, mmun_end);

        spin_lock(fe->ptl);
        if (page)
                put_page(page);
        if (unlikely(!pmd_same(*fe->pmd, orig_pmd))) {
                spin_unlock(fe->ptl);
                mem_cgroup_cancel_charge(new_page, memcg, true);
                put_page(new_page);
                goto out_mn;
        } else {
                pmd_t entry;
                entry = mk_huge_pmd(new_page, vma->vm_page_prot);
                entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
                pmdp_huge_clear_flush_notify(vma, haddr, fe->pmd);
                page_add_new_anon_rmap(new_page, vma, haddr, true);
                mem_cgroup_commit_charge(new_page, memcg, false, true);
                lru_cache_add_active_or_unevictable(new_page, vma);
                set_pmd_at(vma->vm_mm, haddr, fe->pmd, entry);
                update_mmu_cache_pmd(vma, fe->address, fe->pmd);
                if (!page) {
                        add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
                } else {
                        VM_BUG_ON_PAGE(!PageHead(page), page);
                        page_remove_rmap(page, true);
                        put_page(page);
                }
                ret |= VM_FAULT_WRITE;
        }
        spin_unlock(fe->ptl);
out_mn:
        mmu_notifier_invalidate_range_end(vma->vm_mm, mmun_start, mmun_end);
out:
        return ret;
out_unlock:
        spin_unlock(fe->ptl);
        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(pmd_lockptr(mm, pmd));

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

        /* Avoid dumping huge zero page */
        if ((flags & FOLL_DUMP) && is_huge_zero_pmd(*pmd))
                return ERR_PTR(-EFAULT);

        /* Full NUMA hinting faults to serialise migration in fault paths */
        if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))
                goto out;

        page = pmd_page(*pmd);
        VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
        if (flags & FOLL_TOUCH)
                touch_pmd(vma, addr, pmd);
        if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
                /*
                 * We don't mlock() pte-mapped THPs. This way we can avoid
                 * leaking mlocked pages into non-VM_LOCKED VMAs.
                 *
                 * For anon THP:
                 *
                 * In most cases the pmd is the only mapping of the page as we
                 * break COW for the mlock() -- see gup_flags |= FOLL_WRITE for
                 * writable private mappings in populate_vma_page_range().
                 *
                 * The only scenario when we have the page shared here is if we
                 * mlocking read-only mapping shared over fork(). We skip
                 * mlocking such pages.
                 *
                 * For file THP:
                 *
                 * We can expect PageDoubleMap() to be stable under page lock:
                 * for file pages we set it in page_add_file_rmap(), which
                 * requires page to be locked.
                 */

                if (PageAnon(page) && compound_mapcount(page) != 1)
                        goto skip_mlock;
                if (PageDoubleMap(page) || !page->mapping)
                        goto skip_mlock;
                if (!trylock_page(page))
                        goto skip_mlock;
                lru_add_drain();
                if (page->mapping && !PageDoubleMap(page))
                        mlock_vma_page(page);
                unlock_page(page);
        }
skip_mlock:
        page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
        VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page);
        if (flags & FOLL_GET)
                get_page(page);

out:
        return page;
}

/* NUMA hinting page fault entry point for trans huge pmds */
int do_huge_pmd_numa_page(struct fault_env *fe, pmd_t pmd)
{
        struct vm_area_struct *vma = fe->vma;
        struct anon_vma *anon_vma = NULL;
        struct page *page;
        unsigned long haddr = fe->address & HPAGE_PMD_MASK;
        int page_nid = -1, this_nid = numa_node_id();
        int target_nid, last_cpupid = -1;
        bool page_locked;
        bool migrated = false;
        bool was_writable;
        int flags = 0;

        fe->ptl = pmd_lock(vma->vm_mm, fe->pmd);
        if (unlikely(!pmd_same(pmd, *fe->pmd)))
                goto out_unlock;

        /*
         * If there are potential migrations, wait for completion and retry
         * without disrupting NUMA hinting information. Do not relock and
         * check_same as the page may no longer be mapped.
         */
        if (unlikely(pmd_trans_migrating(*fe->pmd))) {
                page = pmd_page(*fe->pmd);
                spin_unlock(fe->ptl);
                wait_on_page_locked(page);
                goto out;
        }

        page = pmd_page(pmd);
        BUG_ON(is_huge_zero_page(page));
        page_nid = page_to_nid(page);
        last_cpupid = page_cpupid_last(page);
        count_vm_numa_event(NUMA_HINT_FAULTS);
        if (page_nid == this_nid) {
                count_vm_numa_event(NUMA_HINT_FAULTS_LOCAL);
                flags |= TNF_FAULT_LOCAL;
        }

        /* See similar comment in do_numa_page for explanation */
        if (!pmd_write(pmd))
                flags |= TNF_NO_GROUP;

        /*
         * Acquire the page lock to serialise THP migrations but avoid dropping
         * page_table_lock if at all possible
         */
        page_locked = trylock_page(page);
        target_nid = mpol_misplaced(page, vma, haddr);
        if (target_nid == -1) {
                /* If the page was locked, there are no parallel migrations */
                if (page_locked)
                        goto clear_pmdnuma;
        }

        /* Migration could have started since the pmd_trans_migrating check */
        if (!page_locked) {
                spin_unlock(fe->ptl);
                wait_on_page_locked(page);
                page_nid = -1;
                goto out;
        }

        /*
         * Page is misplaced. Page lock serialises migrations. Acquire anon_vma
         * to serialises splits
         */
        get_page(page);
        spin_unlock(fe->ptl);
        anon_vma = page_lock_anon_vma_read(page);

        /* Confirm the PMD did not change while page_table_lock was released */
        spin_lock(fe->ptl);
        if (unlikely(!pmd_same(pmd, *fe->pmd))) {
                unlock_page(page);
                put_page(page);
                page_nid = -1;
                goto out_unlock;
        }

        /* Bail if we fail to protect against THP splits for any reason */
        if (unlikely(!anon_vma)) {
                put_page(page);
                page_nid = -1;
                goto clear_pmdnuma;
        }

        /*
         * Migrate the THP to the requested node, returns with page unlocked
         * and access rights restored.
         */
        spin_unlock(fe->ptl);
        migrated = migrate_misplaced_transhuge_page(vma->vm_mm, vma,
                                fe->pmd, pmd, fe->address, page, target_nid);
        if (migrated) {
                flags |= TNF_MIGRATED;
                page_nid = target_nid;
        } else
                flags |= TNF_MIGRATE_FAIL;

        goto out;
clear_pmdnuma:
        BUG_ON(!PageLocked(page));
        was_writable = pmd_write(pmd);
        pmd = pmd_modify(pmd, vma->vm_page_prot);
        pmd = pmd_mkyoung(pmd);
        if (was_writable)
                pmd = pmd_mkwrite(pmd);
        set_pmd_at(vma->vm_mm, haddr, fe->pmd, pmd);
        update_mmu_cache_pmd(vma, fe->address, fe->pmd);
        unlock_page(page);
out_unlock:
        spin_unlock(fe->ptl);

out:
        if (anon_vma)
                page_unlock_anon_vma_read(anon_vma);

        if (page_nid != -1)
                task_numa_fault(last_cpupid, page_nid, HPAGE_PMD_NR, fe->flags);

        return 0;
}

/*
 * Return true if we do MADV_FREE successfully on entire pmd page.
 * Otherwise, return false.
 */
bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
                pmd_t *pmd, unsigned long addr, unsigned long next)
{
        spinlock_t *ptl;
        pmd_t orig_pmd;
        struct page *page;
        struct mm_struct *mm = tlb->mm;
        bool ret = false;

        tlb_remove_check_page_size_change(tlb, HPAGE_PMD_SIZE);

        ptl = pmd_trans_huge_lock(pmd, vma);
        if (!ptl)
                goto out_unlocked;

        orig_pmd = *pmd;
        if (is_huge_zero_pmd(orig_pmd))
                goto out;

        page = pmd_page(orig_pmd);
        /*
         * If other processes are mapping this page, we couldn't discard
         * the page unless they all do MADV_FREE so let's skip the page.
         */
        if (page_mapcount(page) != 1)
                goto out;

        if (!trylock_page(page))
                goto out;

        /*
         * If user want to discard part-pages of THP, split it so MADV_FREE
         * will deactivate only them.
         */
        if (next - addr != HPAGE_PMD_SIZE) {
                get_page(page);
                spin_unlock(ptl);
                split_huge_page(page);
                put_page(page);
                unlock_page(page);
                goto out_unlocked;
        }

        if (PageDirty(page))
                ClearPageDirty(page);
        unlock_page(page);

        if (PageActive(page))
                deactivate_page(page);

        if (pmd_young(orig_pmd) || pmd_dirty(orig_pmd)) {
                orig_pmd = pmdp_huge_get_and_clear_full(tlb->mm, addr, pmd,
                        tlb->fullmm);
                orig_pmd = pmd_mkold(orig_pmd);
                orig_pmd = pmd_mkclean(orig_pmd);

                set_pmd_at(mm, addr, pmd, orig_pmd);
                tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
        }
        ret = true;
out:
        spin_unlock(ptl);
out_unlocked:
        return ret;
}

int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
                 pmd_t *pmd, unsigned long addr)
{
        pmd_t orig_pmd;
        spinlock_t *ptl;

        tlb_remove_check_page_size_change(tlb, HPAGE_PMD_SIZE);

        ptl = __pmd_trans_huge_lock(pmd, vma);
        if (!ptl)
                return 0;
        /*
         * For architectures like ppc64 we look at deposited pgtable
         * when calling pmdp_huge_get_and_clear. So do the
         * pgtable_trans_huge_withdraw after finishing pmdp related
         * operations.
         */
        orig_pmd = pmdp_huge_get_and_clear_full(tlb->mm, addr, pmd,
                        tlb->fullmm);
        tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
        if (vma_is_dax(vma)) {
                spin_unlock(ptl);
                if (is_huge_zero_pmd(orig_pmd))
                        tlb_remove_page_size(tlb, pmd_page(orig_pmd), HPAGE_PMD_SIZE);
        } else if (is_huge_zero_pmd(orig_pmd)) {
                pte_free(tlb->mm, pgtable_trans_huge_withdraw(tlb->mm, pmd));
                atomic_long_dec(&tlb->mm->nr_ptes);
                spin_unlock(ptl);
                tlb_remove_page_size(tlb, pmd_page(orig_pmd), HPAGE_PMD_SIZE);
        } else {
                struct page *page = pmd_page(orig_pmd);
                page_remove_rmap(page, true);
                VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
                VM_BUG_ON_PAGE(!PageHead(page), page);
                if (PageAnon(page)) {
                        pgtable_t pgtable;
                        pgtable = pgtable_trans_huge_withdraw(tlb->mm, pmd);
                        pte_free(tlb->mm, pgtable);
                        atomic_long_dec(&tlb->mm->nr_ptes);
                        add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
                } else {
                        add_mm_counter(tlb->mm, MM_FILEPAGES, -HPAGE_PMD_NR);
                }
                spin_unlock(ptl);
                tlb_remove_page_size(tlb, page, HPAGE_PMD_SIZE);
        }
        return 1;
}

bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr,
                  unsigned long new_addr, unsigned long old_end,
                  pmd_t *old_pmd, pmd_t *new_pmd, bool *need_flush)
{
        spinlock_t *old_ptl, *new_ptl;
        pmd_t pmd;
        struct mm_struct *mm = vma->vm_mm;
        bool force_flush = false;

        if ((old_addr & ~HPAGE_PMD_MASK) ||
            (new_addr & ~HPAGE_PMD_MASK) ||
            old_end - old_addr < HPAGE_PMD_SIZE)
                return false;

        /*
         * 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));
                return false;
        }

        /*
         * We don't have to worry about the ordering of src and dst
         * ptlocks because exclusive mmap_sem prevents deadlock.
         */
        old_ptl = __pmd_trans_huge_lock(old_pmd, vma);
        if (old_ptl) {
                new_ptl = pmd_lockptr(mm, new_pmd);
                if (new_ptl != old_ptl)
                        spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
                pmd = pmdp_huge_get_and_clear(mm, old_addr, old_pmd);
                if (pmd_present(pmd) && pmd_dirty(pmd))
                        force_flush = true;
                VM_BUG_ON(!pmd_none(*new_pmd));

                if (pmd_move_must_withdraw(new_ptl, old_ptl) &&
                                vma_is_anonymous(vma)) {
                        pgtable_t pgtable;
                        pgtable = pgtable_trans_huge_withdraw(mm, old_pmd);
                        pgtable_trans_huge_deposit(mm, new_pmd, pgtable);
                }
                set_pmd_at(mm, new_addr, new_pmd, pmd_mksoft_dirty(pmd));
                if (new_ptl != old_ptl)
                        spin_unlock(new_ptl);
                if (force_flush)
                        flush_tlb_range(vma, old_addr, old_addr + PMD_SIZE);
                else
                        *need_flush = true;
                spin_unlock(old_ptl);
                return true;
        }
        return false;
}

/*
 * Returns
 *  - 0 if PMD could not be locked
 *  - 1 if PMD was locked but protections unchange and TLB flush unnecessary
 *  - HPAGE_PMD_NR is protections changed and TLB flush necessary
 */
int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
                unsigned long addr, pgprot_t newprot, int prot_numa)
{
        struct mm_struct *mm = vma->vm_mm;
        spinlock_t *ptl;
        int ret = 0;

        ptl = __pmd_trans_huge_lock(pmd, vma);
        if (ptl) {
                pmd_t entry;
                bool preserve_write = prot_numa && pmd_write(*pmd);
                ret = 1;

                /*
                 * Avoid trapping faults against the zero page. The read-only
                 * data is likely to be read-cached on the local CPU and
                 * local/remote hits to the zero page are not interesting.
                 */
                if (prot_numa && is_huge_zero_pmd(*pmd)) {
                        spin_unlock(ptl);
                        return ret;
                }

                if (!prot_numa || !pmd_protnone(*pmd)) {
                        entry = pmdp_huge_get_and_clear_notify(mm, addr, pmd);
                        entry = pmd_modify(entry, newprot);
                        if (preserve_write)
                                entry = pmd_mkwrite(entry);
                        ret = HPAGE_PMD_NR;
                        set_pmd_at(mm, addr, pmd, entry);
                        BUG_ON(vma_is_anonymous(vma) && !preserve_write &&
                                        pmd_write(entry));
                }
                spin_unlock(ptl);
        }

        return ret;
}

/*
 * Returns page table lock pointer if a given pmd maps a thp, NULL otherwise.
 *
 * Note that if it returns page table lock pointer, this routine returns without
 * unlocking page table lock. So callers must unlock it.
 */
spinlock_t *__pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma)
{
        spinlock_t *ptl;
        ptl = pmd_lock(vma->vm_mm, pmd);
        if (likely(pmd_trans_huge(*pmd) || pmd_devmap(*pmd)))
                return ptl;
        spin_unlock(ptl);
        return NULL;
}

static void __split_huge_zero_page_pmd(struct vm_area_struct *vma,
                unsigned long haddr, pmd_t *pmd)
{
        struct mm_struct *mm = vma->vm_mm;
        pgtable_t pgtable;
        pmd_t _pmd;
        int i;

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

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

        for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
                pte_t *pte, entry;
                entry = pfn_pte(my_zero_pfn(haddr), vma->vm_page_prot);
                entry = pte_mkspecial(entry);
                pte = pte_offset_map(&_pmd, haddr);
                VM_BUG_ON(!pte_none(*pte));
                set_pte_at(mm, haddr, pte, entry);
                pte_unmap(pte);
        }
        smp_wmb(); /* make pte visible before pmd */
        pmd_populate(mm, pmd, pgtable);
}

static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
                unsigned long haddr, bool freeze)
{
        struct mm_struct *mm = vma->vm_mm;
        struct page *page;
        pgtable_t pgtable;
        pmd_t _pmd;
        bool young, write, dirty, soft_dirty;
        unsigned long addr;
        int i;

        VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
        VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
        VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PMD_SIZE, vma);
        VM_BUG_ON(!pmd_trans_huge(*pmd) && !pmd_devmap(*pmd));

        count_vm_event(THP_SPLIT_PMD);

        if (!vma_is_anonymous(vma)) {
                _pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
                if (vma_is_dax(vma))
                        return;
                page = pmd_page(_pmd);
                if (!PageReferenced(page) && pmd_young(_pmd))
                        SetPageReferenced(page);
                page_remove_rmap(page, true);
                put_page(page);
                add_mm_counter(mm, MM_FILEPAGES, -HPAGE_PMD_NR);
                return;
        } else if (is_huge_zero_pmd(*pmd)) {
                return __split_huge_zero_page_pmd(vma, haddr, pmd);
        }

        page = pmd_page(*pmd);
        VM_BUG_ON_PAGE(!page_count(page), page);
        page_ref_add(page, HPAGE_PMD_NR - 1);
        write = pmd_write(*pmd);
        young = pmd_young(*pmd);
        dirty = pmd_dirty(*pmd);
        soft_dirty = pmd_soft_dirty(*pmd);

        pmdp_huge_split_prepare(vma, haddr, pmd);
        pgtable = pgtable_trans_huge_withdraw(mm, pmd);
        pmd_populate(mm, &_pmd, pgtable);

        for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
                pte_t entry, *pte;
                /*
                 * Note that NUMA hinting access restrictions are not
                 * transferred to avoid any possibility of altering
                 * permissions across VMAs.
                 */
                if (freeze) {
                        swp_entry_t swp_entry;
                        swp_entry = make_migration_entry(page + i, write);
                        entry = swp_entry_to_pte(swp_entry);
                        if (soft_dirty)
                                entry = pte_swp_mksoft_dirty(entry);
                } else {
                        entry = mk_pte(page + i, READ_ONCE(vma->vm_page_prot));
                        entry = maybe_mkwrite(entry, vma);
                        if (!write)
                                entry = pte_wrprotect(entry);
                        if (!young)
                                entry = pte_mkold(entry);
                        if (soft_dirty)
                                entry = pte_mksoft_dirty(entry);
                }
                if (dirty)
                        SetPageDirty(page + i);
                pte = pte_offset_map(&_pmd, addr);
                BUG_ON(!pte_none(*pte));
                set_pte_at(mm, addr, pte, entry);
                atomic_inc(&page[i]._mapcount);
                pte_unmap(pte);
        }

        /*
         * Set PG_double_map before dropping compound_mapcount to avoid
         * false-negative page_mapped().
         */
        if (compound_mapcount(page) > 1 && !TestSetPageDoubleMap(page)) {
                for (i = 0; i < HPAGE_PMD_NR; i++)
                        atomic_inc(&page[i]._mapcount);
        }

        if (atomic_add_negative(-1, compound_mapcount_ptr(page))) {
                /* Last compound_mapcount is gone. */
                __dec_node_page_state(page, NR_ANON_THPS);
                if (TestClearPageDoubleMap(page)) {
                        /* No need in mapcount reference anymore */
                        for (i = 0; i < HPAGE_PMD_NR; i++)
                                atomic_dec(&page[i]._mapcount);
                }
        }

        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_pmd_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, haddr, pmd);
        pmd_populate(mm, pmd, pgtable);

        if (freeze) {
                for (i = 0; i < HPAGE_PMD_NR; i++) {
                        page_remove_rmap(page + i, false);
                        put_page(page + i);
                }
        }
}

void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
                unsigned long address, bool freeze, struct page *page)
{
        spinlock_t *ptl;
        struct mm_struct *mm = vma->vm_mm;
        unsigned long haddr = address & HPAGE_PMD_MASK;

        mmu_notifier_invalidate_range_start(mm, haddr, haddr + HPAGE_PMD_SIZE);
        ptl = pmd_lock(mm, pmd);

        /*
         * If caller asks to setup a migration entries, we need a page to check
         * pmd against. Otherwise we can end up replacing wrong page.
         */
        VM_BUG_ON(freeze && !page);
        if (page && page != pmd_page(*pmd))
                goto out;

        if (pmd_trans_huge(*pmd)) {
                page = pmd_page(*pmd);
                if (PageMlocked(page))
                        clear_page_mlock(page);
        } else if (!pmd_devmap(*pmd))
                goto out;
        __split_huge_pmd_locked(vma, pmd, haddr, freeze);
out:
        spin_unlock(ptl);
        mmu_notifier_invalidate_range_end(mm, haddr, haddr + HPAGE_PMD_SIZE);
}

void split_huge_pmd_address(struct vm_area_struct *vma, unsigned long address,
                bool freeze, struct page *page)
{
        pgd_t *pgd;
        pud_t *pud;
        pmd_t *pmd;

        pgd = pgd_offset(vma->vm_mm, address);
        if (!pgd_present(*pgd))
                return;

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

        pmd = pmd_offset(pud, address);

        __split_huge_pmd(vma, pmd, address, freeze, page);
}

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_pmd_address(vma, start, false, NULL);

        /*
         * 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_pmd_address(vma, end, false, NULL);

        /*
         * 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_pmd_address(next, nstart, false, NULL);
        }
}

static void freeze_page(struct page *page)
{
        enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS |
                TTU_RMAP_LOCKED;
        int i, ret;

        VM_BUG_ON_PAGE(!PageHead(page), page);

        if (PageAnon(page))
                ttu_flags |= TTU_MIGRATION;

        /* We only need TTU_SPLIT_HUGE_PMD once */
        ret = try_to_unmap(page, ttu_flags | TTU_SPLIT_HUGE_PMD);
        for (i = 1; !ret && i < HPAGE_PMD_NR; i++) {
                /* Cut short if the page is unmapped */
                if (page_count(page) == 1)
                        return;

                ret = try_to_unmap(page + i, ttu_flags);
        }
        VM_BUG_ON_PAGE(ret, page + i - 1);
}

static void unfreeze_page(struct page *page)
{
        int i;

        for (i = 0; i < HPAGE_PMD_NR; i++)
                remove_migration_ptes(page + i, page + i, true);
}

static void __split_huge_page_tail(struct page *head, int tail,
                struct lruvec *lruvec, struct list_head *list)
{
        struct page *page_tail = head + tail;

        VM_BUG_ON_PAGE(atomic_read(&page_tail->_mapcount) != -1, page_tail);
        VM_BUG_ON_PAGE(page_ref_count(page_tail) != 0, page_tail);

        /*
         * tail_page->_refcount 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_inc() or
         * 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_inc()/atomic_add().
         */
        if (PageAnon(head)) {
                page_ref_inc(page_tail);
        } else {
                /* Additional pin to radix tree */
                page_ref_add(page_tail, 2);
        }

        page_tail->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
        page_tail->flags |= (head->flags &
                        ((1L << PG_referenced) |
                         (1L << PG_swapbacked) |
                         (1L << PG_mlocked) |
                         (1L << PG_uptodate) |
                         (1L << PG_active) |
                         (1L << PG_locked) |
                         (1L << PG_unevictable) |
                         (1L << PG_dirty)));

        /*
         * After clearing PageTail the gup refcount can be released.
         * Page flags also must be visible before we make the page non-compound.
         */
        smp_wmb();

        clear_compound_head(page_tail);

        if (page_is_young(head))
                set_page_young(page_tail);
        if (page_is_idle(head))
                set_page_idle(page_tail);

        /* ->mapping in first tail page is compound_mapcount */
        VM_BUG_ON_PAGE(tail > 2 && page_tail->mapping != TAIL_MAPPING,
                        page_tail);
        page_tail->mapping = head->mapping;

        page_tail->index = head->index + tail;
        page_cpupid_xchg_last(page_tail, page_cpupid_last(head));
        lru_add_page_tail(head, page_tail, lruvec, list);
}

static void __split_huge_page(struct page *page, struct list_head *list,
                unsigned long flags)
{
        struct page *head = compound_head(page);
        struct zone *zone = page_zone(head);
        struct lruvec *lruvec;
        pgoff_t end = -1;
        int i;

        lruvec = mem_cgroup_page_lruvec(head, zone->zone_pgdat);

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

        if (!PageAnon(page))
                end = DIV_ROUND_UP(i_size_read(head->mapping->host), PAGE_SIZE);

        for (i = HPAGE_PMD_NR - 1; i >= 1; i--) {
                __split_huge_page_tail(head, i, lruvec, list);
                /* Some pages can be beyond i_size: drop them from page cache */
                if (head[i].index >= end) {
                        __ClearPageDirty(head + i);
                        __delete_from_page_cache(head + i, NULL);
                        if (IS_ENABLED(CONFIG_SHMEM) && PageSwapBacked(head))
                                shmem_uncharge(head->mapping->host, 1);
                        put_page(head + i);
                }
        }

        ClearPageCompound(head);
        /* See comment in __split_huge_page_tail() */
        if (PageAnon(head)) {
                page_ref_inc(head);
        } else {
                /* Additional pin to radix tree */
                page_ref_add(head, 2);
                spin_unlock(&head->mapping->tree_lock);
        }

        spin_unlock_irqrestore(zone_lru_lock(page_zone(head)), flags);

        unfreeze_page(head);

        for (i = 0; i < HPAGE_PMD_NR; i++) {
                struct page *subpage = head + i;
                if (subpage == page)
                        continue;
                unlock_page(subpage);

                /*
                 * Subpages 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(subpage);
        }
}

int total_mapcount(struct page *page)
{
        int i, compound, ret;

        VM_BUG_ON_PAGE(PageTail(page), page);

        if (likely(!PageCompound(page)))
                return atomic_read(&page->_mapcount) + 1;

        compound = compound_mapcount(page);
        if (PageHuge(page))
                return compound;
        ret = compound;
        for (i = 0; i < HPAGE_PMD_NR; i++)
                ret += atomic_read(&page[i]._mapcount) + 1;
        /* File pages has compound_mapcount included in _mapcount */
        if (!PageAnon(page))
                return ret - compound * HPAGE_PMD_NR;
        if (PageDoubleMap(page))
                ret -= HPAGE_PMD_NR;
        return ret;
}

/*
 * This calculates accurately how many mappings a transparent hugepage
 * has (unlike page_mapcount() which isn't fully accurate). This full
 * accuracy is primarily needed to know if copy-on-write faults can
 * reuse the page and change the mapping to read-write instead of
 * copying them. At the same time this returns the total_mapcount too.
 *
 * The function returns the highest mapcount any one of the subpages
 * has. If the return value is one, even if different processes are
 * mapping different subpages of the transparent hugepage, they can
 * all reuse it, because each process is reusing a different subpage.
 *
 * The total_mapcount is instead counting all virtual mappings of the
 * subpages. If the total_mapcount is equal to "one", it tells the
 * caller all mappings belong to the same "mm" and in turn the
 * anon_vma of the transparent hugepage can become the vma->anon_vma
 * local one as no other process may be mapping any of the subpages.
 *
 * It would be more accurate to replace page_mapcount() with
 * page_trans_huge_mapcount(), however we only use
 * page_trans_huge_mapcount() in the copy-on-write faults where we
 * need full accuracy to avoid breaking page pinning, because
 * page_trans_huge_mapcount() is slower than page_mapcount().
 */
int page_trans_huge_mapcount(struct page *page, int *total_mapcount)
{
        int i, ret, _total_mapcount, mapcount;

        /* hugetlbfs shouldn't call it */
        VM_BUG_ON_PAGE(PageHuge(page), page);

        if (likely(!PageTransCompound(page))) {
                mapcount = atomic_read(&page->_mapcount) + 1;
                if (total_mapcount)
                        *total_mapcount = mapcount;
                return mapcount;
        }

        page = compound_head(page);

        _total_mapcount = ret = 0;
        for (i = 0; i < HPAGE_PMD_NR; i++) {
                mapcount = atomic_read(&page[i]._mapcount) + 1;
                ret = max(ret, mapcount);
                _total_mapcount += mapcount;
        }
        if (PageDoubleMap(page)) {
                ret -= 1;
                _total_mapcount -= HPAGE_PMD_NR;
        }
        mapcount = compound_mapcount(page);
        ret += mapcount;
        _total_mapcount += mapcount;
        if (total_mapcount)
                *total_mapcount = _total_mapcount;
        return ret;
}

/*
 * This function splits huge page into normal pages. @page can point to any
 * subpage of huge page to split. Split doesn't change the position of @page.
 *
 * Only caller must hold pin on the @page, otherwise split fails with -EBUSY.
 * The huge page must be locked.
 *
 * If @list is null, tail pages will be added to LRU list, otherwise, to @list.
 *
 * Both head page and tail pages will inherit mapping, flags, and so on from
 * the hugepage.
 *
 * GUP pin and PG_locked transferred to @page. Rest subpages can be freed if
 * they are not mapped.
 *
 * Returns 0 if the hugepage is split successfully.
 * Returns -EBUSY if the page is pinned or if anon_vma disappeared from under
 * us.
 */
int split_huge_page_to_list(struct page *page, struct list_head *list)
{
        struct page *head = compound_head(page);
        struct pglist_data *pgdata = NODE_DATA(page_to_nid(head));
        struct anon_vma *anon_vma = NULL;
        struct address_space *mapping = NULL;
        int count, mapcount, extra_pins, ret;
        bool mlocked;
        unsigned long flags;

        VM_BUG_ON_PAGE(is_huge_zero_page(page), page);
        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
        VM_BUG_ON_PAGE(!PageCompound(page), page);

        if (PageAnon(head)) {
                /*
                 * The caller does not necessarily hold an mmap_sem that would
                 * prevent the anon_vma disappearing so we first we take a
                 * reference to it and then lock the anon_vma for write. This
                 * is similar to page_lock_anon_vma_read except the write lock
                 * is taken to serialise against parallel split or collapse
                 * operations.
                 */
                anon_vma = page_get_anon_vma(head);
                if (!anon_vma) {
                        ret = -EBUSY;
                        goto out;
                }
                extra_pins = 0;
                mapping = NULL;
                anon_vma_lock_write(anon_vma);
        } else {
                mapping = head->mapping;

                /* Truncated ? */
                if (!mapping) {
                        ret = -EBUSY;
                        goto out;
                }

                /* Addidional pins from radix tree */
                extra_pins = HPAGE_PMD_NR;
                anon_vma = NULL;
                i_mmap_lock_read(mapping);
        }

        /*
         * Racy check if we can split the page, before freeze_page() will
         * split PMDs
         */
        if (total_mapcount(head) != page_count(head) - extra_pins - 1) {
                ret = -EBUSY;
                goto out_unlock;
        }

        mlocked = PageMlocked(page);
        freeze_page(head);
        VM_BUG_ON_PAGE(compound_mapcount(head), head);

        /* Make sure the page is not on per-CPU pagevec as it takes pin */
        if (mlocked)
                lru_add_drain();

        /* prevent PageLRU to go away from under us, and freeze lru stats */
        spin_lock_irqsave(zone_lru_lock(page_zone(head)), flags);

        if (mapping) {
                void **pslot;

                spin_lock(&mapping->tree_lock);
                pslot = radix_tree_lookup_slot(&mapping->page_tree,
                                page_index(head));
                /*
                 * Check if the head page is present in radix tree.
                 * We assume all tail are present too, if head is there.
                 */
                if (radix_tree_deref_slot_protected(pslot,
                                        &mapping->tree_lock) != head)
                        goto fail;
        }

        /* Prevent deferred_split_scan() touching ->_refcount */
        spin_lock(&pgdata->split_queue_lock);
        count = page_count(head);
        mapcount = total_mapcount(head);
        if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {
                if (!list_empty(page_deferred_list(head))) {
                        pgdata->split_queue_len--;
                        list_del(page_deferred_list(head));
                }
                if (mapping)
                        __dec_node_page_state(page, NR_SHMEM_THPS);
                spin_unlock(&pgdata->split_queue_lock);
                __split_huge_page(page, list, flags);
                ret = 0;
        } else {
                if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
                        pr_alert("total_mapcount: %u, page_count(): %u\n",
                                        mapcount, count);
                        if (PageTail(page))
                                dump_page(head, NULL);
                        dump_page(page, "total_mapcount(head) > 0");
                        BUG();
                }
                spin_unlock(&pgdata->split_queue_lock);
fail:           if (mapping)
                        spin_unlock(&mapping->tree_lock);
                spin_unlock_irqrestore(zone_lru_lock(page_zone(head)), flags);
                unfreeze_page(head);
                ret = -EBUSY;
        }

out_unlock:
        if (anon_vma) {
                anon_vma_unlock_write(anon_vma);
                put_anon_vma(anon_vma);
        }
        if (mapping)
                i_mmap_unlock_read(mapping);
out:
        count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
        return ret;
}

void free_transhuge_page(struct page *page)
{
        struct pglist_data *pgdata = NODE_DATA(page_to_nid(page));
        unsigned long flags;

        spin_lock_irqsave(&pgdata->split_queue_lock, flags);
        if (!list_empty(page_deferred_list(page))) {
                pgdata->split_queue_len--;
                list_del(page_deferred_list(page));
        }
        spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);
        free_compound_page(page);
}

void deferred_split_huge_page(struct page *page)
{
        struct pglist_data *pgdata = NODE_DATA(page_to_nid(page));
        unsigned long flags;

        VM_BUG_ON_PAGE(!PageTransHuge(page), page);

        spin_lock_irqsave(&pgdata->split_queue_lock, flags);
        if (list_empty(page_deferred_list(page))) {
                count_vm_event(THP_DEFERRED_SPLIT_PAGE);
                list_add_tail(page_deferred_list(page), &pgdata->split_queue);
                pgdata->split_queue_len++;
        }
        spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);
}

static unsigned long deferred_split_count(struct shrinker *shrink,
                struct shrink_control *sc)
{
        struct pglist_data *pgdata = NODE_DATA(sc->nid);
        return ACCESS_ONCE(pgdata->split_queue_len);
}

static unsigned long deferred_split_scan(struct shrinker *shrink,
                struct shrink_control *sc)
{
        struct pglist_data *pgdata = NODE_DATA(sc->nid);
        unsigned long flags;
        LIST_HEAD(list), *pos, *next;
        struct page *page;
        int split = 0;

        spin_lock_irqsave(&pgdata->split_queue_lock, flags);
        /* Take pin on all head pages to avoid freeing them under us */
        list_for_each_safe(pos, next, &pgdata->split_queue) {
                page = list_entry((void *)pos, struct page, mapping);
                page = compound_head(page);
                if (get_page_unless_zero(page)) {
                        list_move(page_deferred_list(page), &list);
                } else {
                        /* We lost race with put_compound_page() */
                        list_del_init(page_deferred_list(page));
                        pgdata->split_queue_len--;
                }
                if (!--sc->nr_to_scan)
                        break;
        }
        spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);

        list_for_each_safe(pos, next, &list) {
                page = list_entry((void *)pos, struct page, mapping);
                lock_page(page);
                /* split_huge_page() removes page from list on success */
                if (!split_huge_page(page))
                        split++;
                unlock_page(page);
                put_page(page);
        }

        spin_lock_irqsave(&pgdata->split_queue_lock, flags);
        list_splice_tail(&list, &pgdata->split_queue);
        spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);

        /*
         * Stop shrinker if we didn't split any page, but the queue is empty.
         * This can happen if pages were freed under us.
         */
        if (!split && list_empty(&pgdata->split_queue))
                return SHRINK_STOP;
        return split;
}

static struct shrinker deferred_split_shrinker = {
        .count_objects = deferred_split_count,
        .scan_objects = deferred_split_scan,
        .seeks = DEFAULT_SEEKS,
        .flags = SHRINKER_NUMA_AWARE,
};

#ifdef CONFIG_DEBUG_FS
static int split_huge_pages_set(void *data, u64 val)
{
        struct zone *zone;
        struct page *page;
        unsigned long pfn, max_zone_pfn;
        unsigned long total = 0, split = 0;

        if (val != 1)
                return -EINVAL;

        for_each_populated_zone(zone) {
                max_zone_pfn = zone_end_pfn(zone);
                for (pfn = zone->zone_start_pfn; pfn < max_zone_pfn; pfn++) {
                        if (!pfn_valid(pfn))
                                continue;

                        page = pfn_to_page(pfn);
                        if (!get_page_unless_zero(page))
                                continue;

                        if (zone != page_zone(page))
                                goto next;

                        if (!PageHead(page) || PageHuge(page) || !PageLRU(page))
                                goto next;

                        total++;
                        lock_page(page);
                        if (!split_huge_page(page))
                                split++;
                        unlock_page(page);
next:
                        put_page(page);
                }
        }

        pr_info("%lu of %lu THP split\n", split, total);

        return 0;
}
DEFINE_SIMPLE_ATTRIBUTE(split_huge_pages_fops, NULL, split_huge_pages_set,
                "%llu\n");

static int __init split_huge_pages_debugfs(void)
{
        void *ret;

        ret = debugfs_create_file("split_huge_pages", 0200, NULL, NULL,
                        &split_huge_pages_fops);
        if (!ret)
                pr_warn("Failed to create split_huge_pages in debugfs");
        return 0;
}
late_initcall(split_huge_pages_debugfs);
#endif