static struct page *follow_pmd_mask(struct vm_area_struct *vma,
				    unsigned long address, pud_t *pudp,
				    unsigned int flags,
				    struct follow_page_context *ctx)
{
	pmd_t *pmd, pmdval;
	spinlock_t *ptl;
	struct page *page;
	struct mm_struct *mm = vma->vm_mm;

	pmd = pmd_offset(pudp, address);
	/*
	 * The READ_ONCE() will stabilize the pmdval in a register or
	 * on the stack so that it will stop changing under the code.
	 */
	pmdval = READ_ONCE(*pmd);
	if (pmd_none(pmdval))
		return no_page_table(vma, flags);
	if (pmd_huge(pmdval) && is_vm_hugetlb_page(vma)) {
		page = follow_huge_pmd(mm, address, pmd, flags);
		if (page)
			return page;
		return no_page_table(vma, flags);
	}
	if (is_hugepd(__hugepd(pmd_val(pmdval)))) {
		page = follow_huge_pd(vma, address,
				      __hugepd(pmd_val(pmdval)), flags,
				      PMD_SHIFT);
		if (page)
			return page;
		return no_page_table(vma, flags);
	}
retry:
	if (!pmd_present(pmdval)) {
		if (likely(!(flags & FOLL_MIGRATION)))
			return no_page_table(vma, flags);
		VM_BUG_ON(thp_migration_supported() &&
				  !is_pmd_migration_entry(pmdval));
		if (is_pmd_migration_entry(pmdval))
			pmd_migration_entry_wait(mm, pmd);
		pmdval = READ_ONCE(*pmd);
		/*
		 * MADV_DONTNEED may convert the pmd to null because
		 * mmap_sem is held in read mode
		 */
		if (pmd_none(pmdval))
			return no_page_table(vma, flags);
		goto retry;
	}
	if (pmd_devmap(pmdval)) {
		ptl = pmd_lock(mm, pmd);
		page = follow_devmap_pmd(vma, address, pmd, flags, &ctx->pgmap);
		spin_unlock(ptl);
		if (page)
			return page;
	}
	if (likely(!pmd_trans_huge(pmdval)))
		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);

	if ((flags & FOLL_NUMA) && pmd_protnone(pmdval))
		return no_page_table(vma, flags);

retry_locked:
	ptl = pmd_lock(mm, pmd);
	if (unlikely(pmd_none(*pmd))) {
		spin_unlock(ptl);
		return no_page_table(vma, flags);
	}
	if (unlikely(!pmd_present(*pmd))) {
		spin_unlock(ptl);
		if (likely(!(flags & FOLL_MIGRATION)))
			return no_page_table(vma, flags);
		pmd_migration_entry_wait(mm, pmd);
		goto retry_locked;
	}
	if (unlikely(!pmd_trans_huge(*pmd))) {
		spin_unlock(ptl);
		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
	}
	if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) {
		int ret;
		page = pmd_page(*pmd);
		if (is_huge_zero_page(page)) {
			spin_unlock(ptl);
			ret = 0;
			split_huge_pmd(vma, pmd, address);
			if (pmd_trans_unstable(pmd))
				ret = -EBUSY;
		} else if (flags & FOLL_SPLIT) {
			if (unlikely(!try_get_page(page))) {
				spin_unlock(ptl);
				return ERR_PTR(-ENOMEM);
			}
			spin_unlock(ptl);
			lock_page(page);
			ret = split_huge_page(page);
			unlock_page(page);
			put_page(page);
			if (pmd_none(*pmd))
				return no_page_table(vma, flags);
		} else {  /* flags & FOLL_SPLIT_PMD */
			spin_unlock(ptl);
			split_huge_pmd(vma, pmd, address);
			ret = pte_alloc(mm, pmd) ? -ENOMEM : 0;
		}

		return ret ? ERR_PTR(ret) :
			follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
	}
	page = follow_trans_huge_pmd(vma, address, pmd, flags);
	spin_unlock(ptl);
	ctx->page_mask = HPAGE_PMD_NR - 1;
	return page;
}
static struct page *follow_pmd_mask(struct vm_area_struct *vma,
				    unsigned long address, pud_t *pudp,
				    unsigned int flags,
				    struct follow_page_context *ctx)
{
	pmd_t *pmd, pmdval;
	spinlock_t *ptl;
	struct page *page;
	struct mm_struct *mm = vma->vm_mm;

	pmd = pmd_offset(pudp, address);
	 
	pmdval = READ_ONCE(*pmd);
	if (pmd_none(pmdval))
		return no_page_table(vma, flags);
	if (pmd_huge(pmdval) && is_vm_hugetlb_page(vma)) {
		page = follow_huge_pmd(mm, address, pmd, flags);
		if (page)
			return page;
		return no_page_table(vma, flags);
	}
	if (is_hugepd(__hugepd(pmd_val(pmdval)))) {
		page = follow_huge_pd(vma, address,
				      __hugepd(pmd_val(pmdval)), flags,
				      PMD_SHIFT);
		if (page)
			return page;
		return no_page_table(vma, flags);
	}
retry:
	if (!pmd_present(pmdval)) {
		if (likely(!(flags & FOLL_MIGRATION)))
			return no_page_table(vma, flags);
		VM_BUG_ON(thp_migration_supported() &&
				  !is_pmd_migration_entry(pmdval));
		if (is_pmd_migration_entry(pmdval))
			pmd_migration_entry_wait(mm, pmd);
		pmdval = READ_ONCE(*pmd);
		 
		if (pmd_none(pmdval))
			return no_page_table(vma, flags);
		goto retry;
	}
	if (pmd_devmap(pmdval)) {
		ptl = pmd_lock(mm, pmd);
		page = follow_devmap_pmd(vma, address, pmd, flags, &ctx->pgmap);
		spin_unlock(ptl);
		if (page)
			return page;
	}
	if (likely(!pmd_trans_huge(pmdval)))
		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);

	if ((flags & FOLL_NUMA) && pmd_protnone(pmdval))
		return no_page_table(vma, flags);

retry_locked:
	ptl = pmd_lock(mm, pmd);
	if (unlikely(pmd_none(*pmd))) {
		spin_unlock(ptl);
		return no_page_table(vma, flags);
	}
	if (unlikely(!pmd_present(*pmd))) {
		spin_unlock(ptl);
		if (likely(!(flags & FOLL_MIGRATION)))
			return no_page_table(vma, flags);
		pmd_migration_entry_wait(mm, pmd);
		goto retry_locked;
	}
	if (unlikely(!pmd_trans_huge(*pmd))) {
		spin_unlock(ptl);
		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
	}
	if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) {
		int ret;
		page = pmd_page(*pmd);
		if (is_huge_zero_page(page)) {
			spin_unlock(ptl);
			ret = 0;
			split_huge_pmd(vma, pmd, address);
			if (pmd_trans_unstable(pmd))
				ret = -EBUSY;
		} else if (flags & FOLL_SPLIT) {
			if (unlikely(!try_get_page(page))) {
				spin_unlock(ptl);
				return ERR_PTR(-ENOMEM);
			}
			spin_unlock(ptl);
			lock_page(page);
			ret = split_huge_page(page);
			unlock_page(page);
			put_page(page);
			if (pmd_none(*pmd))
				return no_page_table(vma, flags);
		} else {   
			spin_unlock(ptl);
			split_huge_pmd(vma, pmd, address);
			ret = pte_alloc(mm, pmd) ? -ENOMEM : 0;
		}

		return ret ? ERR_PTR(ret) :
			follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
	}
	page = follow_trans_huge_pmd(vma, address, pmd, flags);
	spin_unlock(ptl);
	ctx->page_mask = HPAGE_PMD_NR - 1;
	return page;
}