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 && !can_follow_write_pmd(*pmd, flags))
		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 (!try_grab_page(page, flags))
		return ERR_PTR(-ENOMEM);

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

	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);

out:
	return page;
}
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 && !can_follow_write_pmd(*pmd, flags))
		goto out;

	 
	if ((flags & FOLL_DUMP) && is_huge_zero_pmd(*pmd))
		return ERR_PTR(-EFAULT);

	 
	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 (!try_grab_page(page, flags))
		return ERR_PTR(-ENOMEM);

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

	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_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);

out:
	return page;
}