static long madvise_willneed(struct vm_area_struct *vma,
			     struct vm_area_struct **prev,
			     unsigned long start, unsigned long end)
{
	struct file *file = vma->vm_file;
	loff_t offset;

	*prev = vma;
#ifdef CONFIG_SWAP
	if (!file) {
		walk_page_range(vma->vm_mm, start, end, &swapin_walk_ops, vma);
		lru_add_drain(); /* Push any new pages onto the LRU now */
		return 0;
	}

	if (shmem_mapping(file->f_mapping)) {
		force_shm_swapin_readahead(vma, start, end,
					file->f_mapping);
		return 0;
	}
#else
	if (!file)
		return -EBADF;
#endif

	if (IS_DAX(file_inode(file))) {
		/* no bad return value, but ignore advice */
		return 0;
	}

	/*
	 * Filesystem's fadvise may need to take various locks.  We need to
	 * explicitly grab a reference because the vma (and hence the
	 * vma's reference to the file) can go away as soon as we drop
	 * mmap_sem.
	 */
	*prev = NULL;	/* tell sys_madvise we drop mmap_sem */
	get_file(file);
	up_read(&current->mm->mmap_sem);
	offset = (loff_t)(start - vma->vm_start)
			+ ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
	vfs_fadvise(file, offset, end - start, POSIX_FADV_WILLNEED);
	fput(file);
	down_read(&current->mm->mmap_sem);
	return 0;
}