static int fuse_write_begin(struct file *file, struct address_space *mapping,
		loff_t pos, unsigned len, unsigned flags,
		struct page **pagep, void **fsdata)
{
	pgoff_t index = pos >> PAGE_SHIFT;
	struct fuse_conn *fc = get_fuse_conn(file_inode(file));
	struct page *page;
	loff_t fsize;
	int err = -ENOMEM;

	WARN_ON(!fc->writeback_cache);

	page = grab_cache_page_write_begin(mapping, index, flags);
	if (!page)
		goto error;

	fuse_wait_on_page_writeback(mapping->host, page->index);

	if (PageUptodate(page) || len == PAGE_SIZE)
		goto success;
	/*
	 * Check if the start this page comes after the end of file, in which
	 * case the readpage can be optimized away.
	 */
	fsize = i_size_read(mapping->host);
	if (fsize <= (pos & PAGE_MASK)) {
		size_t off = pos & ~PAGE_MASK;
		if (off)
			zero_user_segment(page, 0, off);
		goto success;
	}
	err = fuse_do_readpage(file, page);
	if (err)
		goto cleanup;
success:
	*pagep = page;
	return 0;

cleanup:
	unlock_page(page);
	put_page(page);
error:
	return err;
}