//  [c]
int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)
{
	struct super_block *sb = inode->i_sb;
	ext4_lblk_t first_block, stop_block;
	struct address_space *mapping = inode->i_mapping;
	loff_t first_block_offset, last_block_offset;
	handle_t *handle;
	unsigned int credits;
	int ret = 0;

	if (!S_ISREG(inode->i_mode))
		return -EOPNOTSUPP;

	trace_ext4_punch_hole(inode, offset, length, 0);

	/*
	 * Write out all dirty pages to avoid race conditions
	 * Then release them.
	 */
	if (mapping->nrpages && mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
		ret = filemap_write_and_wait_range(mapping, offset,
						   offset + length - 1);
		if (ret)
			return ret;
	}

	mutex_lock(&inode->i_mutex);

	/* No need to punch hole beyond i_size */
	if (offset >= inode->i_size)
		goto out_mutex;

	/*
	 * If the hole extends beyond i_size, set the hole
	 * to end after the page that contains i_size
	 */
	if (offset + length > inode->i_size) {
		length = inode->i_size +
		   PAGE_CACHE_SIZE - (inode->i_size & (PAGE_CACHE_SIZE - 1)) -
		   offset;
	}

	if (offset & (sb->s_blocksize - 1) ||
	    (offset + length) & (sb->s_blocksize - 1)) {
		/*
		 * Attach jinode to inode for jbd2 if we do any zeroing of
		 * partial block
		 */
		ret = ext4_inode_attach_jinode(inode);
		if (ret < 0)
			goto out_mutex;

	}

	/* Wait all existing dio workers, newcomers will block on i_mutex */
	ext4_inode_block_unlocked_dio(inode);
	inode_dio_wait(inode);

	/*
	 * Prevent page faults from reinstantiating pages we have released from
	 * page cache.
	 */
	down_write(&EXT4_I(inode)->i_mmap_sem);
	first_block_offset = round_up(offset, sb->s_blocksize);
	last_block_offset = round_down((offset + length), sb->s_blocksize) - 1;

	/* Now release the pages and zero block aligned part of pages*/
	if (last_block_offset > first_block_offset)
		truncate_pagecache_range(inode, first_block_offset,
					 last_block_offset);

	if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))
		credits = ext4_writepage_trans_blocks(inode);
	else
		credits = ext4_blocks_for_truncate(inode);
	handle = ext4_journal_start(inode, EXT4_HT_TRUNCATE, credits);
	if (IS_ERR(handle)) {
		ret = PTR_ERR(handle);
		ext4_std_error(sb, ret);
		goto out_dio;
	}

	ret = ext4_zero_partial_blocks(handle, inode, offset,
				       length);
	if (ret)
		goto out_stop;

	first_block = (offset + sb->s_blocksize - 1) >>
		EXT4_BLOCK_SIZE_BITS(sb);
	stop_block = (offset + length) >> EXT4_BLOCK_SIZE_BITS(sb);

	/* If there are no blocks to remove, return now */
	if (first_block >= stop_block)
		goto out_stop;

	down_write(&EXT4_I(inode)->i_data_sem);
	ext4_discard_preallocations(inode);

	ret = ext4_es_remove_extent(inode, first_block,
				    stop_block - first_block);
	if (ret) {
		up_write(&EXT4_I(inode)->i_data_sem);
		goto out_stop;
	}

	if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))
		ret = ext4_ext_remove_space(inode, first_block,
					    stop_block - 1);
	else
		ret = ext4_ind_remove_space(handle, inode, first_block,
					    stop_block);

	up_write(&EXT4_I(inode)->i_data_sem);
	if (IS_SYNC(inode))
		ext4_handle_sync(handle);

	inode->i_mtime = inode->i_ctime = ext4_current_time(inode);
	ext4_mark_inode_dirty(handle, inode);
out_stop:
	ext4_journal_stop(handle);
out_dio:
	up_write(&EXT4_I(inode)->i_mmap_sem);
	ext4_inode_resume_unlocked_dio(inode);
out_mutex:
	mutex_unlock(&inode->i_mutex);
	return ret;
}