ext4中inode 7（Reserved group descriptors inode）对应的block块怎么找？

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根据https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout中的介绍，ext4的每个inode对应的block根据inode.i_block[15]进行查找，分两种方式，一种是直接/间接索引，一种是扩展树的方式。
但是编号为7的inode好像都不适用这两种方法，求相关资料。
在我的测试机上，根据我的调试，得到的inode 7的数据内容如下：
{i_mode = 33152, i_uid = 0, i_size_lo = 4243456, i_atime = 1466299725, i_ctime = 1466299725, i_mtime = 1466299725, i_dtime = 0, i_gid = 0, i_links_count = 1, i_blocks_lo = 34496, i_flags = 0, l_i_version = 0, i_block = {0 <repeats 13 times>, 8710, 0}, i_generation = 0, i_file_acl_lo = 0, i_size_high = 1, i_obso_faddr = 0, l_i_blocks_high = 0, l_i_file_acl_high = 0, l_i_uid_high = 0, l_i_gid_high = 0, l_i_checksum_lo = 0, l_i_reserved = 0, i_extra_isize = 28, i_checksum_hi = 0, i_ctime_extra = 0, i_mtime_extra = 0, i_atime_extra = 0, i_crtime = 1466299725, i_crtime_extra = 0, i_version_hi = 0, h_magic = 0}
其中i_flags=0比较奇怪，i_blocks前13个都是0，也比较奇怪。
我想要找到inode 7对应的所有block，该怎么办？