Internals of ext2fs

ext2fs Superblock

As we saw before, the superblock structure has a slot in it for FS-specific data:

      struct super_block {
        ...
        union {
                ...
                struct ext2_sb_info     ext2_sb;
                ...
        } u;
        ...
      };

The struct ext2_sb_info is defined in include/linux/ext2_fs_sb.h:

      struct ext2_sb_info {
        unsigned long s_frag_size;      /* Size of a fragment in bytes */
        unsigned long s_frags_per_block;/* Number of fragments per block */
        unsigned long s_inodes_per_block;/* Number of inodes per block */
        unsigned long s_frags_per_group;/* Number of fragments in a group */
        unsigned long s_blocks_per_group;/* Number of blocks in a group */
        unsigned long s_inodes_per_group;/* Number of inodes in a group */
        ...
      }

ext2_read_super reads this stuff in too, or computes it