Internals of ext2fs

get_sb_bdev calls read_super to read in the superblock structure

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

Superblocks

The kernel reads in the superblock structure with a function read_super
This function dispatches the FS-specific superblock-reading function:

     static struct super_block * read_super(kdev_t dev, struct block_device *bdev,
                                            struct file_system_type *type, int flags,
                                            void *data, int silent)
     {
        struct super_block * s;
        s = get_empty_super();
        ...
        s->s_type = type;
        ...
        if (!type->read_super(s, data, silent))
                goto out_fail;
        ...
        return s;
     }

For ext2fs, this actually invokes ext2_read_super.