ext2_read_super also checks to make sure the FS is really an ext2fs:
es = (struct ext2_super_block *) (((char *)bh->b_data) + offset);
sb->s_magic = le16_to_cpu(es->s_magic);
if (sb->s_magic != EXT2_SUPER_MAGIC) {
if (!silent)
printk ("VFS: Can't find an ext2 filesystem on dev "
"%s.\n", bdevname(dev));
...
return NULL;
}
And that's how mount works