/*** 超级块中可能记录着逻辑块大小，因此我们必须** 以此为准*/blocksize = BLOCK_SIZE << le32_to_cpu(sbi->s_es->s_log_block_size);if (ext2_use_xip(sb) && blocksize != PAGE_SIZE) { if (!silent) ext2_msg(sb, KERN_ERR, "error: unsupported blocksize for xip"); goto failed_mount; } /* If the blocksize doesn't match, re-read the thing.. */ /* 如果块大小和我们之前确定的不太一样 ** 我们有必要重新读一次超级块 ** 因为之前读的可能并不准确 */ if (sb->s_blocksize != blocksize) { brelse(bh); if (!sb_set_blocksize(sb, blocksize)) { ext2_msg(sb, KERN_ERR, "error: blocksize is too small"); goto failed_sbi; } logic_sb_block = (sb_block*BLOCK_SIZE) / blocksize; offset = (sb_block*BLOCK_SIZE) % blocksize; bh = sb_bread(sb, logic_sb_block); if(!bh) { ext2_msg(sb, KERN_ERR, "error: couldn't read" "superblock on 2nd try"); goto failed_sbi; } es = (struct ext2_super_block *) (((char *)bh->b_data) + offset); sbi->s_es = es; if (es->s_magic != cpu_to_le16(EXT2_SUPER_MAGIC)) { ext2_msg(sb, KERN_ERR, "error: magic mismatch"); goto failed_mount; } } /* 计算ext2最大可支持文件的大小*/ sb->s_maxbytes = ext2_max_size(sb->s_blocksize_bits); if (le32_to_cpu(es->s_rev_level) == EXT2_GOOD_OLD_REV) { sbi->s_inode_size = EXT2_GOOD_OLD_INODE_SIZE; sbi->s_first_ino = EXT2_GOOD_OLD_FIRST_INO; } else { sbi->s_inode_size = le16_to_cpu(es->s_inode_size); sbi->s_first_ino = le32_to_cpu(es->s_first_ino); if ((sbi->s_inode_size < EXT2_GOOD_OLD_INODE_SIZE) || !is_power_of_2(sbi->s_inode_size) || (sbi->s_inode_size > blocksize)) { ext2_msg(sb, KERN_ERR, "error: unsupported inode size: %d", sbi->s_inode_size); goto failed_mount; } } /* 对于逻辑块较大的ext2文件系统，为了 ** 减少块内碎片问题，设置了fragment， ** 即每个磁盘块内可再细分成多个fragment ** 这个思想源自FFS，对于1024大小的磁盘块 ** 也就没有必要再划分fragment了 ** 因为最小的fragment大小就是1024字节 */ sbi->s_frag_size = EXT2_MIN_FRAG_SIZE << le32_to_cpu(es->s_log_frag_size); if (sbi->s_frag_size == 0) goto cantfind_ext2; /* 初始化一些静态信息*/ sbi->s_frags_per_block = sb->s_blocksize / sbi->s_frag_size; sbi->s_blocks_per_group = le32_to_cpu(es->s_blocks_per_group); sbi->s_frags_per_group = le32_to_cpu(es->s_frags_per_group); sbi->s_inodes_per_group = le32_to_cpu(es->s_inodes_per_group); if (EXT2_INODE_SIZE(sb) == 0) goto cantfind_ext2; sbi->s_inodes_per_block = sb->s_blocksize / EXT2_INODE_SIZE(sb); if (sbi->s_inodes_per_block == 0 || sbi->s_inodes_per_group == 0) goto cantfind_ext2; sbi->s_itb_per_group = sbi->s_inodes_per_group / sbi->s_inodes_per_block; sbi->s_desc_per_block = sb->s_blocksize / sizeof (struct ext2_group_desc); sbi->s_sbh = bh; sbi->s_mount_state = le16_to_cpu(es->s_state); sbi->s_addr_per_block_bits = ilog2 (EXT2_ADDR_PER_BLOCK(sb)); sbi->s_desc_per_block_bits = ilog2 (EXT2_DESC_PER_BLOCK(sb)); if (sb->s_magic != EXT2_SUPER_MAGIC) goto cantfind_ext2; if (sb->s_blocksize != bh->b_size) { if (!silent) ext2_msg(sb, KERN_ERR, "error: unsupported blocksize"); goto failed_mount; } /* 目前仅支持块大小和fragment size大小相同*/ if (sb->s_blocksize != sbi->s_frag_size) { ext2_msg(sb, KERN_ERR, "error: fragsize %lu != blocksize %lu" "(not supported yet)", sbi->s_frag_size, sb->s_blocksize); goto failed_mount; } if (sbi->s_blocks_per_group > sb->s_blocksize * 8) { ext2_msg(sb, KERN_ERR, "error: #blocks per group too big: %lu", sbi->s_blocks_per_group); goto failed_mount; } if (sbi->s_frags_per_group > sb->s_blocksize * 8) { ext2_msg(sb, KERN_ERR, "error: #fragments per group too big: %lu", sbi->s_frags_per_group); goto failed_mount; } if (sbi->s_inodes_per_group > sb->s_blocksize * 8) { ext2_msg(sb, KERN_ERR, "error: #inodes per group too big: %lu", sbi->s_inodes_per_group); goto failed_mount; } if (EXT2_BLOCKS_PER_GROUP(sb) == 0) goto cantfind_ext2; sbi->s_groups_count = ((le32_to_cpu(es->s_blocks_count) - le32_to_cpu(es->s_first_data_block) - 1) / EXT2_BLOCKS_PER_GROUP(sb)) + 1; db_count = (sbi->s_groups_count + EXT2_DESC_PER_BLOCK(sb) - 1) / EXT2_DESC_PER_BLOCK(sb); sbi->s_group_desc = kmalloc (db_count * sizeof (struct buffer_head *), GFP_KERNEL); if (sbi->s_group_desc == NULL) { ext2_msg(sb, KERN_ERR, "error: not enough memory"); goto failed_mount; } bgl_lock_init(sbi->s_blockgroup_lock); /* 这个数据结构干嘛的现在还不得而知*/ sbi->s_debts = kcalloc(sbi->s_groups_count, sizeof(*sbi->s_debts), GFP_KERNEL); if (!sbi->s_debts) { ext2_msg(sb, KERN_ERR, "error: not enough memory"); goto failed_mount_group_desc; } /* 读出块组描述符信息 */ for (i = 0; i < db_count; i++) { block = descriptor_loc(sb, logic_sb_block, i); sbi->s_group_desc[i] = sb_bread(sb, block); if (!sbi->s_group_desc[i]) { for (j = 0; j < i; j++) brelse (sbi->s_group_desc[j]); ext2_msg(sb, KERN_ERR, "error: unable to read group descriptors"); goto failed_mount_group_desc; } } if (!ext2_check_descriptors (sb)) { ext2_msg(sb, KERN_ERR, "group descriptors corrupted"); goto failed_mount2; } sbi->s_gdb_count = db_count; get_random_bytes(&sbi->s_next_generation, sizeof(u32)); spin_lock_init(&sbi->s_next_gen_lock); /* per fileystem reservation list head & lock */ //init something for reservation windows of every file spin_lock_init(&sbi->s_rsv_window_lock); sbi->s_rsv_window_root = RB_ROOT; /* * Add a single, static dummy reservation to the start of the * reservation window list --- it gives us a placeholder for * append-at-start-of-list which makes the allocation logic * _much_ simpler. */ /* 初始化内存超级块的预留窗口 ** 所谓的预留窗口是在分配数据块的时候 ** 每一次多分配一点，以提高文件数据存储 ** 的连续性 */ sbi->s_rsv_window_head.rsv_start = EXT2_RESERVE_WINDOW_NOT_ALLOCATED; sbi->s_rsv_window_head.rsv_end = EXT2_RESERVE_WINDOW_NOT_ALLOCATED; sbi->s_rsv_window_head.rsv_alloc_hit = 0; sbi->s_rsv_window_head.rsv_goal_size = 0; ext2_rsv_window_add(sb, &sbi->s_rsv_window_head); err = percpu_counter_init(&sbi->s_freeblocks_counter, ext2_count_free_blocks(sb)); if (!err) { err = percpu_counter_init(&sbi->s_freeinodes_counter, ext2_count_free_inodes(sb)); } if (!err) { err = percpu_counter_init(&sbi->s_dirs_counter, ext2_count_dirs(sb)); } if (err) { ext2_msg(sb, KERN_ERR, "error: insufficient memory"); goto failed_mount3; } /* * set up enough so that it can read an inode */ sb->s_op = &ext2_sops; sb->s_export_op = &ext2_export_ops; sb->s_xattr = ext2_xattr_handlers; #ifdef CONFIG_QUOTA sb->dq_op = &dquot_operations; sb->s_qcop = &dquot_quotactl_ops; #endif

/* 读根目录的inode，inode号为默认值2** 读出后保存在内存inode结构中*/root = ext2_iget(sb, EXT2_ROOT_INO);if (IS_ERR(root)) { ret = PTR_ERR(root); goto failed_mount3; } if (!S_ISDIR(root->i_mode) || !root->i_blocks || !root->i_size) { iput(root); ext2_msg(sb, KERN_ERR, "error: corrupt root inode, run e2fsck"); goto failed_mount3; } /* 分配根目录的内存目录项 ** 因为根目录没有父目录这个概念 ** 因此，没法从其父目录中读出其目录 ** 只能在内存中构造一个 */ sb->s_root = d_alloc_root(root); if (!sb->s_root) { iput(root); ext2_msg(sb, KERN_ERR, "error: get root inode failed"); ret = -ENOMEM; goto failed_mount3; } if (EXT2_HAS_COMPAT_FEATURE(sb, EXT3_FEATURE_COMPAT_HAS_JOURNAL)) ext2_msg(sb, KERN_WARNING, "warning: mounting ext3 filesystem as ext2"); if (ext2_setup_super (sb, es, sb->s_flags & MS_RDONLY)) sb->s_flags |= MS_RDONLY; /* 在填充超级块时有可能会修改磁盘超级块 ** 因此有必要作一次写回操作 */ ext2_write_super(sb); return 0;

到此，整个从磁盘读出超级块直至填充内存超级块结构的过程就结束了，整个流程虽然繁杂，但还算简单。