概述
本篇博客中，我们将仔细分析如何从格式化为ext2文件系统的磁盘中读取超级块并填充内存超级块结构，每次将一个格式化了ext2文件系统的磁盘（分区）挂载到挂载点的时候会调用该方法，该方法在操作系统中的实现主要是函数ext2_fill_super。
实现
在ext2系列之前的博客中我们描述了ext2的磁盘划分，所以读取超级块的过程也就显得比较简单，只是在读取完成后可能需要进行一些列的检查等。废话不多说，我们直接来看该函数的实现。我们分为几段来阐述其实现机理。
第一阶段：从磁盘读出超级块
1 static int ext2_fill_super(struct super_block *sb, void *data, int silent) 2 { 3 struct buffer_head * bh; 4 struct ext2_sb_info * sbi; 5 struct ext2_super_block * es; 6 struct inode *root; 7 unsigned long block; 8 unsigned long sb_block = get_sb_block(&data); 9 unsigned long logic_sb_block; 10 unsigned long offset = 0; 11 unsigned long def_mount_opts; 12 long ret = -EINVAL; 13 //default block size is 1024B 14 int blocksize = BLOCK_SIZE; 15 int db_count; 16 int i, j; 17 __le32 features; 18 int err; 19 20 //allocate memory ext2_super_block in memory 21 sbi = kzalloc(sizeof(*sbi), GFP_KERNEL); 22 if (!sbi) 23 return -ENOMEM; 24 25 sbi->s_blockgroup_lock = 26 kzalloc(sizeof(struct blockgroup_lock), GFP_KERNEL); 27 if (!sbi->s_blockgroup_lock) { 28 kfree(sbi); 29 return -ENOMEM; 30 } 31 //sb is vfs super_block 32 //sb->s_fs_info is specific file system super block 33 sb->s_fs_info = sbi; 34 sbi->s_sb_block = sb_block; 35 36 spin_lock_init(&sbi->s_lock); 37 38 /* 39 * See what the current blocksize for the device is, and 40 * use that as the blocksize. Otherwise (or if the blocksize 41 * is smaller than the default) use the default. 42 * This is important for devices that have a hardware 43 * sectorsize that is larger than the default. 44 */ 45 //the block size can't be smaller than BLOCK_SIZE=1024B 46 //and block size must be smaller than PAGE_SIZE = 4096B now 47 blocksize = sb_min_blocksize(sb, BLOCK_SIZE); 48 if (!blocksize) { 49 ext2_msg(sb, KERN_ERR, "error: unable to set blocksize"); 50 goto failed_sbi; 51 } 52 53 /* 54 * If the superblock doesn't start on a hardware sector boundary, 55 * calculate the offset. 56 */ 57 //blocksize may bigger than BLOCK_SIZE=1024B 58 //because we read blocksize bytes data from disk 59 //Block 0 is 1024B and super_block is also 1024B 60 //if blocksize is not 1024B,it must be bigger than 1024B,for example,if blocksize is 2048B 61 //we must read block 0(first 2048B on disk),then we read offset 1024~2047 as super block 62 if (blocksize != BLOCK_SIZE) { 63 logic_sb_block = (sb_block*BLOCK_SIZE) / blocksize; 64 offset = (sb_block*BLOCK_SIZE) % blocksize; 65 } else { 66 logic_sb_block = sb_block; 67 } 68 //read block @logic_sb_block containg super block 69 if (!(bh = sb_bread(sb, logic_sb_block))) { 70 ext2_msg(sb, KERN_ERR, "error: unable to read superblock"); 71 goto failed_sbi; 72 } 73 /* 74 * Note: s_es must be initialized as soon as possible because 75 * some ext2 macro-instructions depend on its value 76 */ 77 es = (struct ext2_super_block *) (((char *)bh->b_data) + offset); 78 //sbi is ext2_super_block in memory while sbi->s_es is ext2_super_block on disk 79 sbi->s_es = es;
第一部分的函数是核心，它负责从ext2的磁盘（分区）上读出超级块，那么这里的问题就产生了：
- 超级块的起始位置在哪？
- 超级块的大小是多少？
- 在实现中我们自己定义的块大小（默认1024）与磁盘设备的块大小如果不一致怎么办？
所以我们看上面的很多代码其实都是在处理这个问题。让我们一一来解答。
首先，超级块位于磁盘（分区）的第二个1024位置上，因为第一个1024字节默认作为引导块，文件系统并不使用。
其次，ext2的超级块大小也为1024字节，这在ext2超级块数据结构的定义中可以看出。
最后，因为读之前我们默认磁盘块大小是1024字节，但磁盘设备定义的块大小可能不同，有可能是2048,4096等等，而我们读磁盘数据的时候是以逻辑块为单位读取的（虽然最终的物理读取是以扇区为单位的），因此，我们必须确定到底块大小是多少，如果决定块大小是1024，那我们只需读出第二个磁盘块即可读出超级块，而如果块大小是2048，那我们读出第一个磁盘块，然后再取1024~2047这一段，下图比较清晰地阐述了这个过程：
另外，我们读出超级块是要缓存在内存中的，而内存中的超级块结构需要在磁盘超级块结构上增添一些管理成员。ext2内存超级块结构为struct ext2_sb_info。
ext2_fill_super所展示的第一段代码所做工作主要有：
- 分配ext2内存超级块结构struct ext2_sb_info，如果分配内存失败，则直接返回-ENOMEM;
- 确定逻辑磁盘块大小，比较默认逻辑块大小和真实逻辑块大小（根据磁盘设备的一些信息确定），将最大者设置为逻辑块大小，但注意：该最大者必须是2的次幂且不可大于4096
- 从磁盘上读出超级块根据2中计算的块大小确定超级块所在逻辑块号和块内偏移，读出超级块，存储在1中分配的内存超级块结构中sbi->s_es = es。
第二阶段：根据磁盘超级块初始化内存超级块的成员
上文描述的第一阶段从磁盘上读出了超级块内容，接下来我们就要根据磁盘上的超级块结构来初始化内存超级块结构，在这个过程中可能还伴随着磁盘超级块内容的检查，确认其是否已经损坏等。
1 sb->s_magic = le16_to_cpu(es->s_magic); 2 3 if (sb->s_magic != EXT2_SUPER_MAGIC) 4 goto cantfind_ext2; 5 6 /* Set defaults before we parse the mount options */ 7 /* 接下来这段根据磁盘超级块 8 ** 结构来设置内存超级块结构的部分选项 9 ** 相比较而言这些选项的重要性没那么高 10 */ 11 def_mount_opts = le32_to_cpu(es->s_default_mount_opts); 12 if (def_mount_opts & EXT2_DEFM_DEBUG) 13 set_opt(sbi->s_mount_opt, DEBUG); 14 if (def_mount_opts & EXT2_DEFM_BSDGROUPS) 15 set_opt(sbi->s_mount_opt, GRPID); 16 if (def_mount_opts & EXT2_DEFM_UID16) 17 set_opt(sbi->s_mount_opt, NO_UID32); 18 #ifdef CONFIG_EXT2_FS_XATTR 19 if (def_mount_opts & EXT2_DEFM_XATTR_USER) 20 set_opt(sbi->s_mount_opt, XATTR_USER); 21 #endif 22 #ifdef CONFIG_EXT2_FS_POSIX_ACL 23 if (def_mount_opts & EXT2_DEFM_ACL) 24 set_opt(sbi->s_mount_opt, POSIX_ACL); 25 #endif 26 /* 这个选项决定了挂载出错时的处理方法 27 ** 如PANIC即指示出错就奔溃... 28 */ 29 if (le16_to_cpu(sbi->s_es->s_errors) == EXT2_ERRORS_PANIC) 30 set_opt(sbi->s_mount_opt, ERRORS_PANIC); 31 else if (le16_to_cpu(sbi->s_es->s_errors) == EXT2_ERRORS_CONTINUE) 32 set_opt(sbi->s_mount_opt, ERRORS_CONT); 33 else 34 set_opt(sbi->s_mount_opt, ERRORS_RO); 35 36 sbi->s_resuid = le16_to_cpu(es->s_def_resuid); 37 sbi->s_resgid = le16_to_cpu(es->s_def_resgid); 38 39 set_opt(sbi->s_mount_opt, RESERVATION); 40 41 if (!parse_options((char *) data, sb)) 42 goto failed_mount; 43 44 sb->s_flags = (sb->s_flags & ~MS_POSIXACL) | 45 ((EXT2_SB(sb)->s_mount_opt & EXT2_MOUNT_POSIX_ACL) ? 46 MS_POSIXACL : 0); 47 48 ext2_xip_verify_sb(sb); /* see if bdev supports xip, unset 49 EXT2_MOUNT_XIP if not */ 50 51 if (le32_to_cpu(es->s_rev_level) == EXT2_GOOD_OLD_REV && 52 (EXT2_HAS_COMPAT_FEATURE(sb, ~0U) || 53 EXT2_HAS_RO_COMPAT_FEATURE(sb, ~0U) || 54 EXT2_HAS_INCOMPAT_FEATURE(sb, ~0U))) 55 ext2_msg(sb, KERN_WARNING, 56 "warning: feature flags set on rev 0 fs, " 57 "running e2fsck is recommended"); 58 /* 59 * Check feature flags regardless of the revision level, since we 60 * previously didn't change the revision level when setting the flags, 61 * so there is a chance incompat flags are set on a rev 0 filesystem. 62 */ 63 features = EXT2_HAS_INCOMPAT_FEATURE(sb, ~EXT2_FEATURE_INCOMPAT_SUPP); 64 if (features) { 65 ext2_msg(sb, KERN_ERR, "error: couldn't mount because of " 66 "unsupported optional features (%x)", 67 le32_to_cpu(features)); 68 goto failed_mount; 69 } 70 if (!(sb->s_flags & MS_RDONLY) && 71 (features = EXT2_HAS_RO_COMPAT_FEATURE(sb, ~EXT2_FEATURE_RO_COMPAT_SUPP))){ 72 ext2_msg(sb, KERN_ERR, "error: couldn't mount RDWR because of " 73 "unsupported optional features (%x)", 74 le32_to_cpu(features)); 75 goto failed_mount; 76 }
相对来说，这部分的代码重要性没那么高，我们无需花费太多的精力，简单阅读下注释即可。
第三阶段：根据磁盘超级块初始化内存超级块的成员（续）
第二阶段初始化内存超级块的只是一些比较简单的选项，到了这个阶段，初始化的东西就比较重要了，它关乎着文件系统的正确性。因此我们作比较详细的分析。
/* ** 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
这里面涉及到的细节问题比较多，但是都比较简单，主要是文件系统各种统计数据的计算等，这里不再赘述，请直接参考代码注释。
第四阶段：构造根目录
当超级块完全读出并构造内存超级块以后，接下来就是构造根目录了，让我们直接看代码：
/* 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。