从ext2文件系统上读出超级块
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。
