ext2文件系统之ext2_lookup函数源代码分析

之前我们讲过vfs虚拟文件系统的lookup文件查找的实现，知道当内核的缓存没有找到的时候，就会调用到inode_operations结构体的lookup函数来实现，我们今天以ext2文件系统为例，来讲解一下对应文件系统的lookup函数的实现。
首先ext2的inode_operation结构体定义在ext2/namei.c，定义如下

const struct inode_operations ext2_dir_inode_operations = {.create= ext2_create,.lookup= ext2_lookup,.link= ext2_link,.unlink= ext2_unlink,.symlink= ext2_symlink,.mkdir= ext2_mkdir,.rmdir= ext2_rmdir,.mknod= ext2_mknod,.rename= ext2_rename,#ifdef CONFIG_EXT2_FS_XATTR.setxattr= generic_setxattr,.getxattr= generic_getxattr,.listxattr= ext2_listxattr,.removexattr= generic_removexattr,#endif.setattr= ext2_setattr,.permission= ext2_permission,};

可以看出来lookup函数是ext2_lookup，这个函数也定义在ext2/namei.c，定义如下

static struct dentry *ext2_lookup(struct inode * dir, struct dentry *dentry, struct nameidata *nd){struct inode * inode;ino_t ino;/*如果目录项的名字长度太长会返回错误*/if (dentry->d_name.len > EXT2_NAME_LEN)return ERR_PTR(-ENAMETOOLONG);/*核心的操作，ext2文件系统通过文件名寻找inode，返回inode的编号*/ino = ext2_inode_by_name(dir, dentry);inode = NULL;if (ino) {/*根据索引节点号来创造inode结构*/inode = iget(dir->i_sb, ino);if (!inode)return ERR_PTR(-EACCES);}/*说明出现错误，inode和dentry断开连接，处理*/return d_splice_alias(inode, dentry);}

然后我们看一下ext2_inode_by_name函数，这个函数定义在fs/ext2/dir.c，定义如下

ino_t ext2_inode_by_name(struct inode * dir, struct dentry *dentry){ino_t res = 0;struct ext2_dir_entry_2 * de;struct page *page;/*关键操作*/de = ext2_find_entry (dir, dentry, &page);if (de) {/*如果返回正确，得到inode号码，然后释放page*/res = le32_to_cpu(de->inode);ext2_put_page(page);}return res;}

我们继续看ext2_find_entry函数，ext2_find_entry函数定义在fs/ext2/dir.c，定义如下

struct ext2_dir_entry_2 * ext2_find_entry (struct inode * dir,struct dentry *dentry, struct page ** res_page){/*父目录的名字和长度*/const char *name = dentry->d_name.name;int namelen = dentry->d_name.len;/*指向下一个目录项的指针，以时间换空间的做法*/unsigned reclen = EXT2_DIR_REC_LEN(namelen);unsigned long start, n;/*返回这个inode在ext2文件系统占用的page数目*/unsigned long npages = dir_pages(dir);struct page *page = NULL;struct ext2_inode_info *ei = EXT2_I(dir);ext2_dirent * de;/*如果这个目录是空的，就直接返回*/if (npages == 0)goto out;/* OFFSET_CACHE */*res_page = NULL;/*开始查找的页数*/start = ei->i_dir_start_lookup;if (start >= npages)start = 0;n = start;/*大循环，一个页一个页的查找*/do {char *kaddr;/*从缓存中寻找，由inode结构体得到对应的页的数据，如果缓存上没有，就去硬盘上读取*/page = ext2_get_page(dir, n);/*查找成功，就在这个页上寻找*/if (!IS_ERR(page)) {/*由page得到虚拟地址*/kaddr = page_address(page);de = (ext2_dirent *) kaddr;/*这个页的最后地址*/kaddr += ext2_last_byte(dir, n) - reclen;/*只要没有到这个页的末尾，就继续循环*/while ((char *) de <= kaddr) {/*如果rec_len为0，就返回错误*/if (de->rec_len == 0) {ext2_error(dir->i_sb, __FUNCTION__,"zero-length directory entry");ext2_put_page(page);goto out;}/*比对，如果一致，就返回找到的目录项*/if (ext2_match (namelen, name, de))goto found;/*当前地址加上rec_len就是下一个地址*/de = ext2_next_entry(de);}/*如果当前目录项查找完毕，没有找到，就释放*/ext2_put_page(page);}/*n标记对应的开始页数*/if (++n >= npages)n = 0;/* next page is past the blocks we've got */if (unlikely(n > (dir->i_blocks >> (PAGE_CACHE_SHIFT - 9)))) {ext2_error(dir->i_sb, __FUNCTION__,"dir %lu size %lld exceeds block count %llu",dir->i_ino, dir->i_size,(unsigned long long)dir->i_blocks);goto out;}} while (n != start);out:return NULL;found:/*找到了，就返回inode的页和ext信息结构体*/*res_page = page;ei->i_dir_start_lookup = n;return de;}

然后我们回去看一下iget函数，就是通过inode号码返回inode结构体的函数，定义在linux/fs.h，定义如下

static inline struct inode *iget(struct super_block *sb, unsigned long ino){/*从挂载的文件系统里寻找inode*/struct inode *inode = iget_locked(sb, ino);/*如果没有找到就调用super_block的函数，直接读取inode*/if (inode && (inode->i_state & I_NEW)) {sb->s_op->read_inode(inode);unlock_new_inode(inode);}return inode;}

调用的super_operations结构体的read_inode函数，super_operations定义在/fs/ext2/super.c里，定义如下

static const struct super_operations ext2_sops = {.alloc_inode= ext2_alloc_inode,.destroy_inode= ext2_destroy_inode,.read_inode= ext2_read_inode,.write_inode= ext2_write_inode,.put_inode= ext2_put_inode,.delete_inode= ext2_delete_inode,.put_super= ext2_put_super,.write_super= ext2_write_super,.statfs= ext2_statfs,.remount_fs= ext2_remount,.clear_inode= ext2_clear_inode,.show_options= ext2_show_options,#ifdef CONFIG_QUOTA.quota_read= ext2_quota_read,.quota_write= ext2_quota_write,#endif};

从上边可以看出来，read_inode函数就是ext2_read_inode函数，这个函数定义在fs/ext2/inode.c里，定义如下

void ext2_read_inode (struct inode * inode){struct ext2_inode_info *ei = EXT2_I(inode);ino_t ino = inode->i_ino;struct buffer_head * bh;/*从super_block块和块号得到ext2_inode结构体*/struct ext2_inode * raw_inode = ext2_get_inode(inode->i_sb, ino, &bh);int n;#ifdef CONFIG_EXT2_FS_POSIX_ACLei->i_acl = EXT2_ACL_NOT_CACHED;ei->i_default_acl = EXT2_ACL_NOT_CACHED;#endifif (IS_ERR(raw_inode)) goto bad_inode; /*把得到的ext2_inode结构体编程inode结构体*/inode->i_mode = le16_to_cpu(raw_inode->i_mode);inode->i_uid = (uid_t)le16_to_cpu(raw_inode->i_uid_low);inode->i_gid = (gid_t)le16_to_cpu(raw_inode->i_gid_low);if (!(test_opt (inode->i_sb, NO_UID32))) {inode->i_uid |= le16_to_cpu(raw_inode->i_uid_high) << 16;inode->i_gid |= le16_to_cpu(raw_inode->i_gid_high) << 16;}inode->i_nlink = le16_to_cpu(raw_inode->i_links_count);inode->i_size = le32_to_cpu(raw_inode->i_size);inode->i_atime.tv_sec = (signed)le32_to_cpu(raw_inode->i_atime);inode->i_ctime.tv_sec = (signed)le32_to_cpu(raw_inode->i_ctime);inode->i_mtime.tv_sec = (signed)le32_to_cpu(raw_inode->i_mtime);inode->i_atime.tv_nsec = inode->i_mtime.tv_nsec = inode->i_ctime.tv_nsec = 0;ei->i_dtime = le32_to_cpu(raw_inode->i_dtime);/* We now have enough fields to check if the inode was active or not. * This is needed because nfsd might try to access dead inodes * the test is that same one that e2fsck uses * NeilBrown 1999oct15 */if (inode->i_nlink == 0 && (inode->i_mode == 0 || ei->i_dtime)) {/* this inode is deleted */brelse (bh);goto bad_inode;}inode->i_blocks = le32_to_cpu(raw_inode->i_blocks);ei->i_flags = le32_to_cpu(raw_inode->i_flags);ei->i_faddr = le32_to_cpu(raw_inode->i_faddr);ei->i_frag_no = raw_inode->i_frag;ei->i_frag_size = raw_inode->i_fsize;ei->i_file_acl = le32_to_cpu(raw_inode->i_file_acl);ei->i_dir_acl = 0;if (S_ISREG(inode->i_mode))inode->i_size |= ((__u64)le32_to_cpu(raw_inode->i_size_high)) << 32;elseei->i_dir_acl = le32_to_cpu(raw_inode->i_dir_acl);ei->i_dtime = 0;inode->i_generation = le32_to_cpu(raw_inode->i_generation);ei->i_state = 0;ei->i_next_alloc_block = 0;ei->i_next_alloc_goal = 0;ei->i_prealloc_count = 0;ei->i_block_group = (ino - 1) / EXT2_INODES_PER_GROUP(inode->i_sb);ei->i_dir_start_lookup = 0;/* * NOTE! The in-memory inode i_data array is in little-endian order * even on big-endian machines: we do NOT byteswap the block numbers! */for (n = 0; n < EXT2_N_BLOCKS; n++)ei->i_data[n] = raw_inode->i_block[n];/*处理特殊文件*/if (S_ISREG(inode->i_mode)) {inode->i_op = &ext2_file_inode_operations;if (ext2_use_xip(inode->i_sb)) {inode->i_mapping->a_ops = &ext2_aops_xip;inode->i_fop = &ext2_xip_file_operations;} else if (test_opt(inode->i_sb, NOBH)) {inode->i_mapping->a_ops = &ext2_nobh_aops;inode->i_fop = &ext2_file_operations;} else {inode->i_mapping->a_ops = &ext2_aops;inode->i_fop = &ext2_file_operations;}} else if (S_ISDIR(inode->i_mode)) {inode->i_op = &ext2_dir_inode_operations;inode->i_fop = &ext2_dir_operations;if (test_opt(inode->i_sb, NOBH))inode->i_mapping->a_ops = &ext2_nobh_aops;elseinode->i_mapping->a_ops = &ext2_aops;} else if (S_ISLNK(inode->i_mode)) {if (ext2_inode_is_fast_symlink(inode))inode->i_op = &ext2_fast_symlink_inode_operations;else {inode->i_op = &ext2_symlink_inode_operations;if (test_opt(inode->i_sb, NOBH))inode->i_mapping->a_ops = &ext2_nobh_aops;elseinode->i_mapping->a_ops = &ext2_aops;}} else {inode->i_op = &ext2_special_inode_operations;if (raw_inode->i_block[0])init_special_inode(inode, inode->i_mode,   old_decode_dev(le32_to_cpu(raw_inode->i_block[0])));else init_special_inode(inode, inode->i_mode,   new_decode_dev(le32_to_cpu(raw_inode->i_block[1])));}brelse (bh);ext2_set_inode_flags(inode);return;bad_inode:make_bad_inode(inode);return;}

接下来看主要的逻辑函数ext2_get_inode，这个函数定义在fs/ext2/inode.c文件，定义如下

static struct ext2_inode *ext2_get_inode(struct super_block *sb, ino_t ino,struct buffer_head **p){struct buffer_head * bh;unsigned long block_group;unsigned long block;unsigned long offset;struct ext2_group_desc * gdp;*p = NULL;/*检验参数是不是合法*/if ((ino != EXT2_ROOT_INO && ino < EXT2_FIRST_INO(sb)) ||    ino > le32_to_cpu(EXT2_SB(sb)->s_es->s_inodes_count))goto Einval;/*把inode的编号除以每一个块组的块的数目就得到想要的块在第几个块组*/block_group = (ino - 1) / EXT2_INODES_PER_GROUP(sb);/*由块组号码得到对应的块组描述符*/gdp = ext2_get_group_desc(sb, block_group, &bh);if (!gdp)goto Egdp;/*块号减一取余每一个块组的块的数目再乘上一个inode的大小，就得到了在对应块内的偏移*/offset = ((ino - 1) % EXT2_INODES_PER_GROUP(sb)) * EXT2_INODE_SIZE(sb);/*从块组描述符得到bg_inode_table字段，也就是这个块组的第一个inode表所在的块号，然后再加上偏移，就得到了块号对应的inode结构体*/block = le32_to_cpu(gdp->bg_inode_table) +(offset >> EXT2_BLOCK_SIZE_BITS(sb));/*算出是对应的块的第几个字节，调用sb_bread函数读取到bufferhead里*/if (!(bh = sb_bread(sb, block)))goto Eio;*p = bh;/*得到了对应的块，然后offset偏移后，使用(struct ext2_inode *)指针转化过后就得到了*/offset &= (EXT2_BLOCK_SIZE(sb) - 1);return (struct ext2_inode *) (bh->b_data + offset);Einval:ext2_error(sb, "ext2_get_inode", "bad inode number: %lu",   (unsigned long) ino);return ERR_PTR(-EINVAL);Eio:ext2_error(sb, "ext2_get_inode",   "unable to read inode block - inode=%lu, block=%lu",   (unsigned long) ino, block);Egdp:return ERR_PTR(-EIO);}

这个函数里边比较重要的是ext2_get_group_desc函数，这个函数定义在fs/ext2/balloc.c，定义如下

struct ext2_group_desc * ext2_get_group_desc(struct super_block * sb,     unsigned int block_group,     struct buffer_head ** bh){/*传入的block_group是inode在第几个块组*/unsigned long group_desc;unsigned long offset;struct ext2_group_desc * desc;struct ext2_sb_info *sbi = EXT2_SB(sb);/*检验参数是否合法*/if (block_group >= sbi->s_groups_count) {ext2_error (sb, "ext2_get_group_desc",    "block_group >= groups_count - "    "block_group = %d, groups_count = %lu",    block_group, sbi->s_groups_count);return NULL;}/*首先得到块组究竟是在第几个块上，块组描述符由连续的几个块组成，EXT2_DESC_PER_BLOCK_BITS(sb)返回一个块含有的块组描述符个数对应的含有二进制位数。比如一个块1024字节，一个块描述符由32字节，所以一个块有32个块描述符，所以就是返回5，因为32编程二进制有5个1，就等于块描述符除以32，得到在第几个块*/group_desc = block_group >> EXT2_DESC_PER_BLOCK_BITS(sb);/*找到块组描述符在块内的偏移*/offset = block_group & (EXT2_DESC_PER_BLOCK(sb) - 1);/*都算好以后就直接从sbi的superblock结构体里边找*/if (!sbi->s_group_desc[group_desc]) {ext2_error (sb, "ext2_get_group_desc",    "Group descriptor not loaded - "    "block_group = %d, group_desc = %lu, desc = %lu",     block_group, group_desc, offset);return NULL;}desc = (struct ext2_group_desc *) sbi->s_group_desc[group_desc]->b_data;if (bh)*bh = sbi->s_group_desc[group_desc];return desc + offset;}