Linux中ext2文件系统的结构

1、ext2产生的历史

  最早的Linux内核是从MINIX系统过渡发展而来的。Linux最早的文件系统就是MINIX文件系统。MINIX文件系统几乎到处都是bug,采用的是16bit偏移量,最大容量为64M,文件名最长为14字符。
  Linux内核0.96版本,包含了虚拟文件系统(vitual file system, VFS),VFS中提供了基础的API用于简化扩展新的文件系统——扩展的文件系统(extended file system, ext)基于VFS API。
  1992年Linux发布的0.96内核版本拥有了Linux真正意义上的文件系统——扩展的文件系统(The Extended Filesystem, ext),ext采用虚拟文件系统(virtual file system, VFS)即基础的API接口简化扩展的文件系统添加于内核。ext使得单个文件的最大数据容量可达2G,文件名的最大长度扩展到了255个字符。但是ext中不支持文件访问atime,文件内容修改mtime,文件属性修改iNode中的ctime相互独立的时间戳。
  为了解决文件的时间戳问题,1993年Linux 0.99版本内核推出,Linux推出了两种新的文件系统:xiafs和ext2(The Second Extended Filesystem).ext2的许多设计理念与Berkeley Fast File System相同,具有扩展的理念,在磁盘数据结构中预留了扩展空间,给将来的新版文件系统的留下了扩展空间。
  之后,ext2成了VFS API中许多新扩展的测试平台。例如VFS内含的POSIX访问控制协议草案和扩展属性协议最先内嵌于ext2,归功于ext2的易扩展和易于理解的中断机制。到了Linux的2.6.17内核版本,ext2中的文件最大可达2TB。目前,ext2在闪盘和固态硬盘领域其影响仍由于日志文件系统(ext3/ext4)。因为ext2没有ext3那样额外的日志写操作。因为频繁的写操作,会降低磁盘的使用寿命,相反固态硬盘的寿命因此得以延长。闪盘因为在挂载的时候没有访问时间戳atime,寿命也得意延长。

翻译自维基百科:https://en.wikipedia.org/wiki/Ext2

 

2、ext2文件系统的结构

在磁盘中,ext2文件系统的结构可以用以下图来描述:

 注意:以上是一个磁盘分区,第一个block,是系统启动扇区(boot sector),其余的分区空间都给了ext2文件系统,ext2将该磁盘分区分成了若干个块组(block group 0-N)。

以下是ext2数据结构(ext2_fs.h):

     1    /*
     2     *  linux/include/linux/ext2_fs.h
     3     *
     4     * Copyright (C) 1992, 1993, 1994, 1995
     5     * Remy Card (card@masi.ibp.fr)
     6     * Laboratoire MASI - Institut Blaise Pascal
     7     * Universite Pierre et Marie Curie (Paris VI)
     8     *
     9     *  from
    10     *
    11     *  linux/include/linux/minix_fs.h
    12     *
    13     *  Copyright (C) 1991, 1992  Linus Torvalds
    14     */
    15    
    16    #ifndef _LINUX_EXT2_FS_H
    17    #define _LINUX_EXT2_FS_H
    18    
    19    #include <linux/types.h>
    20    
    21    /*
    22     * The second extended filesystem constants/structures
    23     */
    24    
    25    /*
    26     * Define EXT2FS_DEBUG to produce debug messages
    27     */
    28    #undef EXT2FS_DEBUG
    29    
    30    /*
    31     * Define EXT2_PREALLOCATE to preallocate data blocks for expanding files
    32     */
    33    #define EXT2_PREALLOCATE
    34    #define EXT2_DEFAULT_PREALLOC_BLOCKS    8
    35    
    36    /*
    37     * The second extended file system version
    38     */
    39    #define EXT2FS_DATE        "95/08/09"
    40    #define EXT2FS_VERSION        "0.5b"
    41    
    42    /*
    43     * Debug code
    44     */
    45    #ifdef EXT2FS_DEBUG
    46    #    define ext2_debug(f, a...)    { \
    47                        printk ("EXT2-fs DEBUG (%s, %d): %s:", \
    48                            __FILE__, __LINE__, __FUNCTION__); \
    49                          printk (f, ## a); \
    50                        }
    51    #else
    52    #    define ext2_debug(f, a...)    /**/
    53    #endif
    54    
    55    /*
    56     * Special inode numbers
    57     */
    58    #define    EXT2_BAD_INO         1    /* Bad blocks inode */
    59    #define EXT2_ROOT_INO         2    /* Root inode */
    60    #define EXT2_ACL_IDX_INO     3    /* ACL inode */
    61    #define EXT2_ACL_DATA_INO     4    /* ACL inode */
    62    #define EXT2_BOOT_LOADER_INO     5    /* Boot loader inode */
    63    #define EXT2_UNDEL_DIR_INO     6    /* Undelete directory inode */
    64    
    65    /* First non-reserved inode for old ext2 filesystems */
    66    #define EXT2_GOOD_OLD_FIRST_INO    11
    67    
    68    /*
    69     * The second extended file system magic number
    70     */
    71    #define EXT2_SUPER_MAGIC    0xEF53
    72    
    73    /*
    74     * Maximal count of links to a file
    75     */
    76    #define EXT2_LINK_MAX        32000
    77    
    78    /*
    79     * Macro-instructions used to manage several block sizes
    80     */
    81    #define EXT2_MIN_BLOCK_SIZE        1024
    82    #define    EXT2_MAX_BLOCK_SIZE        4096
    83    #define EXT2_MIN_BLOCK_LOG_SIZE          10
    84    #ifdef __KERNEL__
    85    # define EXT2_BLOCK_SIZE(s)        ((s)->s_blocksize)
    86    #else
    87    # define EXT2_BLOCK_SIZE(s)        (EXT2_MIN_BLOCK_SIZE << (s)->s_log_block_size)
    88    #endif
    89    #define EXT2_ACLE_PER_BLOCK(s)        (EXT2_BLOCK_SIZE(s) / sizeof (struct ext2_acl_entry))
    90    #define    EXT2_ADDR_PER_BLOCK(s)        (EXT2_BLOCK_SIZE(s) / sizeof (__u32))
    91    #ifdef __KERNEL__
    92    # define EXT2_BLOCK_SIZE_BITS(s)    ((s)->s_blocksize_bits)
    93    #else
    94    # define EXT2_BLOCK_SIZE_BITS(s)    ((s)->s_log_block_size + 10)
    95    #endif
    96    #ifdef __KERNEL__
    97    #define    EXT2_ADDR_PER_BLOCK_BITS(s)    ((s)->u.ext2_sb.s_addr_per_block_bits)
    98    #define EXT2_INODE_SIZE(s)        ((s)->u.ext2_sb.s_inode_size)
    99    #define EXT2_FIRST_INO(s)        ((s)->u.ext2_sb.s_first_ino)
   100    #else
   101    #define EXT2_INODE_SIZE(s)    (((s)->s_rev_level == EXT2_GOOD_OLD_REV) ? \
   102                     EXT2_GOOD_OLD_INODE_SIZE : \
   103                     (s)->s_inode_size)
   104    #define EXT2_FIRST_INO(s)    (((s)->s_rev_level == EXT2_GOOD_OLD_REV) ? \
   105                     EXT2_GOOD_OLD_FIRST_INO : \
   106                     (s)->s_first_ino)
   107    #endif
   108    
   109    /*
   110     * Macro-instructions used to manage fragments
   111     */
   112    #define EXT2_MIN_FRAG_SIZE        1024
   113    #define    EXT2_MAX_FRAG_SIZE        4096
   114    #define EXT2_MIN_FRAG_LOG_SIZE          10
   115    #ifdef __KERNEL__
   116    # define EXT2_FRAG_SIZE(s)        ((s)->u.ext2_sb.s_frag_size)
   117    # define EXT2_FRAGS_PER_BLOCK(s)    ((s)->u.ext2_sb.s_frags_per_block)
   118    #else
   119    # define EXT2_FRAG_SIZE(s)        (EXT2_MIN_FRAG_SIZE << (s)->s_log_frag_size)
   120    # define EXT2_FRAGS_PER_BLOCK(s)    (EXT2_BLOCK_SIZE(s) / EXT2_FRAG_SIZE(s))
   121    #endif
   122    
   123    /*
   124     * ACL structures
   125     */
   126    struct ext2_acl_header    /* Header of Access Control Lists */
   127    {
   128        __u32    aclh_size;
   129        __u32    aclh_file_count;
   130        __u32    aclh_acle_count;
   131        __u32    aclh_first_acle;
   132    };
   133    
   134    struct ext2_acl_entry    /* Access Control List Entry */
   135    {
   136        __u32    acle_size;
   137        __u16    acle_perms;    /* Access permissions */
   138        __u16    acle_type;    /* Type of entry */
   139        __u16    acle_tag;    /* User or group identity */
   140        __u16    acle_pad1;
   141        __u32    acle_next;    /* Pointer on next entry for the */
   142                        /* same inode or on next free entry */
   143    };
   144    
   145    /*
   146     * Structure of a blocks group descriptor
   147     */
   148    struct ext2_group_desc
   149    {
   150        __u32    bg_block_bitmap;        /* Blocks bitmap block */
   151        __u32    bg_inode_bitmap;        /* Inodes bitmap block */
   152        __u32    bg_inode_table;        /* Inodes table block */
   153        __u16    bg_free_blocks_count;    /* Free blocks count */
   154        __u16    bg_free_inodes_count;    /* Free inodes count */
   155        __u16    bg_used_dirs_count;    /* Directories count */
   156        __u16    bg_pad;
   157        __u32    bg_reserved[3];
   158    };
   159    
   160    /*
   161     * Macro-instructions used to manage group descriptors
   162     */
   163    #ifdef __KERNEL__
   164    # define EXT2_BLOCKS_PER_GROUP(s)    ((s)->u.ext2_sb.s_blocks_per_group)
   165    # define EXT2_DESC_PER_BLOCK(s)        ((s)->u.ext2_sb.s_desc_per_block)
   166    # define EXT2_INODES_PER_GROUP(s)    ((s)->u.ext2_sb.s_inodes_per_group)
   167    # define EXT2_DESC_PER_BLOCK_BITS(s)    ((s)->u.ext2_sb.s_desc_per_block_bits)
   168    #else
   169    # define EXT2_BLOCKS_PER_GROUP(s)    ((s)->s_blocks_per_group)
   170    # define EXT2_DESC_PER_BLOCK(s)        (EXT2_BLOCK_SIZE(s) / sizeof (struct ext2_group_desc))
   171    # define EXT2_INODES_PER_GROUP(s)    ((s)->s_inodes_per_group)
   172    #endif
   173    
   174    /*
   175     * Constants relative to the data blocks
   176     */
   177    #define    EXT2_NDIR_BLOCKS        12
   178    #define    EXT2_IND_BLOCK            EXT2_NDIR_BLOCKS
   179    #define    EXT2_DIND_BLOCK            (EXT2_IND_BLOCK + 1)
   180    #define    EXT2_TIND_BLOCK            (EXT2_DIND_BLOCK + 1)
   181    #define    EXT2_N_BLOCKS            (EXT2_TIND_BLOCK + 1)
   182    
   183    /*
   184     * Inode flags
   185     */
   186    #define    EXT2_SECRM_FL            0x00000001 /* Secure deletion */
   187    #define    EXT2_UNRM_FL            0x00000002 /* Undelete */
   188    #define    EXT2_COMPR_FL            0x00000004 /* Compress file */
   189    #define EXT2_SYNC_FL            0x00000008 /* Synchronous updates */
   190    #define EXT2_IMMUTABLE_FL        0x00000010 /* Immutable file */
   191    #define EXT2_APPEND_FL            0x00000020 /* writes to file may only append */
   192    #define EXT2_NODUMP_FL            0x00000040 /* do not dump file */
   193    #define EXT2_NOATIME_FL            0x00000080 /* do not update atime */
   194    /* Reserved for compression usage... */
   195    #define EXT2_DIRTY_FL            0x00000100
   196    #define EXT2_COMPRBLK_FL        0x00000200 /* One or more compressed clusters */
   197    #define EXT2_NOCOMP_FL            0x00000400 /* Don't compress */
   198    #define EXT2_ECOMPR_FL            0x00000800 /* Compression error */
   199    /* End compression flags --- maybe not all used */    
   200    #define EXT2_BTREE_FL            0x00001000 /* btree format dir */
   201    #define EXT2_RESERVED_FL        0x80000000 /* reserved for ext2 lib */
   202    
   203    #define EXT2_FL_USER_VISIBLE        0x00001FFF /* User visible flags */
   204    #define EXT2_FL_USER_MODIFIABLE        0x000000FF /* User modifiable flags */
   205    
   206    /*
   207     * ioctl commands
   208     */
   209    #define    EXT2_IOC_GETFLAGS        _IOR('f', 1, long)
   210    #define    EXT2_IOC_SETFLAGS        _IOW('f', 2, long)
   211    #define    EXT2_IOC_GETVERSION        _IOR('v', 1, long)
   212    #define    EXT2_IOC_SETVERSION        _IOW('v', 2, long)
   213    
   214    /*
   215     * Structure of an inode on the disk
   216     */
   217    struct ext2_inode {
   218        __u16    i_mode;        /* File mode */
   219        __u16    i_uid;        /* Low 16 bits of Owner Uid */
   220        __u32    i_size;        /* Size in bytes */
   221        __u32    i_atime;    /* Access time */
   222        __u32    i_ctime;    /* Creation time */
   223        __u32    i_mtime;    /* Modification time */
   224        __u32    i_dtime;    /* Deletion Time */
   225        __u16    i_gid;        /* Low 16 bits of Group Id */
   226        __u16    i_links_count;    /* Links count */
   227        __u32    i_blocks;    /* Blocks count */
   228        __u32    i_flags;    /* File flags */
   229        union {
   230            struct {
   231                __u32  l_i_reserved1;
   232            } linux1;
   233            struct {
   234                __u32  h_i_translator;
   235            } hurd1;
   236            struct {
   237                __u32  m_i_reserved1;
   238            } masix1;
   239        } osd1;                /* OS dependent 1 */
   240        __u32    i_block[EXT2_N_BLOCKS];/* Pointers to blocks */
   241        __u32    i_generation;    /* File version (for NFS) */
   242        __u32    i_file_acl;    /* File ACL */
   243        __u32    i_dir_acl;    /* Directory ACL */
   244        __u32    i_faddr;    /* Fragment address */
   245        union {
   246            struct {
   247                __u8    l_i_frag;    /* Fragment number */
   248                __u8    l_i_fsize;    /* Fragment size */
   249                __u16    i_pad1;
   250                __u16    l_i_uid_high;    /* these 2 fields    */
   251                __u16    l_i_gid_high;    /* were reserved2[0] */
   252                __u32    l_i_reserved2;
   253            } linux2;
   254            struct {
   255                __u8    h_i_frag;    /* Fragment number */
   256                __u8    h_i_fsize;    /* Fragment size */
   257                __u16    h_i_mode_high;
   258                __u16    h_i_uid_high;
   259                __u16    h_i_gid_high;
   260                __u32    h_i_author;
   261            } hurd2;
   262            struct {
   263                __u8    m_i_frag;    /* Fragment number */
   264                __u8    m_i_fsize;    /* Fragment size */
   265                __u16    m_pad1;
   266                __u32    m_i_reserved2[2];
   267            } masix2;
   268        } osd2;                /* OS dependent 2 */
   269    };
   270    
   271    #define i_size_high    i_dir_acl
   272    
   273    #if defined(__KERNEL__) || defined(__linux__)
   274    #define i_reserved1    osd1.linux1.l_i_reserved1
   275    #define i_frag        osd2.linux2.l_i_frag
   276    #define i_fsize        osd2.linux2.l_i_fsize
   277    #define i_uid_low    i_uid
   278    #define i_gid_low    i_gid
   279    #define i_uid_high    osd2.linux2.l_i_uid_high
   280    #define i_gid_high    osd2.linux2.l_i_gid_high
   281    #define i_reserved2    osd2.linux2.l_i_reserved2
   282    #endif
   283    
   284    #ifdef    __hurd__
   285    #define i_translator    osd1.hurd1.h_i_translator
   286    #define i_frag        osd2.hurd2.h_i_frag;
   287    #define i_fsize        osd2.hurd2.h_i_fsize;
   288    #define i_uid_high    osd2.hurd2.h_i_uid_high
   289    #define i_gid_high    osd2.hurd2.h_i_gid_high
   290    #define i_author    osd2.hurd2.h_i_author
   291    #endif
   292    
   293    #ifdef    __masix__
   294    #define i_reserved1    osd1.masix1.m_i_reserved1
   295    #define i_frag        osd2.masix2.m_i_frag
   296    #define i_fsize        osd2.masix2.m_i_fsize
   297    #define i_reserved2    osd2.masix2.m_i_reserved2
   298    #endif
   299    
   300    /*
   301     * File system states
   302     */
   303    #define    EXT2_VALID_FS            0x0001    /* Unmounted cleanly */
   304    #define    EXT2_ERROR_FS            0x0002    /* Errors detected */
   305    
   306    /*
   307     * Mount flags
   308     */
   309    #define EXT2_MOUNT_CHECK        0x0001    /* Do mount-time checks */
   310    #define EXT2_MOUNT_GRPID        0x0004    /* Create files with directory's group */
   311    #define EXT2_MOUNT_DEBUG        0x0008    /* Some debugging messages */
   312    #define EXT2_MOUNT_ERRORS_CONT        0x0010    /* Continue on errors */
   313    #define EXT2_MOUNT_ERRORS_RO        0x0020    /* Remount fs ro on errors */
   314    #define EXT2_MOUNT_ERRORS_PANIC        0x0040    /* Panic on errors */
   315    #define EXT2_MOUNT_MINIX_DF        0x0080    /* Mimics the Minix statfs */
   316    #define EXT2_MOUNT_NO_UID32        0x0200  /* Disable 32-bit UIDs */
   317    
   318    #define clear_opt(o, opt)        o &= ~EXT2_MOUNT_##opt
   319    #define set_opt(o, opt)            o |= EXT2_MOUNT_##opt
   320    #define test_opt(sb, opt)        ((sb)->u.ext2_sb.s_mount_opt & \
   321                         EXT2_MOUNT_##opt)
   322    /*
   323     * Maximal mount counts between two filesystem checks
   324     */
   325    #define EXT2_DFL_MAX_MNT_COUNT        20    /* Allow 20 mounts */
   326    #define EXT2_DFL_CHECKINTERVAL        0    /* Don't use interval check */
   327    
   328    /*
   329     * Behaviour when detecting errors
   330     */
   331    #define EXT2_ERRORS_CONTINUE        1    /* Continue execution */
   332    #define EXT2_ERRORS_RO            2    /* Remount fs read-only */
   333    #define EXT2_ERRORS_PANIC        3    /* Panic */
   334    #define EXT2_ERRORS_DEFAULT        EXT2_ERRORS_CONTINUE
   335    
   336    /*
   337     * Structure of the super block
   338     */
   339    struct ext2_super_block {
   340        __u32    s_inodes_count;        /* Inodes count */
   341        __u32    s_blocks_count;        /* Blocks count */
   342        __u32    s_r_blocks_count;    /* Reserved blocks count */
   343        __u32    s_free_blocks_count;    /* Free blocks count */
   344        __u32    s_free_inodes_count;    /* Free inodes count */
   345        __u32    s_first_data_block;    /* First Data Block */
   346        __u32    s_log_block_size;    /* Block size */
   347        __s32    s_log_frag_size;    /* Fragment size */
   348        __u32    s_blocks_per_group;    /* # Blocks per group */
   349        __u32    s_frags_per_group;    /* # Fragments per group */
   350        __u32    s_inodes_per_group;    /* # Inodes per group */
   351        __u32    s_mtime;        /* Mount time */
   352        __u32    s_wtime;        /* Write time */
   353        __u16    s_mnt_count;        /* Mount count */
   354        __s16    s_max_mnt_count;    /* Maximal mount count */
   355        __u16    s_magic;        /* Magic signature */
   356        __u16    s_state;        /* File system state */
   357        __u16    s_errors;        /* Behaviour when detecting errors */
   358        __u16    s_minor_rev_level;     /* minor revision level */
   359        __u32    s_lastcheck;        /* time of last check */
   360        __u32    s_checkinterval;    /* max. time between checks */
   361        __u32    s_creator_os;        /* OS */
   362        __u32    s_rev_level;        /* Revision level */
   363        __u16    s_def_resuid;        /* Default uid for reserved blocks */
   364        __u16    s_def_resgid;        /* Default gid for reserved blocks */
   365        /*
   366         * These fields are for EXT2_DYNAMIC_REV superblocks only.
   367         *
   368         * Note: the difference between the compatible feature set and
   369         * the incompatible feature set is that if there is a bit set
   370         * in the incompatible feature set that the kernel doesn't
   371         * know about, it should refuse to mount the filesystem.
   372         * 
   373         * e2fsck's requirements are more strict; if it doesn't know
   374         * about a feature in either the compatible or incompatible
   375         * feature set, it must abort and not try to meddle with
   376         * things it doesn't understand...
   377         */
   378        __u32    s_first_ino;         /* First non-reserved inode */
   379        __u16   s_inode_size;         /* size of inode structure */
   380        __u16    s_block_group_nr;     /* block group # of this superblock */
   381        __u32    s_feature_compat;     /* compatible feature set */
   382        __u32    s_feature_incompat;     /* incompatible feature set */
   383        __u32    s_feature_ro_compat;     /* readonly-compatible feature set */
   384        __u8    s_uuid[16];        /* 128-bit uuid for volume */
   385        char    s_volume_name[16];     /* volume name */
   386        char    s_last_mounted[64];     /* directory where last mounted */
   387        __u32    s_algorithm_usage_bitmap; /* For compression */
   388        /*
   389         * Performance hints.  Directory preallocation should only
   390         * happen if the EXT2_COMPAT_PREALLOC flag is on.
   391         */
   392        __u8    s_prealloc_blocks;    /* Nr of blocks to try to preallocate*/
   393        __u8    s_prealloc_dir_blocks;    /* Nr to preallocate for dirs */
   394        __u16    s_padding1;
   395        __u32    s_reserved[204];    /* Padding to the end of the block */
   396    };
   397    
   398    #ifdef __KERNEL__
   399    #define EXT2_SB(sb)    (&((sb)->u.ext2_sb))
   400    #else
   401    /* Assume that user mode programs are passing in an ext2fs superblock, not
   402     * a kernel struct super_block.  This will allow us to call the feature-test
   403     * macros from user land. */
   404    #define EXT2_SB(sb)    (sb)
   405    #endif
   406    
   407    /*
   408     * Codes for operating systems
   409     */
   410    #define EXT2_OS_LINUX        0
   411    #define EXT2_OS_HURD        1
   412    #define EXT2_OS_MASIX        2
   413    #define EXT2_OS_FREEBSD        3
   414    #define EXT2_OS_LITES        4
   415    
   416    /*
   417     * Revision levels
   418     */
   419    #define EXT2_GOOD_OLD_REV    0    /* The good old (original) format */
   420    #define EXT2_DYNAMIC_REV    1     /* V2 format w/ dynamic inode sizes */
   421    
   422    #define EXT2_CURRENT_REV    EXT2_GOOD_OLD_REV
   423    #define EXT2_MAX_SUPP_REV    EXT2_DYNAMIC_REV
   424    
   425    #define EXT2_GOOD_OLD_INODE_SIZE 128
   426    
   427    /*
   428     * Feature set definitions
   429     */
   430    
   431    #define EXT2_HAS_COMPAT_FEATURE(sb,mask)            \
   432        ( EXT2_SB(sb)->s_es->s_feature_compat & cpu_to_le32(mask) )
   433    #define EXT2_HAS_RO_COMPAT_FEATURE(sb,mask)            \
   434        ( EXT2_SB(sb)->s_es->s_feature_ro_compat & cpu_to_le32(mask) )
   435    #define EXT2_HAS_INCOMPAT_FEATURE(sb,mask)            \
   436        ( EXT2_SB(sb)->s_es->s_feature_incompat & cpu_to_le32(mask) )
   437    #define EXT2_SET_COMPAT_FEATURE(sb,mask)            \
   438        EXT2_SB(sb)->s_es->s_feature_compat |= cpu_to_le32(mask)
   439    #define EXT2_SET_RO_COMPAT_FEATURE(sb,mask)            \
   440        EXT2_SB(sb)->s_es->s_feature_ro_compat |= cpu_to_le32(mask)
   441    #define EXT2_SET_INCOMPAT_FEATURE(sb,mask)            \
   442        EXT2_SB(sb)->s_es->s_feature_incompat |= cpu_to_le32(mask)
   443    #define EXT2_CLEAR_COMPAT_FEATURE(sb,mask)            \
   444        EXT2_SB(sb)->s_es->s_feature_compat &= ~cpu_to_le32(mask)
   445    #define EXT2_CLEAR_RO_COMPAT_FEATURE(sb,mask)            \
   446        EXT2_SB(sb)->s_es->s_feature_ro_compat &= ~cpu_to_le32(mask)
   447    #define EXT2_CLEAR_INCOMPAT_FEATURE(sb,mask)            \
   448        EXT2_SB(sb)->s_es->s_feature_incompat &= ~cpu_to_le32(mask)
   449    
   450    #define EXT2_FEATURE_COMPAT_DIR_PREALLOC    0x0001
   451    #define EXT2_FEATURE_COMPAT_IMAGIC_INODES    0x0002
   452    #define EXT3_FEATURE_COMPAT_HAS_JOURNAL        0x0004
   453    #define EXT2_FEATURE_COMPAT_EXT_ATTR        0x0008
   454    #define EXT2_FEATURE_COMPAT_RESIZE_INO        0x0010
   455    #define EXT2_FEATURE_COMPAT_DIR_INDEX        0x0020
   456    #define EXT2_FEATURE_COMPAT_ANY            0xffffffff
   457    
   458    #define EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER    0x0001
   459    #define EXT2_FEATURE_RO_COMPAT_LARGE_FILE    0x0002
   460    #define EXT2_FEATURE_RO_COMPAT_BTREE_DIR    0x0004
   461    #define EXT2_FEATURE_RO_COMPAT_ANY        0xffffffff
   462    
   463    #define EXT2_FEATURE_INCOMPAT_COMPRESSION    0x0001
   464    #define EXT2_FEATURE_INCOMPAT_FILETYPE        0x0002
   465    #define EXT3_FEATURE_INCOMPAT_RECOVER        0x0004
   466    #define EXT3_FEATURE_INCOMPAT_JOURNAL_DEV    0x0008
   467    #define EXT2_FEATURE_INCOMPAT_ANY        0xffffffff
   468    
   469    #define EXT2_FEATURE_COMPAT_SUPP    0
   470    #define EXT2_FEATURE_INCOMPAT_SUPP    EXT2_FEATURE_INCOMPAT_FILETYPE
   471    #define EXT2_FEATURE_RO_COMPAT_SUPP    (EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER| \
   472                         EXT2_FEATURE_RO_COMPAT_LARGE_FILE| \
   473                         EXT2_FEATURE_RO_COMPAT_BTREE_DIR)
   474    #define EXT2_FEATURE_RO_COMPAT_UNSUPPORTED    ~EXT2_FEATURE_RO_COMPAT_SUPP
   475    #define EXT2_FEATURE_INCOMPAT_UNSUPPORTED    ~EXT2_FEATURE_INCOMPAT_SUPP
   476    
   477    /*
   478     * Default values for user and/or group using reserved blocks
   479     */
   480    #define    EXT2_DEF_RESUID        0
   481    #define    EXT2_DEF_RESGID        0
   482    
   483    /*
   484     * Structure of a directory entry
   485     */
   486    #define EXT2_NAME_LEN 255
   487    
   488    struct ext2_dir_entry {
   489        __u32    inode;            /* Inode number */
   490        __u16    rec_len;        /* Directory entry length */
   491        __u16    name_len;        /* Name length */
   492        char    name[EXT2_NAME_LEN];    /* File name */
   493    };
   494    
   495    /*
   496     * The new version of the directory entry.  Since EXT2 structures are
   497     * stored in intel byte order, and the name_len field could never be
   498     * bigger than 255 chars, it's safe to reclaim the extra byte for the
   499     * file_type field.
   500     */
   501    struct ext2_dir_entry_2 {
   502        __u32    inode;            /* Inode number */
   503        __u16    rec_len;        /* Directory entry length */
   504        __u8    name_len;        /* Name length */
   505        __u8    file_type;
   506        char    name[EXT2_NAME_LEN];    /* File name */
   507    };
   508    
   509    /*
   510     * Ext2 directory file types.  Only the low 3 bits are used.  The
   511     * other bits are reserved for now.
   512     */
   513    enum {
   514        EXT2_FT_UNKNOWN,
   515        EXT2_FT_REG_FILE,
   516        EXT2_FT_DIR,
   517        EXT2_FT_CHRDEV,
   518        EXT2_FT_BLKDEV,
   519        EXT2_FT_FIFO,
   520        EXT2_FT_SOCK,
   521        EXT2_FT_SYMLINK,
   522        EXT2_FT_MAX
   523    };
   524    
   525    /*
   526     * EXT2_DIR_PAD defines the directory entries boundaries
   527     *
   528     * NOTE: It must be a multiple of 4
   529     */
   530    #define EXT2_DIR_PAD             4
   531    #define EXT2_DIR_ROUND             (EXT2_DIR_PAD - 1)
   532    #define EXT2_DIR_REC_LEN(name_len)    (((name_len) + 8 + EXT2_DIR_ROUND) & \
   533                         ~EXT2_DIR_ROUND)
   534    
   535    #ifdef __KERNEL__
   536    /*
   537     * Function prototypes
   538     */
   539    
   540    /*
   541     * Ok, these declarations are also in <linux/kernel.h> but none of the
   542     * ext2 source programs needs to include it so they are duplicated here.
   543     */
   544    # define NORET_TYPE    /**/
   545    # define ATTRIB_NORET  __attribute__((noreturn))
   546    # define NORET_AND     noreturn,
   547    
   548    /* balloc.c */
   549    extern int ext2_bg_has_super(struct super_block *sb, int group);
   550    extern unsigned long ext2_bg_num_gdb(struct super_block *sb, int group);
   551    extern int ext2_new_block (struct inode *, unsigned long,
   552                   __u32 *, __u32 *, int *);
   553    extern void ext2_free_blocks (struct inode *, unsigned long,
   554                      unsigned long);
   555    extern unsigned long ext2_count_free_blocks (struct super_block *);
   556    extern void ext2_check_blocks_bitmap (struct super_block *);
   557    extern struct ext2_group_desc * ext2_get_group_desc(struct super_block * sb,
   558                                unsigned int block_group,
   559                                struct buffer_head ** bh);
   560    
   561    /* dir.c */
   562    extern int ext2_add_link (struct dentry *, struct inode *);
   563    extern ino_t ext2_inode_by_name(struct inode *, struct dentry *);
   564    extern int ext2_make_empty(struct inode *, struct inode *);
   565    extern struct ext2_dir_entry_2 * ext2_find_entry (struct inode *,struct dentry *, struct page **);
   566    extern int ext2_delete_entry (struct ext2_dir_entry_2 *, struct page *);
   567    extern int ext2_empty_dir (struct inode *);
   568    extern struct ext2_dir_entry_2 * ext2_dotdot (struct inode *, struct page **);
   569    extern void ext2_set_link(struct inode *, struct ext2_dir_entry_2 *, struct page *, struct inode *);
   570    
   571    /* fsync.c */
   572    extern int ext2_sync_file (struct file *, struct dentry *, int);
   573    extern int ext2_fsync_inode (struct inode *, int);
   574    
   575    /* ialloc.c */
   576    extern struct inode * ext2_new_inode (const struct inode *, int);
   577    extern void ext2_free_inode (struct inode *);
   578    extern unsigned long ext2_count_free_inodes (struct super_block *);
   579    extern void ext2_check_inodes_bitmap (struct super_block *);
   580    extern unsigned long ext2_count_free (struct buffer_head *, unsigned);
   581    
   582    /* inode.c */
   583    extern void ext2_read_inode (struct inode *);
   584    extern void ext2_write_inode (struct inode *, int);
   585    extern void ext2_put_inode (struct inode *);
   586    extern void ext2_delete_inode (struct inode *);
   587    extern int ext2_sync_inode (struct inode *);
   588    extern void ext2_discard_prealloc (struct inode *);
   589    extern void ext2_truncate (struct inode *);
   590    
   591    /* ioctl.c */
   592    extern int ext2_ioctl (struct inode *, struct file *, unsigned int,
   593                   unsigned long);
   594    
   595    /* super.c */
   596    extern void ext2_error (struct super_block *, const char *, const char *, ...)
   597        __attribute__ ((format (printf, 3, 4)));
   598    extern NORET_TYPE void ext2_panic (struct super_block *, const char *,
   599                       const char *, ...)
   600        __attribute__ ((NORET_AND format (printf, 3, 4)));
   601    extern void ext2_warning (struct super_block *, const char *, const char *, ...)
   602        __attribute__ ((format (printf, 3, 4)));
   603    extern void ext2_update_dynamic_rev (struct super_block *sb);
   604    extern void ext2_put_super (struct super_block *);
   605    extern void ext2_write_super (struct super_block *);
   606    extern int ext2_remount (struct super_block *, int *, char *);
   607    extern struct super_block * ext2_read_super (struct super_block *,void *,int);
   608    extern int ext2_statfs (struct super_block *, struct statfs *);
   609    
   610    /*
   611     * Inodes and files operations
   612     */
   613    
   614    /* dir.c */
   615    extern struct file_operations ext2_dir_operations;
   616    
   617    /* file.c */
   618    extern struct inode_operations ext2_file_inode_operations;
   619    extern struct file_operations ext2_file_operations;
   620    
   621    /* inode.c */
   622    extern struct address_space_operations ext2_aops;
   623    
   624    /* namei.c */
   625    extern struct inode_operations ext2_dir_inode_operations;
   626    
   627    /* symlink.c */
   628    extern struct inode_operations ext2_fast_symlink_inode_operations;
   629    
   630    #endif    /* __KERNEL__ */
   631    
   632    #endif    /* _LINUX_EXT2_FS_H */
ext2_fs.h

 

2.1 超级块(Super Blook)

超级块的数据结构如下:

   336    /*
   337     * Structure of the super block
   338     */
   339    struct ext2_super_block {
   340        __u32    s_inodes_count;        /* Inodes count */
   341        __u32    s_blocks_count;        /* Blocks count */
   342        __u32    s_r_blocks_count;    /* Reserved blocks count */
   343        __u32    s_free_blocks_count;    /* Free blocks count */
   344        __u32    s_free_inodes_count;    /* Free inodes count */
   345        __u32    s_first_data_block;    /* First Data Block */
   346        __u32    s_log_block_size;    /* Block size */
   347        __s32    s_log_frag_size;    /* Fragment size */
   348        __u32    s_blocks_per_group;    /* # Blocks per group */
   349        __u32    s_frags_per_group;    /* # Fragments per group */
   350        __u32    s_inodes_per_group;    /* # Inodes per group */
   351        __u32    s_mtime;        /* Mount time */
   352        __u32    s_wtime;        /* Write time */
   353        __u16    s_mnt_count;        /* Mount count */
   354        __s16    s_max_mnt_count;    /* Maximal mount count */
   355        __u16    s_magic;        /* Magic signature */
   356        __u16    s_state;        /* File system state */
   357        __u16    s_errors;        /* Behaviour when detecting errors */
   358        __u16    s_minor_rev_level;     /* minor revision level */
   359        __u32    s_lastcheck;        /* time of last check */
   360        __u32    s_checkinterval;    /* max. time between checks */
   361        __u32    s_creator_os;        /* OS */
   362        __u32    s_rev_level;        /* Revision level */
   363        __u16    s_def_resuid;        /* Default uid for reserved blocks */
   364        __u16    s_def_resgid;        /* Default gid for reserved blocks */
   365        /*
   366         * These fields are for EXT2_DYNAMIC_REV superblocks only.
   367         *
   368         * Note: the difference between the compatible feature set and
   369         * the incompatible feature set is that if there is a bit set
   370         * in the incompatible feature set that the kernel doesn't
   371         * know about, it should refuse to mount the filesystem.
   372         * 
   373         * e2fsck's requirements are more strict; if it doesn't know
   374         * about a feature in either the compatible or incompatible
   375         * feature set, it must abort and not try to meddle with
   376         * things it doesn't understand...
   377         */
   378        __u32    s_first_ino;         /* First non-reserved inode */
   379        __u16   s_inode_size;         /* size of inode structure */
   380        __u16    s_block_group_nr;     /* block group # of this superblock */
   381        __u32    s_feature_compat;     /* compatible feature set */
   382        __u32    s_feature_incompat;     /* incompatible feature set */
   383        __u32    s_feature_ro_compat;     /* readonly-compatible feature set */
   384        __u8    s_uuid[16];        /* 128-bit uuid for volume */
   385        char    s_volume_name[16];     /* volume name */
   386        char    s_last_mounted[64];     /* directory where last mounted */
   387        __u32    s_algorithm_usage_bitmap; /* For compression */
   388        /*
   389         * Performance hints.  Directory preallocation should only
   390         * happen if the EXT2_COMPAT_PREALLOC flag is on.
   391         */
   392        __u8    s_prealloc_blocks;    /* Nr of blocks to try to preallocate*/
   393        __u8    s_prealloc_dir_blocks;    /* Nr to preallocate for dirs */
   394        __u16    s_padding1;
   395        __u32    s_reserved[204];    /* Padding to the end of the block */
   396    };
struct ext2_super_block

主要的几项描述有:s_inodes_count / s_blocks_count(该分区inode/block总数),s_free_inodes_count / s_free_blocks_count(该分区空闲inode/block数),s_first_data_block(数据块中的起始块位置),s_log_block_size(块大小),s_blocks_per_group(每个块组的块数),s_magic(魔数)

struct ext2_super_block {
	__u32	s_inodes_count;		/* Inodes count */
	__u32	s_blocks_count;		/* Blocks count */
	...
	__u32	s_free_blocks_count;	/* Free blocks count */
	__u32	s_free_inodes_count;	/* Free inodes count */
	__u32	s_first_data_block;	/* First Data Block */
	__u32	s_log_block_size;	/* Block size */
	...
	__u32	s_blocks_per_group;	/* # Blocks per group */
	...
	__u16	s_magic;		/* Magic signature */     # ext2的魔数为0xEF53
	...

说明:每个块组中都有一份超级块的拷贝。当文件系统挂载时,通常只有块组0中的超级块(主备份)会被读取,其他的块组中的超级块只是作为备份,以防文件系统的崩溃。

 

2.2 组描述符表(Group Descriptors Table,GDT)

超级块之后就是组描述符表,是由该分区所有的块的组描述符(Group Descriptor)组成的,每个块组描述符记录了本块组的inode/block bitmap和inode table等。块组描述符数据结构如下所示:

struct ext2_group_desc
{
	__u32	bg_block_bitmap;	/* Blocks bitmap block */
	__u32	bg_inode_bitmap;	/* Inodes bitmap block */
	__u32	bg_inode_table;		/* Inodes table block */
	__u16	bg_free_blocks_count;	/* Free blocks count */
	__u16	bg_free_inodes_count;	/* Free inodes count */
	__u16	bg_used_dirs_count;	/* Directories count */
	__u16	bg_pad;
	__u32	bg_reserved[3];
};

说明:与超级类似,组描述符表也存在各块组中紧接着超级块,其目的与超级块一样,作为备份,防止文件系统的崩溃。

 

2.3 块位图和inode位图(block/inode bitmap)

位图(bitmap)是位(bit)的序列,每一个位代表该位图所在块组中的一个特定的数据块(block bitmap)或inode table中一个特定的inode(inode bitmap)。当bit为0时,表示对应的block/inode空闲;为1时,表示已被占用。

位图始终索引其所在的块组,并且block位图和inode位图均为1block大小,从而限制了该块组的大小。例如一般的block大小为1024bytes,因此一个块组总共有1024*8个block。

 

2.4 inode表(inode table)

inode表由一系列连续的block块组成,每个块中都预定义了一定数量的inode。inode表的起始块位置(块号)存储在组描述符的bg_inode_table字段中。

系统对inode表中的inode进行编号,从1开始,inode的数据结构被定义在ext2_fs.h文件的struct ext2_inode函数中:

   214    /*
   215     * Structure of an inode on the disk
   216     */
   217    struct ext2_inode {
   218        __u16    i_mode;        /* File mode */
   219        __u16    i_uid;        /* Low 16 bits of Owner Uid */
   220        __u32    i_size;        /* Size in bytes */
   221        __u32    i_atime;    /* Access time */
   222        __u32    i_ctime;    /* Creation time */
   223        __u32    i_mtime;    /* Modification time */
   224        __u32    i_dtime;    /* Deletion Time */
   225        __u16    i_gid;        /* Low 16 bits of Group Id */
   226        __u16    i_links_count;    /* Links count */
   227        __u32    i_blocks;    /* Blocks count */
   228        __u32    i_flags;    /* File flags */
   229        union {
   230            struct {
   231                __u32  l_i_reserved1;
   232            } linux1;
   233            struct {
   234                __u32  h_i_translator;
   235            } hurd1;
   236            struct {
   237                __u32  m_i_reserved1;
   238            } masix1;
   239        } osd1;                /* OS dependent 1 */
   240        __u32    i_block[EXT2_N_BLOCKS];/* Pointers to blocks */
   241        __u32    i_generation;    /* File version (for NFS) */
   242        __u32    i_file_acl;    /* File ACL */
   243        __u32    i_dir_acl;    /* Directory ACL */
   244        __u32    i_faddr;    /* Fragment address */
   245        union {
   246            struct {
   247                __u8    l_i_frag;    /* Fragment number */
   248                __u8    l_i_fsize;    /* Fragment size */
   249                __u16    i_pad1;
   250                __u16    l_i_uid_high;    /* these 2 fields    */
   251                __u16    l_i_gid_high;    /* were reserved2[0] */
   252                __u32    l_i_reserved2;
   253            } linux2;
   254            struct {
   255                __u8    h_i_frag;    /* Fragment number */
   256                __u8    h_i_fsize;    /* Fragment size */
   257                __u16    h_i_mode_high;
   258                __u16    h_i_uid_high;
   259                __u16    h_i_gid_high;
   260                __u32    h_i_author;
   261            } hurd2;
   262            struct {
   263                __u8    m_i_frag;    /* Fragment number */
   264                __u8    m_i_fsize;    /* Fragment size */
   265                __u16    m_pad1;
   266                __u32    m_i_reserved2[2];
   267            } masix2;
   268        } osd2;                /* OS dependent 2 */
   269    };
struct ext2_inode

以下是inode数据结构中比较重要的字段:

struct ext2_inode {
        __u16   i_mode;         /* File type and access rights */
        __u16   i_uid;          /* Low 16 bits of Owner Uid */
        __u32   i_size;         /* Size in bytes */
        __u32   i_atime;        /* Access time */
        __u32   i_ctime;        /* Creation time */
        __u32   i_mtime;        /* Modification time */
        __u32   i_dtime;        /* Deletion Time */
        __u16   i_gid;          /* Low 16 bits of Group Id */
        __u16   i_links_count;  /* Links count */
        __u32   i_blocks;       /* Blocks count */
        __u32   i_flags;        /* File flags */
	...
	__u32   i_block[EXT2_N_BLOCKS];  /* Pointers to blocks */
	...
};

i_mode字段中包含了文件的类型和文件的访问权限,被定义在宏文件macro (sys/stat.h)中。

Sign Type Macro
- Regular file S_ISREG(m)
d Directory S_ISDIR(m)
c Character Device   S_ISCHR(m)
b Block Device S_ISBLK(m)
f Fifo S_ISIFO(m)
s Socket S_ISSOCK(m)
l Symbolic Link S_ISLNK(m)
Domain Read Write Exec All
User S_IRUSR S_IWUSR S_IXUSR S_IRWXU
Group S_IRGRP S_IWGRP S_IXGRP S_IRWXG
All S_IROTH S_IWOTH S_IXOTH S_IRWXO
 

i_blocks是该inode指向的文件已使用的block数量;指向数据块的指针存储于字段i_block[EXT2_N_BLOCKS]的数组结构中。

变量EXT2_N_BLOCKS定义在ext2_fs.h的第177行中:

   174	/*
   175	 * Constants relative to the data blocks
   176	 */
   177	#define	EXT2_NDIR_BLOCKS		12
   178	#define	EXT2_IND_BLOCK			EXT2_NDIR_BLOCKS
   179	#define	EXT2_DIND_BLOCK			(EXT2_IND_BLOCK + 1)
   180	#define	EXT2_TIND_BLOCK			(EXT2_DIND_BLOCK + 1)
   181	#define	EXT2_N_BLOCKS			(EXT2_TIND_BLOCK + 1) 

i_block[]数组中有15个指针,它们所代表的含义如下:

  •  i_block[0..11] point directly to the first 12 data blocks of the file.              # 序列号0-11的12个元素(指针)指向文件开头的12个数据块
  •  i_block[12] points to a single indirect block                                               # 第13号元素指向单索引间接块
  •  i_block[13] points to a double indirect block                                              # 第14号元素指向双索引间接块
  •  i_block[14] points to a triple indirect block                                                 # 第15号元素指向三索引间接块

由此我们可以计算出ext2文件系统单个文件的最大容量(假设block大小为1K),数组i_block[]的长度为32bit/8=4byte:

  • 直接索引:12 指针
  • 单间接索引:1024/4=256个直接索引,256 指针
  • 双间接索引:1024/4=256个单间接索引,256*256 指针              
  • 三间接索引:1024/4=256个双间接索引,256*256*256 指针      

因此可得到12K+256K+64M+16G,即大致为16G。如果block的大小为4K,则文件最大可为4T。(注意:真正决定文件大小的是底层的寄存器,寄存器的位数决定了其寻址的能力)

 

2.5 inode表中的inode指向的目录文件

inode指向的目录文件需加以注意,我们可以通过测试S_ISDIR(mode) macro来加以识别:

if (S_ISDIR(inode.i_mode)) ... 

假设inode指向的块是目录实体/home,目录中的内容包含了一系列的文件名和指向inode表中的对应的,如下图:

目录文件的数据结构如下:

struct ext2_dir_entry_2 {
	__u32	inode;			/* Inode number */
	__u16	rec_len;		/* Directory entry length */
	__u8	name_len;		/* Name length */
	__u8	file_type;
	char	name[EXT2_NAME_LEN];	/* File name */
};

字段file_type总共有0-7可能的值,分别代表: 

 

目录内容中的各项(实体)的大小是非固定的,大小取决于文件名称的长度。文件名称最大长度为EXT2_NAME_LEN的值,一般为255;文件名称的实际长度存放于字段name_len;rec_len存储的是本目录实体的大小,该字段自然也就决定了下一目录实体的位置了。

  Example of EXT2 directory

注意:目录中的inode号,指向的是inode表中的inode,指向data block的是i_block[EXT2_N_BLOCKS]的数组中的指针。

 

 2.6 文件操作(创建、查找、修改、删除)的本质

http://cs.smith.edu/~nhowe/Teaching/csc262/oldlabs/ext2.html

https://www.cnblogs.com/f-ck-need-u/p/7016077.html

待完善...

posted @ 2019-06-14 11:08  ant_colonies  阅读(2121)  评论(0编辑  收藏  举报