FS ext2_super_block

super_block：用于记录文件系统和管理信息，特定的文件系统定义了特定的超级块。第一个超级块=start_sect(分区表里ext2文件系统开始的扇区)+2

struct ext2_super_block {
    __le32  s_inodes_count;     /* Inodes count */
    __le32  s_blocks_count;     /* Blocks count */
    __le32  s_r_blocks_count;   /* Reserved blocks count */
    __le32  s_free_blocks_count;    /* Free blocks count */
    __le32  s_free_inodes_count;    /* Free inodes count */
    __le32  s_first_data_block; /* First Data Block */
    __le32  s_log_block_size;   /* Block size */
    __le32  s_log_frag_size;    /* Fragment size */
    __le32  s_blocks_per_group; /* # Blocks per group */
    __le32  s_frags_per_group;  /* # Fragments per group */
    __le32  s_inodes_per_group; /* # Inodes per group */
    __le32  s_mtime;        /* Mount time */
    __le32  s_wtime;        /* Write time */
    __le16  s_mnt_count;        /* Mount count */
    __le16  s_max_mnt_count;    /* Maximal mount count */
    __le16  s_magic;        /* Magic signature */
    __le16  s_state;        /* File system state */
    __le16  s_errors;       /* Behaviour when detecting errors */
    __le16  s_minor_rev_level;  /* minor revision level */
    __le32  s_lastcheck;        /* time of last check */
    __le32  s_checkinterval;    /* max. time between checks */
    __le32  s_creator_os;       /* OS */
    __le32  s_rev_level;        /* Revision level */
    __le16  s_def_resuid;       /* Default uid for reserved blocks */
    __le16  s_def_resgid;       /* Default gid for reserved blocks */
    /*
     * These fields are for EXT2_DYNAMIC_REV superblocks only.
     *
     * Note: the difference between the compatible feature set and
     * the incompatible feature set is that if there is a bit set
     * in the incompatible feature set that the kernel doesn't
     * know about, it should refuse to mount the filesystem.
     * 
     * e2fsck's requirements are more strict; if it doesn't know
     * about a feature in either the compatible or incompatible
     * feature set, it must abort and not try to meddle with
     * things it doesn't understand...
     */
    __le32  s_first_ino;        /* First non-reserved inode */
    __le16   s_inode_size;      /* size of inode structure */
    __le16  s_block_group_nr;   /* block group # of this superblock */
    __le32  s_feature_compat;   /* compatible feature set */
    __le32  s_feature_incompat;     /* incompatible feature set */
    __le32  s_feature_ro_compat;    /* readonly-compatible feature set */
    __u8    s_uuid[16];     /* 128-bit uuid for volume */
    char    s_volume_name[16];  /* volume name */
    char    s_last_mounted[64];     /* directory where last mounted */
    __le32  s_algorithm_usage_bitmap; /* For compression */
    /*
     * Performance hints.  Directory preallocation should only
     * happen if the EXT2_COMPAT_PREALLOC flag is on.
     */
    __u8    s_prealloc_blocks;  /* Nr of blocks to try to preallocate*/
    __u8    s_prealloc_dir_blocks;  /* Nr to preallocate for dirs */
    __u16   s_padding1;
    /*
     * Journaling support valid if EXT3_FEATURE_COMPAT_HAS_JOURNAL set.
     */
    __u8    s_journal_uuid[16]; /* uuid of journal superblock */
    __u32   s_journal_inum;     /* inode number of journal file */
    __u32   s_journal_dev;      /* device number of journal file */
    __u32   s_last_orphan;      /* start of list of inodes to delete */
    __u32   s_hash_seed[4];     /* HTREE hash seed */
    __u8    s_def_hash_version; /* Default hash version to use */
    __u8    s_reserved_char_pad;
    __u16   s_reserved_word_pad;
    __le32  s_default_mount_opts;
    __le32  s_first_meta_bg;    /* First metablock block group */
    __u32   s_reserved[190];    /* Padding to the end of the block */
};

struct ext2_sb_info {
    unsigned long s_frag_size;  /* Size of a fragment in bytes */
    unsigned long s_frags_per_block;/* Number of fragments per block */
    unsigned long s_inodes_per_block;/* Number of inodes per block */
    unsigned long s_frags_per_group;/* Number of fragments in a group */
    unsigned long s_blocks_per_group;/* Number of blocks in a group */
    unsigned long s_inodes_per_group;/* Number of inodes in a group */
    unsigned long s_itb_per_group;  /* Number of inode table blocks per group */
    unsigned long s_gdb_count;  /* Number of group descriptor blocks */
    unsigned long s_desc_per_block; /* Number of group descriptors per block */
    unsigned long s_groups_count;   /* Number of groups in the fs */
    unsigned long s_overhead_last;  /* Last calculated overhead */
    unsigned long s_blocks_last;    /* Last seen block count */
    struct buffer_head * s_sbh; /* Buffer containing the super block */
    struct ext2_super_block * s_es; /* Pointer to the super block in the buffer */
    struct buffer_head ** s_group_desc;
    unsigned long  s_mount_opt;
    unsigned long s_sb_block;
    uid_t s_resuid;
    gid_t s_resgid;
    unsigned short s_mount_state;
    unsigned short s_pad;
    int s_addr_per_block_bits;
    int s_desc_per_block_bits;
    int s_inode_size;
    int s_first_ino;
    spinlock_t s_next_gen_lock;
    u32 s_next_generation;
    unsigned long s_dir_count;
    u8 *s_debts;
    struct percpu_counter s_freeblocks_counter;
    struct percpu_counter s_freeinodes_counter;
    struct percpu_counter s_dirs_counter;
    struct blockgroup_lock *s_blockgroup_lock;
    /* root of the per fs reservation window tree */
    spinlock_t s_rsv_window_lock;
    struct rb_root s_rsv_window_root;
    struct ext2_reserve_window_node s_rsv_window_head;
};

static int __init init_ext2_fs(void)
{
    int err = init_ext2_xattr();
    if (err)
        return err;
    err = init_inodecache();
    if (err)
        goto out1;
        err = register_filesystem(&ext2_fs_type);
    if (err)
        goto out;
    return 0;
out:
    destroy_inodecache();
out1:
    exit_ext2_xattr();
    return err;
}

const struct file_operations ext2_file_operations = {
    .llseek     = generic_file_llseek,
    .read       = do_sync_read,
    .write      = do_sync_write,
    .aio_read   = generic_file_aio_read,
    .aio_write  = generic_file_aio_write,
    .unlocked_ioctl = ext2_ioctl,
#ifdef CONFIG_COMPAT
    .compat_ioctl   = ext2_compat_ioctl,
#endif
    .mmap       = generic_file_mmap,
    .open       = generic_file_open,
    .release    = ext2_release_file,
    .fsync      = simple_fsync,
    .splice_read    = generic_file_splice_read,
    .splice_write   = generic_file_splice_write,
};