What is dentry

dentry, 既directory entry的缩写, 是Linux VFS中用来管理树状目录的结构, 在内存中生成, 并不存在于实际的文件系统中.

dentry的结构

dentry结构的定义(linux/include/dcache.h)如下:

struct dentry {
        /* RCU lookup touched fields */
        unsigned int d_flags;           /* protected by d_lock */
        seqcount_t d_seq;               /* per dentry seqlock */
        struct hlist_bl_node d_hash;    /* lookup hash list */
        struct dentry *d_parent;        /* parent directory */
        struct qstr d_name;
        struct inode *d_inode;          /* Where the name belongs to - NULL is
                                         * negative */
        unsigned char d_iname[DNAME_INLINE_LEN];        /* small names */

        /* Ref lookup also touches following */
        struct lockref d_lockref;       /* per-dentry lock and refcount */
        const struct dentry_operations *d_op;
        struct super_block *d_sb;       /* The root of the dentry tree */
        unsigned long d_time;           /* used by d_revalidate */
        void *d_fsdata;                 /* fs-specific data */

        struct list_head d_lru;         /* LRU list */
        /*
         * d_child and d_rcu can share memory
         */
        union {
                struct list_head d_child;       /* child of parent list */
                struct rcu_head d_rcu;
        } d_u;
        struct list_head d_subdirs;     /* our children */
        struct hlist_node d_alias;      /* inode alias list */
};

这其中比较基本和重要的成员是d_parent, d_child, d_name和d_inode, 分别用来记录上级目录, 下级成员(如果当前dentry为目录文件), 文件名以及所指向的inode.

dentry如何形成树状结构

区别于记录文件数据的inode, dentry只用来记录文件的组织和管理信息. 路径就是由多个dentry连接而成的, 例如/home/user/foo路径中的根目录/, 目录home, 目录user和文件foo各自都是一个dentry结构. 所有的dentry索引起来就形成了文件系统的树状结构.