linux vfs 解析之 dentry

最新推荐文章于 2025-05-31 03:41:52 发布

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本文详细介绍了 Linux 文件系统中的 dentry 数据结构及其在目录项管理中的作用。dentry 结构用于缓存文件系统的目录项信息，提高文件查找效率。文章通过图示展示了 dentry 如何与其他数据结构如 inode 相关联，并解释了 dentry 在不同链表中的位置及意义。

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http://blog.sina.com.cn/s/blog_5219094a01009a4g.html

dentry

相关的数据结构为:

struct dentry {
atomic_t d_count;
unsigned int d_flags;
spinlock_t d_lock;
struct inode *d_inode;

struct dentry *d_parent;
struct qstr d_name;

struct list_head d_lru;
struct list_head d_child;
struct list_head d_subdirs;
struct list_head d_alias;
unsigned long d_time;
struct dentry_operations *d_op;
struct super_block *d_sb;
void *d_fsdata;
  struct rcu_head d_rcu;
struct dcookie_struct *d_cookie;
struct hlist_node d_hash;
int d_mounted;
unsigned char d_iname[DNAME_INLINE_LEN_MIN];
};

dentry对象存在于三个双向链表中:
所有未用的目录项: dentry_unused 链表
正在使用的目录项: 对应inode的 i_dentry 链表
表示父子目录结构的链表

另外,还有一个重要的链表: inode_hashtable(这个暂不介绍).

                                    dentry
                               +--------------+
                               |              |
                               +--------------+
   /-------------------------->|  d_subdirs   |<-----------------------------\
   |                           +--------------+                              |
   |                           |              |                              |
   |                           +--------------+                              |
   |                           |              |                              |
   |                           +--------------+                              |
   |                           |              |                              |
   |                       /-->+--------------+<-----\                       |
   |                       |                         |                       |
   |                       |                         |                       |
   |                       |                         |                       |
   |        dentry         |                         |          dentry       |
   |   +--------------+    |                         |    +--------------+   |
   |   |  d_parent    |----/                         \----|  d_parent    |   |
   |   +--------------+                                   +--------------+   |
   |   |  d_subdirs   |                                   |  d_subdirs   |   |
   |   +--------------+                                   +--------------+   |
   \-->|  d_child     |<---------- ......   ------------->|  d_child     |<--/
       +--------------+                                   +--------------+
       |              |                                   |              |
       +--------------+                                   +--------------+
       |              |                                   |              |
       +--------------+                                   +--------------+

            dentry                                            dentry
       +--------------+                                  +--------------+
       |  d_parent    |                                  |  d_parent    |
       +--------------+                                  +--------------+
       |  d_subdirs   |                                  |  d_subdirs   |
       +--------------+                                  +--------------+
       |  d_child     |                                  |  d_child     |
       +--------------+         /---------------\        +--------------+
/---->|  d_lru       |<--------| dentry_unused |------->|  d_lru       |<--\
|     +--------------+         \---------------/        +--------------+   |
|     |              |                                  |              |   |
|     +--------------+                                  +--------------+   |
|                                                                          |
|                                                                          |
|                                                                          |
|                                                                          |
\-----------------------------   ......   ---------------------------------/


                                   inode
                               +-----------+
                               |           |
                               +-----------+
       /---------------------->|  i_dentry |<---------------------------\
       |                       +-----------+                            |
       |                       |           |                            |
       |                       +-----------+                            |
       |                       |           |                            |
       |                       +--,-----,--+                            |
       |                         /|\   /|\                              |
       |         dentry           |     |              dentry           |
       |    +--------------+      |     |         +--------------+      |
       |    |  d_parent    |      |     |         |  d_parent    |      |
       |    +--------------+      |     |         +--------------+      |
       |    |  d_subdirs   |      |     |         |  d_subdirs   |      |
       |    +--------------+      |     |         +--------------+      |
       |    |  d_child     |      |     |         |  d_child     |      |
       |    +--------------+      |     |         +--------------+      |
       |    |  d_lru       |      |     |         |  d_lru       |      |
       |    +--------------+      |     |         +--------------+      |
       |    |  d_inode     |------/     \---------|  d_inode     |      |
       |    +--------------+                      +--------------+      |
       \--->|  d_alias     |<-----  ......  ----->|  d_alias     |<-----/
            +--------------+                      +--------------+