vfs的supper bolck、inode、dentry和file关系简述

The common file model consists of the following object types:
<1>The superblock object
Stores information concerning a mounted filesystem. For disk-based filesystems, this object usually corresponds to a filesystem control block stored on disk.
<2>The inode object
Stores general information about a specific file. For disk-based filesystems, this object usually corresponds to a file control block stored on disk. Each inode object is associated with an inode number, which uniquely identifies the file within the filesystem.
<3>The file object
Stores information about the interaction between an open file and a process. This information exists only in kernel memory during the period when a process has the file open.
<4>The dentry object
Stores information about the linking of a directory entry (that is, a particular name of the file) with the corresponding file. Each disk-based filesystem stores this information in its own particular way on disk.
Figure 12-2 illustrates with a simple example how processes interact with files. Three different processes have opened the same file, two of them using the same hard link. In this case, each of the three processes uses its own file object, while only two dentry objects are requiredone for each hard link. Both dentry objects refer to the same inode object, which identifies the superblock object and, together with the latter, the common disk file.

Figure 12-2. Interaction between processes and VFS objects

   Note: 1. 从上图可以看出 struct file 是一个dentry，对file的访问最后都是对dentry的访问。