[File System] File System Study Notes (2) --- task_struct

I. task_struct and file system-related information

 

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1. <Sched. h>
2. Struct task_struct {
3. ...
4. /* File system info */
5. Int link_count, total_link_count;
6. ...
7. /* Filesystem information */
8. Struct fs_struct * FS;
9. /* Open File Information */
10. Struct files_struct * files;
11. /* Namespaces */
12. Struct nsproxy * nsproxy;
13. ...
14. }

 

Data related to the file system of the process is stored in FS, which contains the current working directory

 

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1. Fs_struct is mainly used to manage information about a specific process,
2. Struct fs_struct {
3. Atomic_t count;
4. Int umask; // indicates the standard mask, used to set permissions for new files
5. Struct dentry * root, * Pwd, * altroot; // root indicates the root directory of the Process, PWD indicates the directory of the current process, and the shell CD command will change when used
6. Struct vfsmount * rootmnt, * pwdmnt, * altrootmnt; // rootmnt indicates the file system in which the root directory is located,
7. };

 

 

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1. Struct files_struct {
2. Atomic_t count; // number of processes that share the file
3. Struct fdtable * FDT;
4. Struct fdtable fdtab;
5. Int next_fd; // The file descriptor ID used by the process to open a new file next time
6. Struct embedded_fd_set close_on_exec_init;
7. Struct embedded_fd_set open_fds_init;
8. Struct file * fd_array [nr_open_default]; each member is a pointer pointing to each open file struct file instance
9. };

By default, each process in the kernel runs to open the nr_open_default file. The default value is bits_per_long. The value is 32 on 32-bit systems.

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1. Struct fdtable {
2. Unsigned int max_fds; // The maximum number of file objects and file descriptors that the process can process currently.
3. Struct file ** FD;/* Current FD array */
4. Fd_set * close_on_exec;
5. Fd_set * open_fds;
6. Struct rcu_head RCU;
7. Struct files_struct * free_files;
8. Struct fdtable * next;
9. };

Struct file points to the real file information,

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1. Struct file {
2. Struct list_head fu_list;
3. Struct path f_path; // specifies the association between the file name and inode. Information about the file system where the file is located
4. # Define f_dentry f_path.dentry
5. # Define f_vfsmnt f_path.mnt
6. Const struct file_operations * f_op; // various functions called by file operations
7. Atomic_t f_count;
8. Unsigned int f_flags;
9. Mode_t f_mode;
10. Loff_t f_pos; // indicates the location where the process operates on files.
11. Struct fown_struct f_owner; // process information about the file
12. Unsigned int f_uid, f_gid; // your uid GID
13. Struct file_ra_state f_ra; // pre-read feature that specifies whether to pre-read data before actually reading data
14. Unsigned long f_version;
15. ...
16. Struct address_space * f_mapping; // address space ing pointing to the inode instance related to the file
17. ...
18. };
21. Struct path {
22. Struct vfsmount * MNT;
23. Struct dentry * dentry;
24. };

There is no file structure on the hard disk. When a process opens a file, the kernel dynamically creates a file object. The same file has different file objects in different processes.

Appendix:

Process to open a file:
1. the user-layer open () function finally calls the sys_open () function (FS/open. c) The sys_open () function mainly uses the do_flip_open () function to find the inode of the file. The do_flip_open () function first calls the open_namei () function, and the open_namei () function calls path_lookup () the function looks for the inode of the file and performs several additional checks. If a new file system item is created, the function also needs to be applied to the umask (current-> FS-> umask) process) by default. The do_flip_open () function then calls the nameidata_to_flip () function to initialize the pre-read structure, place the newly created file instance on the super fast s_files chain, and call open () in the file_operations structure of the underlying file system () function.
The final control is transferred back to the user process. before returning the file descriptor, fd_install must place the file instance in the files-> fd array of the task_struct process.

[File System] File System Study Notes (2) --- task_struct