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Linux虚拟文件系统之文件系统卸载（sys_umount()) . [复制链接]

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发表于 2012-02-13 14:22 |只看该作者 |倒序浏览

Linux虚拟文件系统之文件系统卸载（sys_umount()) .

Linux中卸载文件系统由umount系统调用实现，入口函数为sys_umount()。较于文件系统的安装较为简单，下面是具体的实现。

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01./*sys_umont系统调用*/
02.SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
03.{
04. struct path path;
05. int retval;
06. /*找到装载点的vfsmount实例和dentry实例，二者包装
07. 在一个nameidata结构中*/
08. retval = user_path(name, &path);
09. if (retval)
10. goto out;
11. retval = -EINVAL;
12. /*如果查找的最终目录不是文件系统的挂载点*/
13. if (path.dentry != path.mnt->mnt_root)
14. goto dput_and_out;
15. /*如果要卸载的文件系统还没有安装在命名空间中*/
16. if (!check_mnt(path.mnt))
17. goto dput_and_out;
18.
19. retval = -EPERM;
20. /*如果用户不具有卸载文件系统的特权*/
21. if (!capable(CAP_SYS_ADMIN))
22. goto dput_and_out;
23. /*实际umount工作*/
24. retval = do_umount(path.mnt, flags);
25.dput_and_out:
26. /* we mustn't call path_put() as that would clear mnt_expiry_mark */
27. dput(path.dentry);
28. mntput_no_expire(path.mnt);
29.out:
30. return retval;
31.}
/*sys_umont系统调用*/
SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
{
struct path path;
int retval;
/*找到装载点的vfsmount实例和dentry实例，二者包装
在一个nameidata结构中*/
retval = user_path(name, &path);
if (retval)
goto out;
retval = -EINVAL;
/*如果查找的最终目录不是文件系统的挂载点*/
if (path.dentry != path.mnt->mnt_root)
goto dput_and_out;
/*如果要卸载的文件系统还没有安装在命名空间中*/
if (!check_mnt(path.mnt))
goto dput_and_out;
retval = -EPERM;
/*如果用户不具有卸载文件系统的特权*/
if (!capable(CAP_SYS_ADMIN))
goto dput_and_out;
/*实际umount工作*/
retval = do_umount(path.mnt, flags);
dput_and_out:
/* we mustn't call path_put() as that would clear mnt_expiry_mark */
dput(path.dentry);
mntput_no_expire(path.mnt);
out:
return retval;
}

复制代码

卸载实际工作

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01.static int do_umount(struct vfsmount *mnt, int flags)
02.{
03. /*从vfsmount对象的mnt_sb字段检索超级块对象sb的地址*/
04. struct super_block *sb = mnt->mnt_sb;
05. int retval;
06. /*初始化umount_list，该链表在后面的释放中会做临时链表
07. 用*/
08. LIST_HEAD(umount_list);
09.
10. retval = security_sb_umount(mnt, flags);
11. if (retval)
12. return retval;
13.
14. /*
15. * Allow userspace to request a mountpoint be expired rather than
16. * unmounting unconditionally. Unmount only happens if:
17. * (1) the mark is already set (the mark is cleared by mntput())
18. * (2) the usage count == 1 [parent vfsmount] + 1 [sys_umount]
19. */
20. /*如果设置了MNT_EXPIRE标志，即要标记挂载点“到期”*/
21. if (flags & MNT_EXPIRE) {
22. /*若要卸载的文件系统是根文件系统或者同时设置了
23. MNT_FORCE或MNT_DETACH，则返回-EINVAL*/
24. if (mnt == current->fs->root.mnt ||
25. flags & (MNT_FORCE | MNT_DETACH))
26. return -EINVAL;
27. /*检查vfsmount的引用计数，若不为2，则返回-EBUSY，
28. 要卸载的文件系统在卸载的时候不能有引用者，
29. 这个2代表vfsmount的父vfsmount和sys_umount()对本对象的引用*/
30. if (atomic_read(&mnt->mnt_count) != 2)
31. return -EBUSY;
32. /*设置vfsmount对象的mnt_expiry_mark字段为1。*/
33. if (!xchg(&mnt->mnt_expiry_mark, 1))
34. return -EAGAIN;
35. }
36.
37. /*
38. * If we may have to abort operations to get out of this
39. * mount, and they will themselves hold resources we must
40. * allow the fs to do things. In the Unix tradition of
41. * 'Gee thats tricky lets do it in userspace' the umount_begin
42. * might fail to complete on the first run through as other tasks
43. * must return, and the like. Thats for the mount program to worry
44. * about for the moment.
45. */
46. /*如果用户要求强制卸载操作，则调用umount_begin
47. 超级块操作中断任何正在进行的安装操作*/
48. /*当然如果特定的文件系统定义了下面函数则调用它*/
49. if (flags & MNT_FORCE && sb->s_op->umount_begin) {
50. sb->s_op->umount_begin(sb);
51. }
52.
53. /*
54. * No sense to grab the lock for this test, but test itself looks
55. * somewhat bogus. Suggestions for better replacement?
56. * Ho-hum... In principle, we might treat that as umount + switch
57. * to rootfs. GC would eventually take care of the old vfsmount.
58. * Actually it makes sense, especially if rootfs would contain a
59. * /reboot - static binary that would close all descriptors and
60. * call reboot(9). Then init(8) could umount root and exec /reboot.
61. */
62. /*如果要卸载的文件系统是根文件系统，且用户
63. 并不要求真正地把它卸载下来（即设置了MNT_DETACH标志，
64. 这个标志仅仅标记挂载点为不能再访问，知道挂载不busy
65. 时才卸载），则调用do_remount_sb()重新安装根文件系统为只
66. 读并终止，并返回do_remount_sb()的返回值。*/
67. if (mnt == current->fs->root.mnt && !(flags & MNT_DETACH)) {
68. /*
69. * Special case for "unmounting" root ...
70. * we just try to remount it readonly.
71. */
72. down_write(&sb->s_umount);
73. if (!(sb->s_flags & MS_RDONLY))
74. retval = do_remount_sb(sb, MS_RDONLY, NULL, 0);
75. up_write(&sb->s_umount);
76. return retval;
77. }
78.
79. down_write(&namespace_sem);
80. /*为进行写操作而获取当前进程的namespace_sem读/写信号量和vfsmount_lock自旋锁*/
81. spin_unlock(&vfsmount_lock);
82. spin_lock(&vfsmount_lock);
83. event++;
84.
85. if (!(flags & MNT_DETACH))
86. shrink_submounts(mnt, &umount_list);
87.
88. retval = -EBUSY;
89. /*如果已安装文件系统不包含任何子安装文件系统的安装点，或者用户要求强制
90. 卸载文件系统，则调用umount_tree()卸载文件系统（及其所有子文件系统）。*/
91. if (flags & MNT_DETACH || !propagate_mount_busy(mnt, 2)) {
92. if (!list_empty(&mnt->mnt_list))
93. /*完成实际的底层的卸载文件系统的任务。首先他将mnt的所有孩子移动至kill链表中，
94. 也就是传递进去的umount_list，然后将kill链表中的所有的vfsmount对象的一些字段设为无效状态。
95. */
96. umount_tree(mnt, 1, &umount_list);
97. retval = 0;
98. }
99.
100. if (retval)
101. security_sb_umount_busy(mnt);
102. /*释放vfsmount_lock自旋锁和当前进程的namespace_sem读/写信号量*/
103. up_write(&namespace_sem);
104. /*减小相应文件系统根目录的目录项对象和已经安装文件系统
105. 描述符的引用计数器值，这些计数器值由path_lookup()增加*/
106. release_mounts(&umount_list);
107. return retval;
108.}
static int do_umount(struct vfsmount *mnt, int flags)
{
/*从vfsmount对象的mnt_sb字段检索超级块对象sb的地址*/
struct super_block *sb = mnt->mnt_sb;
int retval;
/*初始化umount_list，该链表在后面的释放中会做临时链表
用*/
LIST_HEAD(umount_list);
retval = security_sb_umount(mnt, flags);
if (retval)
return retval;
/*
* Allow userspace to request a mountpoint be expired rather than
* unmounting unconditionally. Unmount only happens if:
* (1) the mark is already set (the mark is cleared by mntput())
* (2) the usage count == 1 [parent vfsmount] + 1 [sys_umount]
*/
/*如果设置了MNT_EXPIRE标志，即要标记挂载点“到期”*/
if (flags & MNT_EXPIRE) {
/*若要卸载的文件系统是根文件系统或者同时设置了
MNT_FORCE或MNT_DETACH，则返回-EINVAL*/
if (mnt == current->fs->root.mnt ||
flags & (MNT_FORCE | MNT_DETACH))
return -EINVAL;
/*检查vfsmount的引用计数，若不为2，则返回-EBUSY，
要卸载的文件系统在卸载的时候不能有引用者，
这个2代表vfsmount的父vfsmount和sys_umount()对本对象的引用*/
if (atomic_read(&mnt->mnt_count) != 2)
return -EBUSY;
/*设置vfsmount对象的mnt_expiry_mark字段为1。*/
if (!xchg(&mnt->mnt_expiry_mark, 1))
return -EAGAIN;
}
/*
* If we may have to abort operations to get out of this
* mount, and they will themselves hold resources we must
* allow the fs to do things. In the Unix tradition of
* 'Gee thats tricky lets do it in userspace' the umount_begin
* might fail to complete on the first run through as other tasks
* must return, and the like. Thats for the mount program to worry
* about for the moment.
*/
/*如果用户要求强制卸载操作，则调用umount_begin
超级块操作中断任何正在进行的安装操作*/
/*当然如果特定的文件系统定义了下面函数则调用它*/
if (flags & MNT_FORCE && sb->s_op->umount_begin) {
sb->s_op->umount_begin(sb);
}
/*
* No sense to grab the lock for this test, but test itself looks
* somewhat bogus. Suggestions for better replacement?
* Ho-hum... In principle, we might treat that as umount + switch
* to rootfs. GC would eventually take care of the old vfsmount.
* Actually it makes sense, especially if rootfs would contain a
* /reboot - static binary that would close all descriptors and
* call reboot(9). Then init(8) could umount root and exec /reboot.
*/
/*如果要卸载的文件系统是根文件系统，且用户
并不要求真正地把它卸载下来（即设置了MNT_DETACH标志，
这个标志仅仅标记挂载点为不能再访问，知道挂载不busy
时才卸载），则调用do_remount_sb()重新安装根文件系统为只
读并终止，并返回do_remount_sb()的返回值。*/
if (mnt == current->fs->root.mnt && !(flags & MNT_DETACH)) {
/*
* Special case for "unmounting" root ...
* we just try to remount it readonly.
*/
down_write(&sb->s_umount);
if (!(sb->s_flags & MS_RDONLY))
retval = do_remount_sb(sb, MS_RDONLY, NULL, 0);
up_write(&sb->s_umount);
return retval;
}
down_write(&namespace_sem);
/*为进行写操作而获取当前进程的namespace_sem读/写信号量和vfsmount_lock自旋锁*/
spin_unlock(&vfsmount_lock);
spin_lock(&vfsmount_lock);
event++;
if (!(flags & MNT_DETACH))
shrink_submounts(mnt, &umount_list);
retval = -EBUSY;
/*如果已安装文件系统不包含任何子安装文件系统的安装点，或者用户要求强制
卸载文件系统，则调用umount_tree()卸载文件系统（及其所有子文件系统）。*/
if (flags & MNT_DETACH || !propagate_mount_busy(mnt, 2)) {
if (!list_empty(&mnt->mnt_list))
/*完成实际的底层的卸载文件系统的任务。首先他将mnt的所有孩子移动至kill链表中，
也就是传递进去的umount_list，然后将kill链表中的所有的vfsmount对象的一些字段设为无效状态。
*/
umount_tree(mnt, 1, &umount_list);
retval = 0;
}
if (retval)
security_sb_umount_busy(mnt);
/*释放vfsmount_lock自旋锁和当前进程的namespace_sem读/写信号量*/
up_write(&namespace_sem);
/*减小相应文件系统根目录的目录项对象和已经安装文件系统
描述符的引用计数器值，这些计数器值由path_lookup()增加*/
release_mounts(&umount_list);
return retval;
}

复制代码

从内核链表中脱离

[cpp] view plaincopyprint?

01./*完成实际的底层的卸载文件系统的任务。首先他将mnt的所有子移动至kill链表中，
02.也就是传递进去的umount_list，然后将kill链表中的所有的vfsmount对象的一些字段设为无效状态。
03.*/
04.void umount_tree(struct vfsmount *mnt, int propagate, struct list_head *kill)
05.{
06. struct vfsmount *p;
07.
08. for (p = mnt; p; p = next_mnt(p, mnt))
09. list_move(&p->mnt_hash, kill);
10.
11. if (propagate)
12. propagate_umount(kill);
13.
14. list_for_each_entry(p, kill, mnt_hash) {
15. list_del_init(&p->mnt_expire);
16. list_del_init(&p->mnt_list);
17. __touch_mnt_namespace(p->mnt_ns);
18. p->mnt_ns = NULL;
19. list_del_init(&p->mnt_child);
20. if (p->mnt_parent != p) {
21. p->mnt_parent->mnt_ghosts++;
22. p->mnt_mountpoint->d_mounted--;
23. }
24. change_mnt_propagation(p, MS_PRIVATE);
25. }
26.}
/*完成实际的底层的卸载文件系统的任务。首先他将mnt的所有子移动至kill链表中，
也就是传递进去的umount_list，然后将kill链表中的所有的vfsmount对象的一些字段设为无效状态。
*/
void umount_tree(struct vfsmount *mnt, int propagate, struct list_head *kill)
{
struct vfsmount *p;
for (p = mnt; p; p = next_mnt(p, mnt))
list_move(&p->mnt_hash, kill);
if (propagate)
propagate_umount(kill);
list_for_each_entry(p, kill, mnt_hash) {
list_del_init(&p->mnt_expire);
list_del_init(&p->mnt_list);
__touch_mnt_namespace(p->mnt_ns);
p->mnt_ns = NULL;
list_del_init(&p->mnt_child);
if (p->mnt_parent != p) {
p->mnt_parent->mnt_ghosts++;
p->mnt_mountpoint->d_mounted--;
}
change_mnt_propagation(p, MS_PRIVATE);
}
}