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。

/*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;
}

卸载实际工作

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;
}

从内核链表中脱离

/*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);
    }
}

释放引用计数

void release_mounts(struct list_head *head)
{
    struct vfsmount *mnt;
    while (!list_empty(head)) {
        mnt = list_first_entry(head, struct vfsmount, mnt_hash);
        list_del_init(&mnt->mnt_hash);
        if (mnt->mnt_parent != mnt) {
            struct dentry *dentry;
            struct vfsmount *m;
            spin_lock(&vfsmount_lock);
            dentry = mnt->mnt_mountpoint;
            m = mnt->mnt_parent;
            mnt->mnt_mountpoint = mnt->mnt_root;
            mnt->mnt_parent = mnt;
            m->mnt_ghosts--;
            spin_unlock(&vfsmount_lock);
            /*下面两个函数为减小引用计数，减到0时释放*/
            dput(dentry);
            mntput(m);
        }
        /*vfsmount对象所占的内存空间最终在mntput()函数中释放*/
        mntput(mnt);
    }
}