asmlinkage long sys_chdir(const char __user * filename)
{
	/*路徑名查詢函式*/
	struct nameidata nd;
	int error;
	/*之前我們分析過，根據得到的檔名，找到對應的dentry和mnt結構體放入nameidata結構體裡邊*/
	error = __user_walk(filename,
			    LOOKUP_FOLLOW|LOOKUP_DIRECTORY|LOOKUP_CHDIR, &nd);
	if (error)
		goto out;
	/*vfs層許可權檢查，前邊也分析過*/
	error = vfs_permission(&nd, MAY_EXEC);
	if (error)
		goto dput_and_out;
	/*主要的工作函式，設定程序當前工作路徑*/
	set_fs_pwd(current->fs, nd.mnt, nd.dentry);


dput_and_out:
	/*釋放建立的nameidata結構體*/
	path_release(&nd);
out:
	return error;
}
 

void set_fs_pwd(struct fs_struct *fs, struct vfsmount *mnt,
		struct dentry *dentry)
{
	/*指向當前程序結構體裡的fs_struct裡的dentry和mnt結構體，就是原來的路徑*/
	struct dentry *old_pwd;
	struct vfsmount *old_pwdmnt;


	write_lock(&fs->lock);
	old_pwd = fs->pwd;
	old_pwdmnt = fs->pwdmnt;
	/*把程序結構體task_struct裡的pwd和pwdmnt指向新的dentry和vfsmount結構體*/
	fs->pwdmnt = mntget(mnt);
	fs->pwd = dget(dentry);
	write_unlock(&fs->lock);
	/*釋放原先的dentry和vfsmount*/
	if (old_pwd) {
		dput(old_pwd);
		mntput(old_pwdmnt);
	}
}