UserManager 調(diào)用 createUser 成功后，會廣播消息 Intent.ACTION_USER_ADDED。
StorageManagerSerive 會收到這個通知，然后通知 vold 服務(wù)。

if (Intent.ACTION_USER_ADDED.equals(action)) {
       final UserManager um = mContext.getSystemService(UserManager.class);
       final int userSerialNumber = um.getUserSerialNumber(userId);
       mConnector.execute("volume", "user_added", userId, userSerialNumber);
} 

system\vold\CommandListener.cpp

    } else if (cmd == "user_added" && argc > 3) {
        // user_added [user] [serial]
        return sendGenericOkFail(cli, vm->onUserAdded(atoi(argv[2]), atoi(argv[3])));
    } else if (cmd == "user_removed" && argc > 2) {
        // user_removed [user]
        return sendGenericOkFail(cli, vm->onUserRemoved(atoi(argv[2])));

    } else if (cmd == "user_started" && argc > 2) {
        // user_started [user]
        return sendGenericOkFail(cli, vm->onUserStarted(atoi(argv[2])));

    } else if (cmd == "user_stopped" && argc > 2) {
        // user_stopped [user]
        return sendGenericOkFail(cli, vm->onUserStopped(atoi(argv[2])));

當(dāng)這個用戶start時，就會調(diào)用 VolumeManager 的 onUserStarted 函數(shù)：

int VolumeManager::onUserStarted(userid_t userId) {
    // Note that sometimes the system will spin up processes from Zygote
    // before actually starting the user, so we're okay if Zygote
    // already created this directory.
    std::string path(StringPrintf("%s/%d", kPathUserMount, userId));
    fs_prepare_dir(path.c_str(), 0755, AID_ROOT, AID_ROOT);

    mStartedUsers.insert(userId);
    if (mPrimary) {
        linkPrimary(userId);
    }
    return 0;
}

fs_prepare_dir就負(fù)責(zé)創(chuàng)建這個用戶的sd卡專有目錄。