Android存储系统之源码篇

http://gityuan.com/2016/07/17/android-io/

/framework/base/services/java/com/android/server/SystemServer.java
/framework/base/services/core/java/com/android/server/MountService.java
/framework/base/services/core/java/com/android/server/NativeDaemonConnector.java
/framework/base/services/core/java/com/android/server/NativeDaemonEvent.java
/framework/base/core/java/android/os/storage/IMountService.java
/framework/base/core/java/android/os/storage/IMountServiceListener.java
/framework/base/core/java/android/os/storage/StorageManager.java

/system/vold/Main.cpp
/system/vold/VolumeManager.cpp
/system/vold/NetlinkManager.cpp
/system/vold/NetlinkHandler.cpp
/system/vold/CommandListener.cpp
/system/vold/VoldCommand.cpp
/system/vold/VolumeBase.cpp
/system/vold/PublicVolume.cpp
/system/vold/EmulatedVolume.cpp
/system/vold/PublicVolume.cpp
/system/vold/Disk.cpp

/system/core/libsysutils/src/NetlinkListener.cpp
/system/core/libsysutils/src/SocketListener.cpp
/system/core/libsysutils/src/FrameworkListener.cpp
/system/core/libsysutils/src/FrameworkCommand.cpp
/system/core/include/sysutils/NetlinkListener.h
/system/core/include/sysutils/SocketListener.h
/system/core/include/sysutils/FrameworkListener.h
/system/core/include/sysutils/FrameworkCommand.h

一、概述

本文主要介绍跟存储相关的模块MountService和Vold的整体流程与架构设计.

MountService：Android Binder服务，运行在system_server进程，用于跟Vold进行消息通信，比如

MountService

向

Vold

发送挂载SD卡的命令,或者接收到来自

Vold

的外设热插拔事件。
Vold:全称为Volume Daemon，用于管理外部存储设备的Native守护进程，这是一个非常重要的守护进程，由NetlinkManager，VolumeManager，CommandListener这3部分组成。

二、MountService

MountService运行在

system_server

进程，在系统启动到阶段PHASE_WAIT_FOR_DEFAULT_DISPLAY后，进入

startOtherServices

会启动MountService.

2.1 启动

[-> SystemServer.java]

private void startOtherServices() {
...
IMountService mountService = null;
//启动MountService服务，【见小节2.2】
mSystemServiceManager.startService(MOUNT_SERVICE_CLASS);
//等价new IMountService.Stub.Proxy()，即获取MountService的proxy对象
mountService = IMountService.Stub.asInterface(
ServiceManager.getService("mount"));
...

mActivityManagerService.systemReady(new Runnable() {
public void run() {
//启动到阶段550【见小节2.7】
mSystemServiceManager.startBootPhase(
SystemService.PHASE_ACTIVITY_MANAGER_READY);
...
});
}

NotificationManagerService依赖于MountService，比如media/usb通知事件，所以需要先启动MountService。此处MOUNT_SERVICE_CLASS=

com.android.server.MountService$Lifecycle

.

2.2 startService

mSystemServiceManager.startService(MOUNT_SERVICE_CLASS)主要完成3件事：

创建MOUNT_SERVICE_CLASS所指类的Lifecycle对象；
将该对象添加SystemServiceManager的

mServices

服务列表；
最后调用Lifecycle的onStart()方法，主要工作量这个过程，如下：
[-> MountService.java]

class MountService extends IMountService.Stub
implements INativeDaemonConnectorCallbacks, Watchdog.Monitor {
public static class Lifecycle extends SystemService {
public void onStart() {
//创建MountService对象【见小节2.3】
mMountService = new MountService(getContext());
//登记Binder服务
publishBinderService("mount", mMountService);
}
...
}
...
}

创建MountService对象，并向Binder服务的大管家ServiceManager登记，该服务名为“mount”，对应服务对象为mMountService。登记之后，其他地方当需要MountService的服务时便可以通过服务名来向ServiceManager来查询具体的MountService服务。

2.3 MountService

[-> MountService.java]

public MountService(Context context) {
sSelf = this;

mContext = context;
//FgThread线程名为“"android.fg"，创建IMountServiceListener回调方法【见小节2.4】
mCallbacks = new Callbacks(FgThread.get().getLooper());
//获取PKMS的Client端对象
mPms = (PackageManagerService) ServiceManager.getService("package");
//创建“MountService”线程
HandlerThread hthread = new HandlerThread(TAG);
hthread.start();

mHandler = new MountServiceHandler(hthread.getLooper());
//IoThread线程名为"android.io"，创建OBB操作的handler
mObbActionHandler = new ObbActionHandler(IoThread.get().getLooper());

File dataDir = Environment.getDataDirectory();
File systemDir = new File(dataDir, "system");
mLastMaintenanceFile = new File(systemDir, LAST_FSTRIM_FILE);
//判断/data/system/last-fstrim文件，不存在则创建，存在则更新最后修改时间
if (!mLastMaintenanceFile.exists()) {
(new FileOutputStream(mLastMaintenanceFile)).close();
...
} else {
mLastMaintenance = mLastMaintenanceFile.lastModified();
}
...
//将MountServiceInternalImpl登记到sLocalServiceObjects
LocalServices.addService(MountServiceInternal.class, mMountServiceInternal);
//创建用于VoldConnector的NDC对象【见小节2.5】
mConnector = new NativeDaemonConnector(this, "vold", MAX_CONTAINERS * 2, VOLD_TAG, 25,
null);
mConnector.setDebug(true);
//创建线程名为"VoldConnector"的线程，用于跟vold通信【见小节2.6】
Thread thread = new Thread(mConnector, VOLD_TAG);
thread.start();

//创建用于CryptdConnector工作的NDC对象
mCryptConnector = new NativeDaemonConnector(this, "cryptd",
MAX_CONTAINERS * 2, CRYPTD_TAG, 25, null);
mCryptConnector.setDebug(true);
//创建线程名为"CryptdConnector"的线程，用于加密
Thread crypt_thread = new Thread(mCryptConnector, CRYPTD_TAG);
crypt_thread.start();

//注册监听用户添加、删除的广播
final IntentFilter userFilter = new IntentFilter();
userFilter.addAction(Intent.ACTION_USER_ADDED);
userFilter.addAction(Intent.ACTION_USER_REMOVED);
mContext.registerReceiver(mUserReceiver, userFilter, null, mHandler);

//内部私有volume的路径为/data，该volume通过dumpsys mount是不会显示的
addInternalVolume();

//默认为false
if (WATCHDOG_ENABLE) {
Watchdog.getInstance().addMonitor(this);
}
}

其主要功能依次是：

创建ICallbacks回调方法,FgThread线程名为”android.fg”，此处用到的Looper便是线程”android.fg”中的Looper;
创建并启动线程名为”MountService”的handlerThread；
创建OBB操作的handler,IoThread线程名为”android.io”，此处用到的的Looper便是线程”android.io”中的Looper;
创建NativeDaemonConnector对象
创建并启动线程名为”VoldConnector”的线程；
创建并启动线程名为”CryptdConnector”的线程；
注册监听用户添加、删除的广播；
从这里便可知道共创建了3个线程：”MountService”,”VoldConnector”,”CryptdConnector”，另外还会使用到系统进程中的两个线程”android.fg”和”android.io”. 这便是在文章开头进程架构图中Java framework层进程的创建情况.

接下来再分别看看MountService创建过程中的Callbacks实例化, NativeDaemonConnector实例化,以及”vold”线程的运行.

2.4 Callbacks

class MountService {
...
private static class Callbacks extends Handler {
private final RemoteCallbackList<IMountServiceListener>
mCallbacks = new RemoteCallbackList<>();
public Callbacks(Looper looper) {
super(looper);
}
...
}
}

创建Callbacks时的Looper为FgThread.get().getLooper()，其中

FgThread

采用单例模式，是一个线程名为”android.fg”的HandlerThread。另外，Callbacks对象有一个成员变量

mCallbacks

，如下：

[-> RemoteCallbackList.java]

public class RemoteCallbackList<E extends IInterface> {
ArrayMap<IBinder, Callback> mCallbacks
= new ArrayMap<IBinder, Callback>();

//Binder死亡通知
private final class Callback implements IBinder.DeathRecipient {
public void binderDied() {
...
}
}

//注册死亡回调
public boolean register(E callback, Object cookie) {
synchronized (mCallbacks) {
...
IBinder binder = callback.asBinder();
Callback cb = new Callback(callback, cookie);
binder.linkToDeath(cb, 0);
mCallbacks.put(binder, cb);
...
}
}
...
}

通过

register()

方法添加IMountServiceListener对象信息到

mCallbacks

成员变量。RemoteCallbackList的内部类Callback继承于IBinder.DeathRecipient，很显然这是死亡通知，当binder服务端进程死亡后，回调binderDied方法通知binder客户端进行相应地处理。

2.5 NativeDaemonConnector

[-> NativeDaemonConnector.java]

NativeDaemonConnector(INativeDaemonConnectorCallbacks callbacks, String socket,
int responseQueueSize, String logTag, int maxLogSize, PowerManager.WakeLock wl) {
this(callbacks, socket, responseQueueSize, logTag, maxLogSize, wl,
FgThread.get().getLooper());
}

NativeDaemonConnector(INativeDaemonConnectorCallbacks callbacks, String socket,
int responseQueueSize, String logTag, int maxLogSize, PowerManager.WakeLock wl,
Looper looper) {
mCallbacks = callbacks;
//socket名为"vold"
mSocket = socket;
//对象响应个数为500
mResponseQueue = new ResponseQueue(responseQueueSize);
mWakeLock = wl;
if (mWakeLock != null) {
mWakeLock.setReferenceCounted(true);
}
mLooper = looper;
mSequenceNumber = new AtomicInteger(0);
//TAG为"VoldConnector"
TAG = logTag != null ? logTag : "NativeDaemonConnector";
mLocalLog = new LocalLog(maxLogSize);
}

mLooper为FgThread.get().getLooper()，即运行在”android.fg”线程；
mResponseQueue对象中成员变量

mPendingCmds

数据类型为LinkedList，记录着vold进程上报的响应事件，事件个数上限为500。

2.6 NDC.run

[-> NativeDaemonConnector.java]

final class NativeDaemonConnector implements Runnable, Handler.Callback, Watchdog.Monitor {
public void run() {
mCallbackHandler = new Handler(mLooper, this);

while (true) {
try {
//监听vold的socket【见小节2.13】
listenToSocket();
} catch (Exception e) {
loge("Error in NativeDaemonConnector: " + e);
SystemClock.sleep(5000);
}
}
}
}

在线程

VoldConnector

中建立了名为

vold

的socket的客户端，通过循环方式不断监听Vold服务端发送过来的消息。
另外，同理还有一个线程

CryptdConnector也采用类似的方式，建立了

cryptd`的socket客户端，监听Vold中另个线程发送过来的消息。到此,MountService与NativeDaemonConnector都已经启动，那么接下来到系统启动到达阶段PHASE_ACTIVITY_MANAGER_READY，则调用到onBootPhase方法。

2.7 onBootPhase

[-> MountService.java ::Lifecycle]

由于MountService的内部Lifecycle已添加SystemServiceManager的

mServices

服务列表；系统启动到

PHASE_ACTIVITY_MANAGER_READY

时会回调

mServices

中的

onBootPhase

方法

public static class Lifecycle extends SystemService {
public void onBootPhase(int phase) {
if (phase == SystemService.PHASE_ACTIVITY_MANAGER_READY) {
mMountService.systemReady();
}
}
}

再调用MountService.systemReady方法，该方法主要是通过

mHandler

发送消息。

private void systemReady() {
mSystemReady = true;
mHandler.obtainMessage(H_SYSTEM_READY).sendToTarget();
}

此处

mHandler = new MountServiceHandler(hthread.getLooper())

,采用的是线程”MountService”中的Looper。到此system_server主线程通过handler向线程”MountService”发送

H_SYSTEM_READY

消息，接下来进入线程”MountService”的MountServiceHandler对象(简称MSH)的handleMessage()来处理相关的消息。

2.8 MSH.handleMessage

[-> MountService.java ::MountServiceHandler]

class MountServiceHandler extends Handler {
public void handleMessage(Message msg) {
switch (msg.what) {
case H_SYSTEM_READY: {
handleSystemReady(); //【见小节2.9】
break;
}
...
}
}
}

2.9 handleSystemReady

[-> MountService.java]

private void handleSystemReady() {
synchronized (mLock) {
//【见小节2.10】
resetIfReadyAndConnectedLocked();
}

//计划执行日常的fstrim操作【】
MountServiceIdler.scheduleIdlePass(mContext);
}

2.10 resetIfReadyAndConnectedLocked

[-> MountService.java]

private void resetIfReadyAndConnectedLocked() {
Slog.d(TAG, "Thinking about reset, mSystemReady=" + mSystemReady
+ ", mDaemonConnected=" + mDaemonConnected);
//当系统启动到阶段550，并且已经与vold守护进程建立连接，则执行reset
if (mSystemReady && mDaemonConnected) {
killMediaProvider();
mDisks.clear();
mVolumes.clear();

//将/data为路径的private volume添加到mVolumes
addInternalVolume();

try {
//【见小节2.11】
mConnector.execute("volume", "reset");

//告知所有已经存在和启动的users
final UserManager um = mContext.getSystemService(UserManager.class);
final List<UserInfo> users = um.getUsers();
for (UserInfo user : users) {
mConnector.execute("volume", "user_added", user.id, user.serialNumber);
}
for (int userId : mStartedUsers) {
mConnector.execute("volume", "user_started", userId);
}
} catch (NativeDaemonConnectorException e) {
Slog.w(TAG, "Failed to reset vold", e);
}
}
}

2.11 NDC.execute

[-> NativeDaemonConnector.java]

public NativeDaemonEvent execute(String cmd, Object... args)
throws NativeDaemonConnectorException {
return execute(DEFAULT_TIMEOUT, cmd, args);
}

其中

DEFAULT_TIMEOUT=1min

，即命令执行超时时长为1分钟。经过层层调用，executeForList()

public NativeDaemonEvent[] executeForList(long timeoutMs, String cmd, Object... args)
throws NativeDaemonConnectorException {
final long startTime = SystemClock.elapsedRealtime();

final ArrayList<NativeDaemonEvent> events = Lists.newArrayList();

final StringBuilder rawBuilder = new StringBuilder();
final StringBuilder logBuilder = new StringBuilder();

//mSequenceNumber初始化值为0，每执行一次该方法则进行加1操作
final int sequenceNumber = mSequenceNumber.incrementAndGet();

makeCommand(rawBuilder, logBuilder, sequenceNumber, cmd, args);

//例如：“3 volume reset”
final String rawCmd = rawBuilder.toString();
final String logCmd = logBuilder.toString();

log("SND -> {" + logCmd + "}");

synchronized (mDaemonLock) {
//将cmd写入到socket的输出流
mOutputStream.write(rawCmd.getBytes(StandardCharsets.UTF_8));
...
}

NativeDaemonEvent event = null;
do {
//【见小节2.12】
event = mResponseQueue.remove(sequenceNumber, timeoutMs, logCmd);
events.add(event);
//当收到的事件响应码属于[100,200)区间，则继续等待后续事件上报
} while (event.isClassContinue());

final long endTime = SystemClock.elapsedRealtime();
//对于执行时间超过500ms则会记录到log
if (endTime - startTime > WARN_EXECUTE_DELAY_MS) {
loge("NDC Command {" + logCmd + "} took too long (" + (endTime - startTime) + "ms)");
}
...
return events.toArray(new NativeDaemonEvent[events.size()]);
}

首先，将带执行的命令mSequenceNumber执行加1操作，再将cmd(例如

3 volume reset

)写入到socket的输出流，通过循环与poll机制等待执行底层响应该操作结果，否则直到1分钟超时才结束该方法。即便收到底层的响应码，如果响应码属于[100,200)区间，则继续阻塞等待后续事件上报。

2.12 ResponseQueue.remove

[-> MountService.java ::ResponseQueue]

private static class ResponseQueue {
public NativeDaemonEvent remove(int cmdNum, long timeoutMs, String logCmd) {
PendingCmd found = null;
synchronized (mPendingCmds) {
//从mPendingCmds查询cmdNum
for (PendingCmd pendingCmd : mPendingCmds) {
if (pendingCmd.cmdNum == cmdNum) {
found = pendingCmd;
break;
}
}
//如果已有的mPendingCmds中查询不到，则创建一个新的PendingCmd
if (found == null) {
found = new PendingCmd(cmdNum, logCmd);
mPendingCmds.add(found);
}
found.availableResponseCount--;
if (found.availableResponseCount == 0) mPendingCmds.remove(found);
}
NativeDaemonEvent result = null;
try {
//采用poll轮询方式等待底层上报该事件，直到1分钟超时
result = found.responses.poll(timeoutMs, TimeUnit.MILLISECONDS);
} catch (InterruptedException e) {}
return result;
}
}

这里用到poll，先来看看

responses = new ArrayBlockingQueue<NativeDaemonEvent>(10)

,这是一个长度为10的可阻塞队列。
这里的poll也是阻塞的方式来轮询事件。

responses.poll

[-> ArrayBlockingQueue.java]

public E poll(long timeout, TimeUnit unit) throws InterruptedException {
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
//可中断的锁等待
lock.lockInterruptibly();
try {
//当队列长度为空，循环等待
while (count == 0) {
if (nanos <= 0)
return null;
nanos = notEmpty.awaitNanos(nanos);
}
return extract();
} finally {
lock.unlock();
}
}

小知识：这里用到了ReentrantLock同步锁，该锁跟synchronized有功能有很相似，用于多线程并发访问。那么ReentrantLock与synchronized相比,

ReentrantLock优势：

ReentrantLock功能更为强大，比如有时间锁等候，可中断锁等候(lockInterruptibly)，锁投票等功能；
ReentrantLock性能更好些；
ReentrantLock提供可轮询的锁请求(tryLock)，相对不容易产生死锁；而synchronized只要进入，要么成功获取，要么一直阻塞等待。
ReentrantLock的劣势：

lock必须在finally块显式地释放，否则如果代码抛出Exception，锁将一直得不到释放；对于synchronized而言，JVM或者ART虚拟机都会确保该锁能自动释放。
synchronized锁，在dump线程转储时会记录锁信息，对于分析调试大有裨益；对于Lock来说，只是普通类，虚拟机无法识别。
再回到ResponseQueue.remove(),该方法中mPendingCmds中的内容是哪里添加的呢？其实是在NDC.listenToSocket循环监听到消息时添加的，则接下来看看监听过程。

2.13 listenToSocket

[-> NativeDaemonConnector.java]

private void listenToSocket() throws IOException {
LocalSocket socket = null;

try {
socket = new LocalSocket();
LocalSocketAddress address = determineSocketAddress();
//建立与"/dev/socket/vold"的socket连接
socket.connect(address);

InputStream inputStream = socket.getInputStream();
synchronized (mDaemonLock) {
mOutputStream = socket.getOutputStream();
}
//建立连接后，回调MS.onDaemonConnected【见小节2.15】
mCallbacks.onDaemonConnected();

byte[] buffer = new byte[BUFFER_SIZE];
int start = 0;

while (true) {
int count = inputStream.read(buffer, start, BUFFER_SIZE - start);
...

for (int i = 0; i < count; i++) {
if (buffer[i] == 0) {
final String rawEvent = new String(
buffer, start, i - start, StandardCharsets.UTF_8);

boolean releaseWl = false;
try {
//解析socket服务端发送的event
final NativeDaemonEvent event = NativeDaemonEvent.parseRawEvent(
rawEvent);

log("RCV <- {" + event + "}");
//当事件的响应码区间为[600,700)，则发送消息交由mCallbackHandler处理
if (event.isClassUnsolicited()) {
if (mCallbacks.onCheckHoldWakeLock(event.getCode())
&& mWakeLock != null) {
mWakeLock.acquire();
releaseWl = true;
}
if (mCallbackHandler.sendMessage(mCallbackHandler.obtainMessage(
event.getCode(), event.getRawEvent()))) {
releaseWl = false;
}
} else {
//对于其他的响应码则添加到mResponseQueue队列【见小节2.14】
mResponseQueue.add(event.getCmdNumber(), event);
}
} catch (IllegalArgumentException e) {
log("Problem parsing message " + e);
} finally {
if (releaseWl) {
mWakeLock.acquire();
}
}
start = i + 1;
}
}
...
}
} catch (IOException ex) {
throw ex;
} finally {
//收尾清理类工作，关闭mOutputStream, socket
...
}
}

这里有一个动作是mResponseQueue.add()，通过该方法便能触发ResponseQueue.poll阻塞操作继续往下执行。

2.14 ResponseQueue.add

[-> NativeDaemonConnector.java]

private static class ResponseQueue {
public void add(int cmdNum, NativeDaemonEvent response) {
PendingCmd found = null;
synchronized (mPendingCmds) {
for (PendingCmd pendingCmd : mPendingCmds) {
if (pendingCmd.cmdNum == cmdNum) {
found = pendingCmd;
break;
}
}
//没有找到则创建相应的PendingCmd
if (found == null) {
while (mPendingCmds.size() >= mMaxCount) {
PendingCmd pendingCmd = mPendingCmds.remove();
}
found = new PendingCmd(cmdNum, null);
mPendingCmds.add(found);
}
found.availableResponseCount++;
if (found.availableResponseCount == 0) mPendingCmds.remove(found);
}
try {
found.responses.put(response);
} catch (InterruptedException e) { }
}
}

responses.put

[-> ArrayBlockingQueue.java]

public void put(E e) throws InterruptedException {
checkNotNull(e);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
//当队列满了则等待
while (count == items.length)
notFull.await();
insert(e);
} finally {
lock.unlock();
}
}

看完了如何向mPendingCmds中增加待处理的命令,再来回过来看看,当当listenToSocket刚开始监听前,收到Native的Daemon连接后的执行操作.

2.15 MS.onDaemonConnected

[-> MountService.java]

public void onDaemonConnected() {
mDaemonConnected = true;
mHandler.obtainMessage(H_DAEMON_CONNECTED).sendToTarget();
}

当前主线程发送消息

H_DAEMON_CONNECTED

给线程MountService`，该线程收到消息后调用MountServiceHandler的handleMessage()相应分支后，进而调用handleDaemonConnected()方法。

private void handleDaemonConnected() {
synchronized (mLock) {
resetIfReadyAndConnectedLocked();
}

//类型为CountDownLatch，用于多线程同步，阻塞await()直到计数器为零
mConnectedSignal.countDown();
if (mConnectedSignal.getCount() != 0) {
return;
}

//调用PMS来加载ASECs
mPms.scanAvailableAsecs();

//用于通知ASEC扫描已完成
mAsecsScanned.countDown();
}

这里的PMS.scanAvailableAsecs()经过层层调用，最终核心工作还是通过MountService.getSecureContainerList。

[-> MountService.java]

public String[] getSecureContainerList() {
enforcePermission(android.Manifest.permission.ASEC_ACCESS);
//等待mConnectedSignal计数锁达到零
waitForReady();
//当没有挂载Primary卷设备，则弹出警告
warnOnNotMounted();

try {
//向vold进程发送asec list命令
return NativeDaemonEvent.filterMessageList(
mConnector.executeForList("asec", "list"), VoldResponseCode.AsecListResult);
} catch (NativeDaemonConnectorException e) {
return new String[0];
}
}

2.16 小节

这里以一张简单的流程图来说明上述过程:

三、Vold

介绍完了Java framework层的MountService以及NativeDaemonConnector,往下走来到了Vold的世界.Vold是由开机过程中解析init.rc时启动：

on post-fs-data
start vold //启动vold服务

Vold的service定义如下：

service vold /system/bin/vold
class core
socket vold stream 0660 root mount
socket cryptd stream 0660 root mount
ioprio be 2

接下来便进入Vold的main(),在开启新的征途之前,为了不被代码弄晕,先来用一幅图来介绍下这些核心类之间的关系以及主要方法,以方便更好的往下阅读.

3.1 main

[-> system/vold/Main.cpp]

int main(int argc, char** argv) {
setenv("ANDROID_LOG_TAGS", "*:v", 1);
android::base::InitLogging(argv, android::base::LogdLogger(android::base::SYSTEM));

VolumeManager *vm;
CommandListener *cl;
CryptCommandListener *ccl;
NetlinkManager *nm;

//解析参数，设置contenxt
parse_args(argc, argv);
...

fcntl(android_get_control_socket("vold"), F_SETFD, FD_CLOEXEC);
fcntl(android_get_control_socket("cryptd"), F_SETFD, FD_CLOEXEC);

mkdir("/dev/block/vold", 0755);

//用于cryptfs检查，并mount加密的文件系统
klog_set_level(6);

//创建单例对象VolumeManager 【见小节3.2.1】
if (!(vm = VolumeManager::Instance())) {
exit(1);
}

//创建单例对象NetlinkManager 【见小节3.3.1】
if (!(nm = NetlinkManager::Instance())) {
exit(1);
}

if (property_get_bool("vold.debug", false)) {
vm->setDebug(true);
}

// 创建CommandListener对象 【见小节3.4.1】
cl = new CommandListener();
// 创建CryptCommandListener对象 【见小节3.5.1】
ccl = new CryptCommandListener();

//【见小节3.2.2】
vm->setBroadcaster((SocketListener *) cl);
//【见小节3.3.2】
nm->setBroadcaster((SocketListener *) cl);

if (vm->start()) { //【见小节3.2.3】
exit(1);
}

process_config(vm)； //【见小节3.2.4】

if (nm->start()) {  //【见小节3.3.3】
exit(1);
}

coldboot("/sys/block");

//启动响应命令的监听器 //【见小节3.4.2】
if (cl->startListener()) {
exit(1);
}

if (ccl->startListener()) {
exit(1);
}

//Vold成为监听线程
while(1) {
sleep(1000);
}

exit(0);
}

该方法的主要功能是创建下面4个对象并启动

VolumeManager
NetlinkManager （NetlinkHandler）
CommandListener
CryptCommandListener
接下来分别说说几个类：

3.2 VolumeManager

3.2.1 创建

[-> VolumeManager.cpp]

VolumeManager *VolumeManager::Instance() {
if (!sInstance)
sInstance = new VolumeManager();
return sInstance;
}

创建单例模式的VolumeManager对象

VolumeManager::VolumeManager() {
mDebug = false;
mActiveContainers = new AsecIdCollection();
mBroadcaster = NULL;
mUmsSharingCount = 0;
mSavedDirtyRatio = -1;
//当UMS获取时，则设置dirty ratio为0
mUmsDirtyRatio = 0;
}

3.2.2 vm->setBroadcaster

void setBroadcaster(SocketListener *sl) {
mBroadcaster = sl;
}

将新创建的

CommandListener

对象sl赋值给vm对象的成员变量

mBroadcaster

3.2.3 vm->start

int VolumeManager::start() {
//卸载所有设备，以提供最干净的环境
unmountAll();

//创建Emulated内部存储
mInternalEmulated = std::shared_ptr<android::vold::VolumeBase>(
new android::vold::EmulatedVolume("/data/media"));
mInternalEmulated->create();
return 0;
}

mInternalEmulated的据类型为

EmulatedVolume

，设备路径为

/data/media

，id和label为“emulated”，mMountFlags=0。

EmulatedVolume

继承于

VolumeBase

3.2.3.1 unmountAll

int VolumeManager::unmountAll() {
std::lock_guard<std::mutex> lock(mLock);

//卸载内部存储
if (mInternalEmulated != nullptr) {
mInternalEmulated->unmount();
}
//卸载外部存储
for (auto disk : mDisks) {
disk->unmountAll();
}

FILE* fp = setmntent("/proc/mounts", "r");
if (fp == NULL) {
return -errno;
}

std::list<std::string> toUnmount;
mntent* mentry;
while ((mentry = getmntent(fp)) != NULL) {
if (strncmp(mentry->mnt_dir, "/mnt/", 5) == 0
|| strncmp(mentry->mnt_dir, "/storage/", 9) == 0) {
toUnmount.push_front(std::string(mentry->mnt_dir));
}
}
endmntent(fp);

for (auto path : toUnmount) {
//强制卸载
android::vold::ForceUnmount(path);
}

return 0;
}

此处打开的”/proc/mounts”每一行内容依次是文件名，目录，类型，操作。例如：

/dev/fuse /mnt/runtime/default/emulated fuse rw,nosuid,nodev,...

该方法的主要工作是卸载：

内部存储mInternalEmulated；
外部存储mDisks，比如sdcard等；
“/proc/mounts”中目录包含mnt或者storage的路径；
卸载内部存储:

status_t EmulatedVolume::doUnmount() {
if (mFusePid > 0) {
kill(mFusePid, SIGTERM);
TEMP_FAILURE_RETRY(waitpid(mFusePid, nullptr, 0));
mFusePid = 0;
}

KillProcessesUsingPath(getPath());
//强制卸载fuse路径
ForceUnmount(mFuseDefault);
ForceUnmount(mFuseRead);
ForceUnmount(mFuseWrite);

rmdir(mFuseDefault.c_str());
rmdir(mFuseRead.c_str());
rmdir(mFuseWrite.c_str());

mFuseDefault.clear();
mFuseRead.clear();
mFuseWrite.clear();

return OK;
}

KillProcessesUsingPath

的功能很强大，通过文件path来查看其所在进程，并杀掉相应进程。当以下5处任意一处存在与path相同的地方，则会杀掉相应的进程：

proc/<pid>/fd

，打开文件；

proc/<pid>/maps

 打开文件映射；

proc/<pid>/cwd

 链接文件；

proc/<pid>/root

 链接文件；

proc/<pid>/exe

 链接文件；

3.2.3.2 EV.create
[-> VolumeBase.cpp]

status_t VolumeBase::create() {
mCreated = true;
status_t res = doCreate();
//通知VolumeCreated事件
notifyEvent(ResponseCode::VolumeCreated,
StringPrintf("%d \"%s\" \"%s\"", mType, mDiskId.c_str(), mPartGuid.c_str()));
//设置为非挂载状态
setState(State::kUnmounted);
return res;
}

void VolumeBase::notifyEvent(int event, const std::string& value) {
if (mSilent) return;
//通过socket向MountService发送创建volume的命令(650)
VolumeManager::Instance()->getBroadcaster()->sendBroadcast(event,
StringPrintf("%s %s", getId().c_str(), value.c_str()).c_str(), false);
}

3.2.4 process_config(vm)

[-> system/vold/Main.cpp]

static int process_config(VolumeManager *vm) {
//获取Fstab路径
std::string path(android::vold::DefaultFstabPath());
fstab = fs_mgr_read_fstab(path.c_str());
...

bool has_adoptable = false;
for (int i = 0; i < fstab->num_entries; i++) {
if (fs_mgr_is_voldmanaged(&fstab->recs[i])) {
if (fs_mgr_is_nonremovable(&fstab->recs[i])) {
LOG(WARNING) << "nonremovable no longer supported; ignoring volume";
continue;
}

std::string sysPattern(fstab->recs[i].blk_device);
std::string nickname(fstab->recs[i].label);
int flags = 0;

if (fs_mgr_is_encryptable(&fstab->recs[i])) {
flags |= android::vold::Disk::Flags::kAdoptable;
has_adoptable = true;
}
if (fs_mgr_is_noemulatedsd(&fstab->recs[i])
|| property_get_bool("vold.debug.default_primary", false)) {
flags |= android::vold::Disk::Flags::kDefaultPrimary;
}

vm->addDiskSource(std::shared_ptr<VolumeManager::DiskSource>(
new VolumeManager::DiskSource(sysPattern, nickname, flags)));
}
}
property_set("vold.has_adoptable", has_adoptable ? "1" : "0");
return 0;
}

Fstab路径：首先通过

getprop ro.hardware

，比如高通芯片则为

qcom

那么Fstab路径就是

/fstab.qcom

，那么该文件的具体内容，例如(当然这个不同手机会有所不同)：

src	mnt_point	type	mnt_flags and options	fs_mgr_flags
/dev/block/bootdevice/by-name/system	/system	ext4	ro,barrier=1,discard	wait,verify
/dev/block/bootdevice/by-name/userdata	/data	ext4	nosuid,nodev,barrier=1,noauto_da_alloc,discard	wait,check,forceencrypt=footer
/dev/block/bootdevice/by-name/cust	/cust	ext4	nosuid,nodev,barrier=1	wait,check
/devices/soc.0/7864900.sdhci/mmc_host*	/storage/sdcard1	vfat	nosuid,nodev	wait,voldmanaged=sdcard1:auto,noemulatedsd,encryptable=footer
/dev/block/bootdevice/by-name/config	/frp	emmc	defaults defaults
/devices/platform/msm_hsusb*	/storage/usbotg	vfat	nosuid,nodev	wait,voldmanaged=usbotg:auto,encryptable=footer

3.3 NetlinkManager

3.3.1 创建

[-> NetlinkManager.cpp]

NetlinkManager *NetlinkManager::Instance() {
if (!sInstance)
sInstance = new NetlinkManager();
return sInstance;
}

3.3.2 nm->setBroadcaster

void setBroadcaster(SocketListener *sl) {
mBroadcaster = sl;
}

3.3.3 nm->start

int NetlinkManager::start() {
struct sockaddr_nl nladdr;
int sz = 64 * 1024;
int on = 1;

memset(&nladdr, 0, sizeof(nladdr));
nladdr.nl_family = AF_NETLINK;
nladdr.nl_pid = getpid(); //记录当前进程的pid
nladdr.nl_groups = 0xffffffff;

//创建event socket
if ((mSock = socket(PF_NETLINK, SOCK_DGRAM | SOCK_CLOEXEC,
NETLINK_KOBJECT_UEVENT)) < 0) {
return -1;
}

//设置uevent的SO_RCVBUFFORCE选项
if (setsockopt(mSock, SOL_SOCKET, SO_RCVBUFFORCE, &sz, sizeof(sz)) < 0) {
goto out;
}

//设置uevent的SO_PASSCRED选项
if (setsockopt(mSock, SOL_SOCKET, SO_PASSCRED, &on, sizeof(on)) < 0) {
goto out;
}
//绑定uevent socket
if (bind(mSock, (struct sockaddr *) &nladdr, sizeof(nladdr)) < 0) {
goto out;
}

//创建NetlinkHandler【见小节3.3.4】
mHandler = new NetlinkHandler(mSock);
//启动NetlinkHandler【见小节3.3.5】
if (mHandler->start()) {
goto out;
}

return 0;

out:
close(mSock);
return -1;
}

3.3.4 NetlinkHandler

NetlinkHandler继承于

NetlinkListener

，

NetlinkListener

继承于

SocketListener

。new
NetlinkHandler(mSock)中参数mSock是用于与Kernel进行通信的socket对象。由于这个继承关系，当NetlinkHandler初始化时会调用基类的初始化，最终调用到：

[-> SocketListener.cpp]

SocketListener::SocketListener(int socketFd, bool listen) {
//listen=false
init(NULL, socketFd, listen, false);
}

void SocketListener::init(const char *socketName, int socketFd, bool listen, bool useCmdNum) {
mListen = listen;
mSocketName = socketName;
//用于监听Kernel发送过程的uevent事件
mSock = socketFd;
mUseCmdNum = useCmdNum;
//初始化同步锁
pthread_mutex_init(&mClientsLock, NULL);
//创建socket通信的client端
mClients = new SocketClientCollection();
}

到此，mListen = false; mSocketName = NULL; mUseCmdNum = false。 另外，这里用到的同步锁，用于控制多线程并发访问。 接着在来看看start过程：

3.3.5 NH->start

[-> NetlinkHandler.cpp]

int NetlinkHandler::start() {
return this->startListener();
}

[-> SocketListener.cpp]

int SocketListener::startListener() {
return startListener(4);
}

int SocketListener::startListener(int backlog) {
...
//mListen =false
if (mListen && listen(mSock, backlog) < 0) {
return -1;
} else if (!mListen)
//创建SocketClient对象，并加入到mClients队列
mClients->push_back(new SocketClient(mSock, false, mUseCmdNum));

//创建匿名管道
if (pipe(mCtrlPipe)) {
return -1;
}

//创建工作线程，线程运行函数threadStart【见小节3.3.6】
if (pthread_create(&mThread, NULL, SocketListener::threadStart, this)) {
return -1;
}

return 0;
}

mCtrlPipe是匿名管道，这是一个二元数组，mCtrlPipe[0]从管道读数据，mCtrlPipe[1]从管道写数据。

3.3.6 threadStart

[-> SocketListener.cpp]

void *SocketListener::threadStart(void *obj) {
SocketListener *me = reinterpret_cast<SocketListener *>(obj);
//【见小节3.3.7】
me->runListener();
pthread_exit(NULL); //线程退出
return NULL;
}

3.3.7 SL->runListener

[-> SocketListener.cpp]

void SocketListener::runListener() {
SocketClientCollection pendingList;
while(1) {
SocketClientCollection::iterator it;
fd_set read_fds;
int rc = 0;
int max = -1;

FD_ZERO(&read_fds);

if (mListen) {
max = mSock;
FD_SET(mSock, &read_fds);
}

FD_SET(mCtrlPipe[0], &read_fds);
if (mCtrlPipe[0] > max)
max = mCtrlPipe[0];

pthread_mutex_lock(&mClientsLock);
for (it = mClients->begin(); it != mClients->end(); ++it) {
// NB: calling out to an other object with mClientsLock held (safe)
int fd = (*it)->getSocket();
FD_SET(fd, &read_fds);
if (fd > max) {
max = fd;
}
}
pthread_mutex_unlock(&mClientsLock);
SLOGV("mListen=%d, max=%d, mSocketName=%s", mListen, max, mSocketName);
if ((rc = select(max + 1, &read_fds, NULL, NULL, NULL)) < 0) {
if (errno == EINTR)
continue;
SLOGE("select failed (%s) mListen=%d, max=%d", strerror(errno), mListen, max);
sleep(1);
continue;
} else if (!rc)
continue;

if (FD_ISSET(mCtrlPipe[0], &read_fds)) {
char c = CtrlPipe_Shutdown;
TEMP_FAILURE_RETRY(read(mCtrlPipe[0], &c, 1));
if (c == CtrlPipe_Shutdown) {
break;
}
continue;
}
if (mListen && FD_ISSET(mSock, &read_fds)) {
struct sockaddr addr;
socklen_t alen;
int c;

do {
alen = sizeof(addr);
c = accept(mSock, &addr, &alen);
SLOGV("%s got %d from accept", mSocketName, c);
} while (c < 0 && errno == EINTR);
if (c < 0) {
SLOGE("accept failed (%s)", strerror(errno));
sleep(1);
continue;
}
fcntl(c, F_SETFD, FD_CLOEXEC);
pthread_mutex_lock(&mClientsLock);
mClients->push_back(new SocketClient(c, true, mUseCmdNum));
pthread_mutex_unlock(&mClientsLock);
}

/* Add all active clients to the pending list first */
pendingList.clear();
pthread_mutex_lock(&mClientsLock);
for (it = mClients->begin(); it != mClients->end(); ++it) {
SocketClient* c = *it;
// NB: calling out to an other object with mClientsLock held (safe)
int fd = c->getSocket();
if (FD_ISSET(fd, &read_fds)) {
pendingList.push_back(c);
c->incRef();
}
}
pthread_mutex_unlock(&mClientsLock);

/* Process the pending list, since it is owned by the thread,
* there is no need to lock it */
while (!pendingList.empty()) {
/* Pop the first item from the list */
it = pendingList.begin();
SocketClient* c = *it;
pendingList.erase(it);
/* Process it, if false is returned, remove from list */
if (!onDataAvailable(c)) {
release(c, false);
}
c->decRef();
}
}
}

3.4 CommandListener

3.4.1 创建

[-> CommandListener.cpp]

CommandListener::CommandListener() :
FrameworkListener("vold", true) {
registerCmd(new DumpCmd());
registerCmd(new VolumeCmd());
registerCmd(new AsecCmd());
registerCmd(new ObbCmd());
registerCmd(new StorageCmd());
registerCmd(new FstrimCmd());
}

3.4.1.1 FrameworkListener
[-> FrameworkListener.cpp]

FrameworkListener::FrameworkListener(const char *socketName, bool withSeq) :
SocketListener(socketName, true, withSeq) {
init(socketName, withSeq);
}

void FrameworkListener::init(const char *socketName UNUSED, bool withSeq) {
mCommands = new FrameworkCommandCollection();
errorRate = 0;
mCommandCount = 0;
mWithSeq = withSeq; //true
}

3.4.1.2 SocketListener
[-> SocketListener.cpp]

SocketListener::SocketListener(const char *socketName, bool listen, bool useCmdNum) {
init(socketName, -1, listen, useCmdNum);
}

void SocketListener::init(const char *socketName, int socketFd, bool listen, bool useCmdNum) {
mListen = listen; //true
mSocketName = socketName; //"vold"
mSock = socketFd; // -1
mUseCmdNum = useCmdNum; //true
pthread_mutex_init(&mClientsLock, NULL);
mClients = new SocketClientCollection();
}

socket名为“vold”


3.4.1.3 registerCmd

void FrameworkListener::registerCmd(FrameworkCommand *cmd) {
mCommands->push_back(cmd);
}

CommandListener::VolumeCmd::VolumeCmd() :
VoldCommand("volume") {
}

创建这些对象 DumpCmd，VolumeCmd，AsecCmd，ObbCmd，StorageCmd，FstrimCmd，并都加入到mCommands队列。

3.4.2 cl->startListener

int SocketListener::startListener() {
return startListener(4);
}

int SocketListener::startListener(int backlog) {

if (!mSocketName && mSock == -1) {
...
} else if (mSocketName) {
//获取“vold”所对应的句柄
if ((mSock = android_get_control_socket(mSocketName)) < 0) {
return -1;
}
fcntl(mSock, F_SETFD, FD_CLOEXEC);
}

//CL开始监听
if (mListen && listen(mSock, backlog) < 0) {
return -1;
}
...
//创建匿名管道
if (pipe(mCtrlPipe)) {
return -1;
}

//创建工作线程
if (pthread_create(&mThread, NULL, SocketListener::threadStart, this)) {
return -1;
}

return 0;
}

四、小结

Linux Kernel：通过

uevent

向Vold的NetlinkManager发送Uevent事件；
NetlinkManager：接收来自Kernel的

Uevent

事件，再转发给VolumeManager；
VolumeManager：接收来自NetlinkManager的事件，再转发给CommandListener进行处理；
CommandListener：接收来自VolumeManager的事件，通过

socket

通信方式发送给MountService；
MountService：接收来自CommandListener的事件。