30 Android存储系统之源码篇
16lz
2021-12-04
/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 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 { ArrayMap mCallbacks = new ArrayMap(); //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 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 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,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:
volume_reset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.
vold volume3.1 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( 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 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 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/，打开文件；/fd -
proc/打开文件映射；/maps -
proc/链接文件；/cwd -
proc/链接文件；/root -
proc/链接文件；/exe
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( 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(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。
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