Android SDCard Mount 流程分析（二）
2012-04-12 10:09 Terry_龙 阅读(15090) 评论(1) 编辑 收藏 举报上一篇关于Mount的分析，分析了main的作用和一些挂载系统的分析。下面深入分析Mount的流程走法。
Mount流程分为两个部分
- 主动挂载（插入SDCARD或者USB硬盘时系统自动挂载）
- 手动挂载（卸载SDCARD或者USB硬盘后，再点击加载设备的手动挂载）
不同挂载走的流程并不相同，比如手动挂载是由上层发命令给vold 执行挂动作，而主动挂载是由kernel 分命令给vold 再由vold 发挂载消息给上层，上层得到挂载消息和状态后再发命令给vold 执行挂载。主动挂载较之复杂些。不过虽然流程不一样，但最终还是要调用Volume的挂载函数，下面将详细介绍两者的行走的流程。
由于会涉及SDCARD或者USB硬盘，其中调用的方法就不详细说明，这里只说出当插入SDCARD或者USB硬盘会走的流程。
主动挂载
主动挂载时，会走向DirectVolume类，调用DirectVolume::mountVol方法，代码如下：
int DirectVolume::mountVol() {
char errmsg[255];
dev_t deviceNodes[64];
int i, n = 0;
if (getState() == Volume::State_NoMedia) {
snprintf(errmsg, sizeof(errmsg),
"Volume %s %s mount failed - no media",
getLabel(), getMountpoint());
mVm->getBroadcaster()->sendBroadcast(
ResponseCode::VolumeMountFailedNoMedia,
errmsg, false);
errno = ENODEV;
return -1;
} else if (getState() != Volume::State_Idle) {
errno = EBUSY;
return -1;
}
n = getDeviceNodes((dev_t *) &deviceNodes, 64);
if (!n) {
SLOGE("Failed to get device nodes (%s)\n", strerror(errno));
return -1;
}
bool mounted = false;
for (i = 0; i < n; i++) {
mDevNodeIndex = deviceNodes[i];
//XXX: hack mountpoint
if ( mMountpointParsed ) { free(mMountpointParsed); mMountpointParsed = NULL; }
mMountpointParsed = getParsedMountPoint(mMountpoint, i);
if (isMountpointMounted(getMountpoint())) {
SLOGW("Volume is idle but appears to be mounted - fixing");
setState(Volume::State_Mounted);
// mCurrentlyMountedKdev = XXX
errno = EBUSY;
continue;
}
if (!Volume::mountVol()) {
mounted = true;
}
mState = Volume::State_Idle;
}
if ( mMountpointParsed ) { free(mMountpointParsed); mMountpointParsed = NULL; }
if ( mounted ) {
// at least on partition has been mounted successful, mark disk as mounted
setState(Volume::State_Mounted);
return 0;
}
SLOGE("Volume %s found no suitable devices for mounting :(\n", getLabel());
setState(Volume::State_Idle);
return -1;
}
代码加亮部分，蓝色部分，会循环整个设备节点系统目录位于(/dev/block/vold)，然后调用红色部分代码，调用Volume的挂载方法。
这里，无论是SDCARD或者USB硬盘在主动挂载时，都会走DirectVolume。
手动挂载
手动挂载是由上层发Mount 命令，代码位于MountService里面的doMountVolume方法，具体如何实现我们先不深究，它这里通过发送socket(mount)命令到Vold 的CommandListener里面的CommandListener::VolumeCmd::runCommand方法进入代码这里：
else if (!strcmp(argv[1], "mount")) {
if (argc != 3) {
cli->sendMsg(ResponseCode::CommandSyntaxError, "Usage: volume mount <path>", false);
return 0;
}
if(!strcmp(argv[2],"firstMount")){
VolumeCollection::iterator i;
if(mVolumes!=NULL){
for (i = mVolumes->begin(); i != mVolumes->end(); ++i) {
if (strcmp("/sdcard", (*i)->getMountpoint())) {
vm->mountVolume((*i)->getMountpoint());
}
}
}
}else{
vm->mountVolume(argv[2]);
}
}
if (argc != 3) {
cli->sendMsg(ResponseCode::CommandSyntaxError, "Usage: volume mount <path>", false);
return 0;
}
if(!strcmp(argv[2],"firstMount")){
VolumeCollection::iterator i;
if(mVolumes!=NULL){
for (i = mVolumes->begin(); i != mVolumes->end(); ++i) {
if (strcmp("/sdcard", (*i)->getMountpoint())) {
vm->mountVolume((*i)->getMountpoint());
}
}
}
}else{
vm->mountVolume(argv[2]);
}
}
这里执行挂载动作，看上面蓝色代码是为了系统第一次启动上层发送命令firstMount给CommandListener执行挂载USB硬盘的动作，红色代码即是核心要挂载的方法，调用的VolumeManage的mountVolume 方法，只需传入挂载点。该方法代码是：
int VolumeManager::mountVolume(const char *label) {
Volume *v = lookupVolume(label);
if (!v) {
errno = ENOENT;
return -1;
}
return v->mountVol();
}
Volume *v = lookupVolume(label);
if (!v) {
errno = ENOENT;
return -1;
}
return v->mountVol();
}
可以看出，这里同样调用的是Volume的mountVol方法，殊途同归，接下来着重看一下Volume类里面这个mountVol方法，究竟干了些啥。
Volume::mountVol 方法深究
别的先不管，来看一下代码
int Volume::mountVol() {
int rc = 0;
char errmsg[255];
const char *mountPath;
char devicePath[255];
sprintf(devicePath, "/dev/block/vold/%d:%d", MAJOR(mDevNodeIndex),
MINOR(mDevNodeIndex));//得到设备节点,如:/dev/block/vold/8:1
SLOGI("%s being considered for volume %s ...major : %d minor: %d\n", devicePath, getLabel(),
MAJOR(mDevNodeIndex),MINOR(mDevNodeIndex));
errno = 0;
setState(Volume::State_Checking);//设置状态为checking整型为3
// TODO: find a way to read the filesystem ID
bool isFatFs = true;
bool isNtfsFS = true;
//检查设备格式是否为Fat32
if (Fat::check(devicePath)) {
if (errno == ENODATA) {
SLOGW("%s does not contain a FAT filesystem\n", devicePath);
isFatFs = false;
} else {
errno = EIO;
/* Badness - abort the mount */
SLOGE("%s failed FS checks (%s)", devicePath, strerror(errno));
setState(Volume::State_Idle);
return -1;
}
}
//创建挂载目录
// create mountpoint
if (mkdir(getMountpoint(), 0755)) {
if (errno != EEXIST) {
SLOGE("Failed to create mountpoint %s (%s)", getMountpoint(), strerror(errno));
return -1;
}
}
/*
* Mount the device on our internal staging mountpoint so we can
* muck with it before exposing it to non priviledged users.
*/
errno = 0;
//如果为sdcard则挂载到/mnt/secure/staging，否则挂载到挂载点
if(!strcmp(getLabel(),"sdcard"))
mountPath="/mnt/secure/staging";
else
mountPath=getMountpoint();
//接下来就是不同格式不同的挂载，这里支持两种格式：fat32,Ntfs
if ( isFatFs ) {
if (Fat::doMount(devicePath,mountPath, false, false, 1000, 1015, 0702, true)) {
SLOGE("%s failed to mount via VFAT (%s)\n", devicePath, strerror(errno));
isFatFs = false;
}
isNtfsFS = false;
}
if ( isNtfsFS ) {
if (Ntfs::doMount(devicePath, mountPath, true)) {
SLOGE("%s failed to mount via NTFS (%s)\n", devicePath, strerror(errno));
isNtfsFS = false;
}
}
if ( !isFatFs && !isNtfsFS ) {
// unsupported filesystem
return -1;
}
SLOGI("Device %s, target %s mounted @ /mnt/secure/staging", devicePath, getMountpoint());
if ( !strcmp(getLabel(), "sdcard") ) {
protectFromAutorunStupidity();
if (createBindMounts()) {
SLOGE("Failed to create bindmounts (%s)", strerror(errno));
umount("/mnt/secure/staging");
setState(Volume::State_Idle);
return -1;
}
}
/*
* Now that the bindmount trickery is done, atomically move the
* whole subtree to expose it to non priviledged users.
* 如果为sdcard则将/mnt/secure/staging 目录移动到挂载点，并将该目录unmount
*/
if(!strcmp(getLabel(),"sdcard")){
if (doMoveMount("/mnt/secure/staging", getMountpoint(), false)) {
SLOGE("Failed to move mount (%s)", strerror(errno));
umount("/mnt/secure/staging");
setState(Volume::State_Idle);
return -1;
}
}
setState(Volume::State_Mounted);//设置状态到MountService
mCurrentlyMountedKdev = mDevNodeIndex;
return 0;
}
int rc = 0;
char errmsg[255];
const char *mountPath;
char devicePath[255];
sprintf(devicePath, "/dev/block/vold/%d:%d", MAJOR(mDevNodeIndex),
MINOR(mDevNodeIndex));//得到设备节点,如:/dev/block/vold/8:1
SLOGI("%s being considered for volume %s ...major : %d minor: %d\n", devicePath, getLabel(),
MAJOR(mDevNodeIndex),MINOR(mDevNodeIndex));
errno = 0;
setState(Volume::State_Checking);//设置状态为checking整型为3
// TODO: find a way to read the filesystem ID
bool isFatFs = true;
bool isNtfsFS = true;
//检查设备格式是否为Fat32
if (Fat::check(devicePath)) {
if (errno == ENODATA) {
SLOGW("%s does not contain a FAT filesystem\n", devicePath);
isFatFs = false;
} else {
errno = EIO;
/* Badness - abort the mount */
SLOGE("%s failed FS checks (%s)", devicePath, strerror(errno));
setState(Volume::State_Idle);
return -1;
}
}
//创建挂载目录
// create mountpoint
if (mkdir(getMountpoint(), 0755)) {
if (errno != EEXIST) {
SLOGE("Failed to create mountpoint %s (%s)", getMountpoint(), strerror(errno));
return -1;
}
}
/*
* Mount the device on our internal staging mountpoint so we can
* muck with it before exposing it to non priviledged users.
*/
errno = 0;
//如果为sdcard则挂载到/mnt/secure/staging，否则挂载到挂载点
if(!strcmp(getLabel(),"sdcard"))
mountPath="/mnt/secure/staging";
else
mountPath=getMountpoint();
//接下来就是不同格式不同的挂载，这里支持两种格式：fat32,Ntfs
if ( isFatFs ) {
if (Fat::doMount(devicePath,mountPath, false, false, 1000, 1015, 0702, true)) {
SLOGE("%s failed to mount via VFAT (%s)\n", devicePath, strerror(errno));
isFatFs = false;
}
isNtfsFS = false;
}
if ( isNtfsFS ) {
if (Ntfs::doMount(devicePath, mountPath, true)) {
SLOGE("%s failed to mount via NTFS (%s)\n", devicePath, strerror(errno));
isNtfsFS = false;
}
}
if ( !isFatFs && !isNtfsFS ) {
// unsupported filesystem
return -1;
}
SLOGI("Device %s, target %s mounted @ /mnt/secure/staging", devicePath, getMountpoint());
if ( !strcmp(getLabel(), "sdcard") ) {
protectFromAutorunStupidity();
if (createBindMounts()) {
SLOGE("Failed to create bindmounts (%s)", strerror(errno));
umount("/mnt/secure/staging");
setState(Volume::State_Idle);
return -1;
}
}
/*
* Now that the bindmount trickery is done, atomically move the
* whole subtree to expose it to non priviledged users.
* 如果为sdcard则将/mnt/secure/staging 目录移动到挂载点，并将该目录unmount
*/
if(!strcmp(getLabel(),"sdcard")){
if (doMoveMount("/mnt/secure/staging", getMountpoint(), false)) {
SLOGE("Failed to move mount (%s)", strerror(errno));
umount("/mnt/secure/staging");
setState(Volume::State_Idle);
return -1;
}
}
setState(Volume::State_Mounted);//设置状态到MountService
mCurrentlyMountedKdev = mDevNodeIndex;
return 0;
}
注意：原生的代码可能跟上面贴出来的代码有点不同，上面的代码是增加了Ntfs-3g挂载的支持和多分区挂载的支持，但基本流程是相同的。
代码有详细的注释，这里要注意的是：sdcard和USB的支持不同，sdcard 挂载时需要先挂载到临时目录/mnt/secure/staging，然后再移动到最终需要挂载的挂载点，而USB硬盘特别是多分区的支持，不用先挂载到临时目录，而是可以支持挂载到想要挂载的挂载点，这里是比较需要注意到的地方(在这里栽过跟头，会出现“随机性的挂载失败”)。
ok.
