Android 匿名内存解析
有了binder机制为什么还需要匿名内存来实现IPC呢？我觉得很大的原因就是binder传输是有大小限制的，不说应用层的限制。在驱动中binder的传输大小被限制在了4M，分享一张图片可能就超过了这个限制。匿名内存的主要解决思路就是通过binder传输文件描述符，使得两个进程都能访问同一个地址来实现共享。
MemoryFile使用
在平常开发中android提供了MemoryFile来实现匿名内存。看下最简单的实现。
Service端
const val GET_ASH_MEMORY = 1000
class MyService : Service() {
val ashData = "AshDemo".toByteArray()
override fun onBind(intent: Intent): IBinder {
return object : Binder() {
override fun onTransact(code: Int, data: Parcel, reply: Parcel?, flags: Int): Boolean {
when(code){
GET_ASH_MEMORY->{//收到客户端请求的时候会烦
val descriptor = createMemoryFile()
reply?.writeParcelable(descriptor, 0)
reply?.writeInt(ashData.size)
return true
}
else->{
return super.onTransact(code, data, reply, flags)
}
}
}
}
}
private fun createMemoryFile(): ParcelFileDescriptor? {
val file = MemoryFile("AshFile", 1024)//创建MemoryFile
val descriptorMethod = file.javaClass.getDeclaredMethod("getFileDescriptor")
val fd=descriptorMethod.invoke(file)//反射拿到fd
file.writeBytes(ashData, 0, 0,ashData.size)//写入字符串
return ParcelFileDescriptor.dup(fd as FileDescriptor?)//返回一个封装的fd
}
}
Server的功能很简单收到GET_ASH_MEMORY请求的时候创建一个MemoryFile，往里写入一个字符串的byte数组，然后将fd和字符长度写入reply中返回给客户端。
Client端
class MainActivity : AppCompatActivity() {
val connect = object :ServiceConnection{
override fun onServiceConnected(name: ComponentName?, service: IBinder?) {
val reply = Parcel.obtain()
val sendData = Parcel.obtain()
service?.transact(GET_ASH_MEMORY, sendData, reply, 0)//传输信号GET_ASH_MEMORY
val pfd = reply.readParcelable(javaClass.classLoader)
val descriptor = pfd?.fileDescriptor//拿到fd
val size = reply.readInt()//拿到长度
val input = FileInputStream(descriptor)
val bytes = input.readBytes()
val message = String(bytes, 0, size, Charsets.UTF_8)//生成string
Toast.makeText(this@MainActivity,message,Toast.LENGTH_SHORT).show()
}
override fun onServiceDisconnected(name: ComponentName?) {
}
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
findViewById(R.id.intent).setOnClickListener {
//启动服务
bindService(Intent(this,MyService::class.java),connect, Context.BIND_AUTO_CREATE)
}
}
}
客户端也很简单，启动服务，发送一个获取MemoryFile的请求，然后通过reply拿到fd和长度，用FileInputStream读取fd中的内容，最后通过toast可以验证这个message已经拿到了。
AshMemory 创建原理
public MemoryFile(String name, int length) throws IOException {
try {
mSharedMemory = SharedMemory.create(name, length);
mMapping = mSharedMemory.mapReadWrite();
} catch (ErrnoException ex) {
ex.rethrowAsIOException();
}
}
MemoryFile就是对SharedMemory的一层封装，具体的工能都是SharedMemory实现的。看SharedMemory的实现。
public static @NonNull SharedMemory create(@Nullable String name, int size)
throws ErrnoException {
if (size <= 0) {
throw new IllegalArgumentException("Size must be greater than zero");
}
return new SharedMemory(nCreate(name, size));
}
private static native FileDescriptor nCreate(String name, int size) throws ErrnoException;
通过一个JNI获得fd，从这里可以推断出java层也只是一个封装，拿到的已经是创建好的fd。
//frameworks/base/core/jni/android_os_SharedMemory.cpp
jobject SharedMemory_nCreate(JNIEnv* env, jobject, jstring jname, jint size) {
const char* name = jname ? env->GetStringUTFChars(jname, nullptr) : nullptr;
int fd = ashmem_create_region(name, size);//创建匿名内存块
int err = fd < 0 ? errno : 0;
if (name) {
env->ReleaseStringUTFChars(jname, name);
}
if (fd < 0) {
jniThrowErrnoException(env, "SharedMemory_create", err);
return nullptr;
}
jobject jifd = jniCreateFileDescriptor(env, fd);//创建java fd返回
if (jifd == nullptr) {
close(fd);
}
return jifd;
}
通过cutils中的ashmem_create_region函数实现的创建
//system/core/libcutils/ashmem-dev.cpp
int ashmem_create_region(const char *name, size_t size)
{
int ret, save_errno;
if (has_memfd_support()) {//老版本兼容用
return memfd_create_region(name ? name : "none", size);
}
int fd = __ashmem_open();//打开Ashmem驱动
if (fd < 0) {
return fd;
}
if (name) {
char buf[ASHMEM_NAME_LEN] = {0};
strlcpy(buf, name, sizeof(buf));
ret = TEMP_FAILURE_RETRY(ioctl(fd, ASHMEM_SET_NAME, buf));//通过ioctl设置名字
if (ret < 0) {
goto error;
}
}
ret = TEMP_FAILURE_RETRY(ioctl(fd, ASHMEM_SET_SIZE, size));//通过ioctl设置大小
if (ret < 0) {
goto error;
}
return fd;
error:
save_errno = errno;
close(fd);
errno = save_errno;
return ret;
}
标准的驱动交互操作
1.open打开驱动
2.通过ioctl与驱动进行交互
下面看下open的流程
static int __ashmem_open()
{
int fd;
pthread_mutex_lock(&__ashmem_lock);
fd = __ashmem_open_locked();
pthread_mutex_unlock(&__ashmem_lock);
return fd;
}
/* logistics of getting file descriptor for ashmem */
static int __ashmem_open_locked()
{
static const std::string ashmem_device_path = get_ashmem_device_path();//拿到Ashmem驱动路径
if (ashmem_device_path.empty()) {
return -1;
}
int fd = TEMP_FAILURE_RETRY(open(ashmem_device_path.c_str(), O_RDWR | O_CLOEXEC));
return fd;
}
回到MemoryFile的构造函数中，拿到了驱动的fd之后调用了mapReadWrite
public @NonNull ByteBuffer mapReadWrite() throws ErrnoException {
return map(OsConstants.PROT_READ | OsConstants.PROT_WRITE, 0, mSize);
}
public @NonNull ByteBuffer map(int prot, int offset, int length) throws ErrnoException {
checkOpen();
validateProt(prot);
if (offset < 0) {
throw new IllegalArgumentException("Offset must be >= 0");
}
if (length <= 0) {
throw new IllegalArgumentException("Length must be > 0");
}
if (offset + length > mSize) {
throw new IllegalArgumentException("offset + length must not exceed getSize()");
}
long address = Os.mmap(0, length, prot, OsConstants.MAP_SHARED, mFileDescriptor, offset);//调用了系统的mmap
boolean readOnly = (prot & OsConstants.PROT_WRITE) == 0;
Runnable unmapper = new Unmapper(address, length, mMemoryRegistration.acquire());
return new DirectByteBuffer(length, address, mFileDescriptor, unmapper, readOnly);
}
到这里就有一个疑问，Linux就有共享内存，android为什么要自己搞一套，只能看下Ashmemory驱动的实现了。
驱动第一步看init和file_operations
static int __init ashmem_init(void)
{
int ret = -ENOMEM;
ashmem_area_cachep = kmem_cache_create("ashmem_area_cache",
sizeof(struct ashmem_area),
0, 0, NULL);//创建
if (!ashmem_area_cachep) {
pr_err("failed to create slab cache\n");
goto out;
}
ashmem_range_cachep = kmem_cache_create("ashmem_range_cache",
sizeof(struct ashmem_range),
0, SLAB_RECLAIM_ACCOUNT, NULL);//创建
if (!ashmem_range_cachep) {
pr_err("failed to create slab cache\n");
goto out_free1;
}
ret = misc_register(&ashmem_misc);//注册为了一个misc设备
........
return ret;
}
创建了两个内存分配器ashmem_area_cachep和ashmem_range_cachep用于分配ashmem_area和ashmem_range
//common/drivers/staging/android/ashmem.c
static const struct file_operations ashmem_fops = {
.owner = THIS_MODULE,
.open = ashmem_open,
.release = ashmem_release,
.read_iter = ashmem_read_iter,
.llseek = ashmem_llseek,
.mmap = ashmem_mmap,
.unlocked_ioctl = ashmem_ioctl,
#ifdef CONFIG_COMPAT
.compat_ioctl = compat_ashmem_ioctl,
#endif
#ifdef CONFIG_PROC_FS
.show_fdinfo = ashmem_show_fdinfo,
#endif
};
open调用的就是ashmem_open
static int ashmem_open(struct inode *inode, struct file *file)
{
struct ashmem_area *asma;
int ret;
ret = generic_file_open(inode, file);
if (ret)
return ret;
asma = kmem_cache_zalloc(ashmem_area_cachep, GFP_KERNEL);//分配一个ashmem_area
if (!asma)
return -ENOMEM;
INIT_LIST_HEAD(&asma->unpinned_list);//初始化unpinned_list
memcpy(asma->name, ASHMEM_NAME_PREFIX, ASHMEM_NAME_PREFIX_LEN);//初始化一个名字
asma->prot_mask = PROT_MASK;
file->private_data = asma;
return 0;
}
ioctl设置名字和长度调用的就是ashmem_ioctl
static long ashmem_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
struct ashmem_area *asma = file->private_data;
long ret = -ENOTTY;
switch (cmd) {
case ASHMEM_SET_NAME:
ret = set_name(asma, (void __user *)arg);
break;
case ASHMEM_SET_SIZE:
ret = -EINVAL;
mutex_lock(&ashmem_mutex);
if (!asma->file) {
ret = 0;
asma->size = (size_t)arg;
}
mutex_unlock(&ashmem_mutex);
break;
}
........
}
实现也都很简单就是改变了一下asma里的值。接下来就是重点mmap了，具体是怎么分配内存的。
static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)
{
static struct file_operations vmfile_fops;
struct ashmem_area *asma = file->private_data;
int ret = 0;
mutex_lock(&ashmem_mutex);
/* user needs to SET_SIZE before mapping */
if (!asma->size) {//判断设置了size
ret = -EINVAL;
goto out;
}
/* requested mapping size larger than object size */
if (vma->vm_end - vma->vm_start > PAGE_ALIGN(asma->size)) {//判断大小是否超过了虚拟内存
ret = -EINVAL;
goto out;
}
/* requested protection bits must match our allowed protection mask */
if ((vma->vm_flags & ~calc_vm_prot_bits(asma->prot_mask, 0)) &
calc_vm_prot_bits(PROT_MASK, 0)) {//权限判断
ret = -EPERM;
goto out;
}
vma->vm_flags &= ~calc_vm_may_flags(~asma->prot_mask);
if (!asma->file) {//是否创建过临时文件，没创建过进入
char *name = ASHMEM_NAME_DEF;
struct file *vmfile;
struct inode *inode;
if (asma->name[ASHMEM_NAME_PREFIX_LEN] != '\0')
name = asma->name;
/* ... and allocate the backing shmem file */
vmfile = shmem_file_setup(name, asma->size, vma->vm_flags);//调用linux函数在tmpfs中创建临时文件
if (IS_ERR(vmfile)) {
ret = PTR_ERR(vmfile);
goto out;
}
vmfile->f_mode |= FMODE_LSEEK;
inode = file_inode(vmfile);
lockdep_set_class(&inode->i_rwsem, &backing_shmem_inode_class);
asma->file = vmfile;
/*
* override mmap operation of the vmfile so that it can't be
* remapped which would lead to creation of a new vma with no
* asma permission checks. Have to override get_unmapped_area
* as well to prevent VM_BUG_ON check for f_ops modification.
*/
if (!vmfile_fops.mmap) {//设置了临时文件的文件操作，防止有其他程序mmap这个临时文件
vmfile_fops = *vmfile->f_op;
vmfile_fops.mmap = ashmem_vmfile_mmap;
vmfile_fops.get_unmapped_area =
ashmem_vmfile_get_unmapped_area;
}
vmfile->f_op = &vmfile_fops;
}
get_file(asma->file);
/*
* XXX - Reworked to use shmem_zero_setup() instead of
* shmem_set_file while we're in staging. -jstultz
*/
if (vma->vm_flags & VM_SHARED) {//这块内存是不是需要跨进程
ret = shmem_zero_setup(vma);//设置文件
if (ret) {
fput(asma->file);
goto out;
}
} else {
/**
实现就是把vm_ops设置为NULL
static inline void vma_set_anonymous(struct vm_area_struct *vma)
{
vma->vm_ops = NULL;
}
*/
vma_set_anonymous(vma);
}
vma_set_file(vma, asma->file);
/* XXX: merge this with the get_file() above if possible */
fput(asma->file);
out:
mutex_unlock(&ashmem_mutex);
return ret;
}
函数很长，但是思路还是很清晰的。创建临时文件，设置文件操作。其中调用的都是linux的系统函数了，看真正设置的shmem_zero_setup函数
int shmem_zero_setup(struct vm_area_struct *vma)
{
struct file *file;
loff_t size = vma->vm_end - vma->vm_start;
/*
* Cloning a new file under mmap_lock leads to a lock ordering conflict
* between XFS directory reading and selinux: since this file is only
* accessible to the user through its mapping, use S_PRIVATE flag to
* bypass file security, in the same way as shmem_kernel_file_setup().
*/
file = shmem_kernel_file_setup("dev/zero", size, vma->vm_flags);
if (IS_ERR(file))
return PTR_ERR(file);
if (vma->vm_file)
fput(vma->vm_file);
vma->vm_file = file;
vma->vm_ops = &shmem_vm_ops;//很重要的操作将这块虚拟内存的vm_ops设置为shmem_vm_ops
if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
((vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK) <
(vma->vm_end & HPAGE_PMD_MASK)) {
khugepaged_enter(vma, vma->vm_flags);
}
return 0;
}
static const struct vm_operations_struct shmem_vm_ops = {
.fault = shmem_fault,//Linux的共享内存实现的基础
.map_pages = filemap_map_pages,
#ifdef CONFIG_NUMA
.set_policy = shmem_set_policy,
.get_policy = shmem_get_policy,
#endif
};
到这里共享内存的初始化就结束了。
AshMemory 读写
//frameworks/base/core/java/android/os/MemoryFile.java
public void writeBytes(byte[] buffer, int srcOffset, int destOffset, int count)
throws IOException {
beginAccess();
try {
mMapping.position(destOffset);
mMapping.put(buffer, srcOffset, count);
} finally {
endAccess();
}
}
private void beginAccess() throws IOException {
checkActive();
if (mAllowPurging) {
if (native_pin(mSharedMemory.getFileDescriptor(), true)) {
throw new IOException("MemoryFile has been purged");
}
}
}
private void endAccess() throws IOException {
if (mAllowPurging) {
native_pin(mSharedMemory.getFileDescriptor(), false);
}
}
其中beginAccess和endAccess是对应的。调用的都是native_pin是一个native函数，一个参数是true一个是false。pin的作用就是锁住这块内存不被系统回收，当不使用的时候就解锁。
static jboolean android_os_MemoryFile_pin(JNIEnv* env, jobject clazz, jobject fileDescriptor,
jboolean pin) {
int fd = jniGetFDFromFileDescriptor(env, fileDescriptor);
int result = (pin ? ashmem_pin_region(fd, 0, 0) : ashmem_unpin_region(fd, 0, 0));
if (result < 0) {
jniThrowException(env, "java/io/IOException", NULL);
}
return result == ASHMEM_WAS_PURGED;
}
调用的ashmem_pin_region和ashmem_unpin_region来实现解锁和解锁。实现还是在ashmem-dev.cpp
//system/core/libcutils/ashmem-dev.cpp
int ashmem_pin_region(int fd, size_t offset, size_t len)
{
.......
ashmem_pin pin = { static_cast(offset), static_cast(len) };
return __ashmem_check_failure(fd, TEMP_FAILURE_RETRY(ioctl(fd, ASHMEM_PIN, &pin)));
}
通过的也是ioclt通知的驱动。加锁的细节就不展开了。具体的写入就是利用linux的共享内存机制实现的共享。
Linux共享机制简介
共享简单的实现方式就是通过mmap同一个文件来实现。但是真实文件的读写速度实在是太慢了，所以利用tmpfs这个虚拟文件系统，创建了一个虚拟文件来读写。同时这块虚拟内存在上面也写到重写了vm_ops。当有进程操作这个虚拟内存的时候会触发缺页错误，接着会去查找Page缓存，由于是第一次所以没有缓存，读取物理内存，同时加入Page缓存，当第二个进程进来的时也触发缺页错误时就能找到Page缓存了，那么他们操作的就是同一块物理内存了。
总结
看完之后发现AshMemory是基于Linux的共享内存实现的。做了几点改造
- 首先把一整块内存变成了一个个region，这样在不用的时候可以解锁来让系统回收。
- 将Linux共享内存的整数标记共享内存，而AshMemory是用的fd，让它可以利用binder机制的fd传输。
- 读写设置都做了加锁的处理，减少了用户使用的难度。
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