Android 匿名内存深入分析

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MemoryFileval descriptorMethod = file.javaClass.getDeclaredMethod("getFileDescriptor")val fd=descriptorMethod.invoke(file)//反射拿到fdfile.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_MEMORYval pfd = reply.readParcelable<ParcelFileDescriptor>(javaClass.classLoader)val descriptor = pfd?.fileDescriptor//拿到fdval 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stringToast.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<TextView>(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mmapboolean 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_areaif (!asma)return -ENOMEM;
INIT_LIST_HEAD(&asma->unpinned_list);//初始化unpinned_listmemcpy(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sizeret = -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NULLstatic 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<uint32_t>(offset), static_cast<uint32_t>(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。