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577 lines
21 KiB
Markdown
577 lines
21 KiB
Markdown
<audio id="audio" title="41 | IPC（中）：不同项目组之间抢资源，如何协调？" controls="" preload="none"><source id="mp3" src="https://static001.geekbang.org/resource/audio/5e/d6/5e5caec562b544ec17f47848e115cdd6.mp3"></audio>
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了解了如何使用共享内存和信号量集合之后，今天我们来解析一下，内核里面都做了什么。
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不知道你有没有注意到，咱们讲消息队列、共享内存、信号量的机制的时候，我们其实能够从中看到一些统一的规律：**它们在使用之前都要生成key，然后通过key得到唯一的id，并且都是通过xxxget函数。**
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在内核里面，这三种进程间通信机制是使用统一的机制管理起来的，都叫ipcxxx。
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为了维护这三种进程间通信进制，在内核里面，我们声明了一个有三项的数组。
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我们通过这段代码，来具体看一看。
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```
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struct ipc_namespace {
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......
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struct ipc_ids ids[3];
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......
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}
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#define IPC_SEM_IDS 0
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#define IPC_MSG_IDS 1
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#define IPC_SHM_IDS 2
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#define sem_ids(ns) ((ns)->ids[IPC_SEM_IDS])
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#define msg_ids(ns) ((ns)->ids[IPC_MSG_IDS])
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#define shm_ids(ns) ((ns)->ids[IPC_SHM_IDS])
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```
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根据代码中的定义，第0项用于信号量，第1项用于消息队列，第2项用于共享内存，分别可以通过sem_ids、msg_ids、shm_ids来访问。
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这段代码里面有ns，全称叫namespace。可能不容易理解，你现在可以将它认为是将一台Linux服务器逻辑的隔离为多台Linux服务器的机制，它背后的原理是一个相当大的话题，我们需要在容器那一章详细讲述。现在，你就可以简单的认为没有namespace，整个Linux在一个namespace下面，那这些ids也是整个Linux只有一份。
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接下来，我们再来看struct ipc_ids里面保存了什么。
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首先，in_use表示当前有多少个ipc；其次，seq和next_id用于一起生成ipc唯一的id，因为信号量，共享内存，消息队列，它们三个的id也不能重复；ipcs_idr是一棵基数树，我们又碰到它了，一旦涉及从一个整数查找一个对象，它都是最好的选择。
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```
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struct ipc_ids {
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int in_use;
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unsigned short seq;
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struct rw_semaphore rwsem;
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struct idr ipcs_idr;
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int next_id;
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};
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struct idr {
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struct radix_tree_root idr_rt;
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unsigned int idr_next;
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};
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```
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也就是说，对于sem_ids、msg_ids、shm_ids各有一棵基数树。那这棵树里面究竟存放了什么，能够统一管理这三类ipc对象呢？
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通过下面这个函数ipc_obtain_object_idr，我们可以看出端倪。这个函数根据id，在基数树里面找出来的是struct kern_ipc_perm。
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```
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struct kern_ipc_perm *ipc_obtain_object_idr(struct ipc_ids *ids, int id)
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{
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struct kern_ipc_perm *out;
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int lid = ipcid_to_idx(id);
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out = idr_find(&ids->ipcs_idr, lid);
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return out;
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}
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```
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如果我们看用于表示信号量、消息队列、共享内存的结构，就会发现，这三个结构的第一项都是struct kern_ipc_perm。
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```
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struct sem_array {
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struct kern_ipc_perm sem_perm; /* permissions .. see ipc.h */
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time_t sem_ctime; /* create/last semctl() time */
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struct list_head pending_alter; /* pending operations */
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/* that alter the array */
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struct list_head pending_const; /* pending complex operations */
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/* that do not alter semvals */
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struct list_head list_id; /* undo requests on this array */
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int sem_nsems; /* no. of semaphores in array */
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int complex_count; /* pending complex operations */
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unsigned int use_global_lock;/* >0: global lock required */
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struct sem sems[];
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} __randomize_layout;
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struct msg_queue {
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struct kern_ipc_perm q_perm;
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time_t q_stime; /* last msgsnd time */
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time_t q_rtime; /* last msgrcv time */
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time_t q_ctime; /* last change time */
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unsigned long q_cbytes; /* current number of bytes on queue */
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unsigned long q_qnum; /* number of messages in queue */
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unsigned long q_qbytes; /* max number of bytes on queue */
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pid_t q_lspid; /* pid of last msgsnd */
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pid_t q_lrpid; /* last receive pid */
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struct list_head q_messages;
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struct list_head q_receivers;
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struct list_head q_senders;
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} __randomize_layout;
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struct shmid_kernel /* private to the kernel */
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{
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struct kern_ipc_perm shm_perm;
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struct file *shm_file;
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unsigned long shm_nattch;
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unsigned long shm_segsz;
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time_t shm_atim;
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time_t shm_dtim;
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time_t shm_ctim;
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pid_t shm_cprid;
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pid_t shm_lprid;
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struct user_struct *mlock_user;
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/* The task created the shm object. NULL if the task is dead. */
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struct task_struct *shm_creator;
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struct list_head shm_clist; /* list by creator */
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} __randomize_layout;
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```
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也就是说，我们完全可以通过struct kern_ipc_perm的指针，通过进行强制类型转换后，得到整个结构。做这件事情的函数如下：
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```
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static inline struct sem_array *sem_obtain_object(struct ipc_namespace *ns, int id)
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{
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struct kern_ipc_perm *ipcp = ipc_obtain_object_idr(&sem_ids(ns), id);
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return container_of(ipcp, struct sem_array, sem_perm);
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}
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static inline struct msg_queue *msq_obtain_object(struct ipc_namespace *ns, int id)
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{
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struct kern_ipc_perm *ipcp = ipc_obtain_object_idr(&msg_ids(ns), id);
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return container_of(ipcp, struct msg_queue, q_perm);
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}
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static inline struct shmid_kernel *shm_obtain_object(struct ipc_namespace *ns, int id)
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{
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struct kern_ipc_perm *ipcp = ipc_obtain_object_idr(&shm_ids(ns), id);
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return container_of(ipcp, struct shmid_kernel, shm_perm);
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}
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```
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通过这种机制，我们就可以将信号量、消息队列、共享内存抽象为ipc类型进行统一处理。你有没有觉得，这有点儿面向对象编程中抽象类和实现类的意思？没错，如果你试图去了解C++中类的实现机制，其实也是这么干的。
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<img src="https://static001.geekbang.org/resource/image/08/af/082b742753d862cfeae520fb02aa41af.png" alt="">
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有了抽象类，接下来我们来看共享内存和信号量的具体实现。
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## 如何创建共享内存？
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首先，我们来看创建共享内存的的系统调用。
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```
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SYSCALL_DEFINE3(shmget, key_t, key, size_t, size, int, shmflg)
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{
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struct ipc_namespace *ns;
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static const struct ipc_ops shm_ops = {
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.getnew = newseg,
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.associate = shm_security,
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.more_checks = shm_more_checks,
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};
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struct ipc_params shm_params;
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ns = current->nsproxy->ipc_ns;
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shm_params.key = key;
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shm_params.flg = shmflg;
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shm_params.u.size = size;
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return ipcget(ns, &shm_ids(ns), &shm_ops, &shm_params);
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}
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```
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这里面调用了抽象的ipcget、参数分别为共享内存对应的shm_ids、对应的操作shm_ops以及对应的参数shm_params。
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如果key设置为IPC_PRIVATE则永远创建新的，如果不是的话，就会调用ipcget_public。ipcget的具体代码如下：
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```
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int ipcget(struct ipc_namespace *ns, struct ipc_ids *ids,
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const struct ipc_ops *ops, struct ipc_params *params)
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{
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if (params->key == IPC_PRIVATE)
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return ipcget_new(ns, ids, ops, params);
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else
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return ipcget_public(ns, ids, ops, params);
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}
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static int ipcget_public(struct ipc_namespace *ns, struct ipc_ids *ids, const struct ipc_ops *ops, struct ipc_params *params)
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{
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struct kern_ipc_perm *ipcp;
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int flg = params->flg;
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int err;
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ipcp = ipc_findkey(ids, params->key);
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if (ipcp == NULL) {
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if (!(flg & IPC_CREAT))
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err = -ENOENT;
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else
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err = ops->getnew(ns, params);
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} else {
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if (flg & IPC_CREAT && flg & IPC_EXCL)
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err = -EEXIST;
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else {
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err = 0;
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if (ops->more_checks)
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err = ops->more_checks(ipcp, params);
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......
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}
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}
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return err;
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}
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```
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在ipcget_public中，我们会按照key，去查找struct kern_ipc_perm。如果没有找到，那就看是否设置了IPC_CREAT；如果设置了，就创建一个新的。如果找到了，就将对应的id返回。
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我们这里重点看，如何按照参数shm_ops，创建新的共享内存，会调用newseg。
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```
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static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
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{
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key_t key = params->key;
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int shmflg = params->flg;
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size_t size = params->u.size;
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int error;
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struct shmid_kernel *shp;
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size_t numpages = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
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struct file *file;
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char name[13];
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vm_flags_t acctflag = 0;
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......
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shp = kvmalloc(sizeof(*shp), GFP_KERNEL);
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......
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shp->shm_perm.key = key;
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shp->shm_perm.mode = (shmflg & S_IRWXUGO);
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shp->mlock_user = NULL;
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shp->shm_perm.security = NULL;
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......
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file = shmem_kernel_file_setup(name, size, acctflag);
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......
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shp->shm_cprid = task_tgid_vnr(current);
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shp->shm_lprid = 0;
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shp->shm_atim = shp->shm_dtim = 0;
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shp->shm_ctim = get_seconds();
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shp->shm_segsz = size;
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shp->shm_nattch = 0;
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shp->shm_file = file;
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shp->shm_creator = current;
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error = ipc_addid(&shm_ids(ns), &shp->shm_perm, ns->shm_ctlmni);
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......
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list_add(&shp->shm_clist, &current->sysvshm.shm_clist);
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......
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file_inode(file)->i_ino = shp->shm_perm.id;
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ns->shm_tot += numpages;
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error = shp->shm_perm.id;
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......
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return error;
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}
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```
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**newseg函数的第一步，通过kvmalloc在直接映射区分配一个struct shmid_kernel结构。**这个结构就是用来描述共享内存的。这个结构最开始就是上面说的struct kern_ipc_perm结构。接下来就是填充这个struct shmid_kernel结构，例如key、权限等。
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**newseg函数的第二步，共享内存需要和文件进行关联**。**为什么要做这个呢？我们在讲内存映射的时候讲过，虚拟地址空间可以和物理内存关联，但是物理内存是某个进程独享的。虚拟地址空间也可以映射到一个文件，文件是可以跨进程共享的。
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咱们这里的共享内存需要跨进程共享，也应该借鉴文件映射的思路。只不过不应该映射一个硬盘上的文件，而是映射到一个内存文件系统上的文件。mm/shmem.c里面就定义了这样一个基于内存的文件系统。这里你一定要注意区分shmem和shm的区别，前者是一个文件系统，后者是进程通信机制。
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在系统初始化的时候，shmem_init注册了shmem文件系统shmem_fs_type，并且挂在到了shm_mnt下面。
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```
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int __init shmem_init(void)
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{
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int error;
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error = shmem_init_inodecache();
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error = register_filesystem(&shmem_fs_type);
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shm_mnt = kern_mount(&shmem_fs_type);
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......
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return 0;
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}
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static struct file_system_type shmem_fs_type = {
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.owner = THIS_MODULE,
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.name = "tmpfs",
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.mount = shmem_mount,
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.kill_sb = kill_litter_super,
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.fs_flags = FS_USERNS_MOUNT,
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};
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```
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接下来，newseg函数会调用shmem_kernel_file_setup，其实就是在shmem文件系统里面创建一个文件。
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```
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/**
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* shmem_kernel_file_setup - get an unlinked file living in tmpfs which must be kernel internal.
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* @name: name for dentry (to be seen in /proc/<pid>/maps
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* @size: size to be set for the file
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* @flags: VM_NORESERVE suppresses pre-accounting of the entire object size */
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struct file *shmem_kernel_file_setup(const char *name, loff_t size, unsigned long flags)
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{
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return __shmem_file_setup(name, size, flags, S_PRIVATE);
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}
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static struct file *__shmem_file_setup(const char *name, loff_t size,
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unsigned long flags, unsigned int i_flags)
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{
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struct file *res;
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struct inode *inode;
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struct path path;
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struct super_block *sb;
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struct qstr this;
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......
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this.name = name;
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this.len = strlen(name);
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this.hash = 0; /* will go */
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sb = shm_mnt->mnt_sb;
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path.mnt = mntget(shm_mnt);
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path.dentry = d_alloc_pseudo(sb, &this);
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d_set_d_op(path.dentry, &anon_ops);
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......
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inode = shmem_get_inode(sb, NULL, S_IFREG | S_IRWXUGO, 0, flags);
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inode->i_flags |= i_flags;
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d_instantiate(path.dentry, inode);
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inode->i_size = size;
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......
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res = alloc_file(&path, FMODE_WRITE | FMODE_READ,
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&shmem_file_operations);
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return res;
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}
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```
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__shmem_file_setup会创建新的shmem文件对应的dentry和inode，并将它们两个关联起来，然后分配一个struct file结构，来表示新的shmem文件，并且指向独特的shmem_file_operations。
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```
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static const struct file_operations shmem_file_operations = {
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.mmap = shmem_mmap,
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.get_unmapped_area = shmem_get_unmapped_area,
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#ifdef CONFIG_TMPFS
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.llseek = shmem_file_llseek,
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.read_iter = shmem_file_read_iter,
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.write_iter = generic_file_write_iter,
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.fsync = noop_fsync,
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.splice_read = generic_file_splice_read,
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.splice_write = iter_file_splice_write,
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.fallocate = shmem_fallocate,
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#endif
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};
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```
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**newseg函数的第三步，通过ipc_addid将新创建的struct shmid_kernel结构挂到shm_ids里面的基数树上，并返回相应的id，并且将struct shmid_kernel挂到当前进程的sysvshm队列中。**
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至此，共享内存的创建就完成了。
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## 如何将共享内存映射到虚拟地址空间？
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从上面的代码解析中，我们知道，共享内存的数据结构struct shmid_kernel，是通过它的成员struct file *shm_file，来管理内存文件系统shmem上的内存文件的。无论这个共享内存是否被映射，shm_file都是存在的。
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接下来，我们要将共享内存映射到虚拟地址空间中。调用的是shmat，对应的系统调用如下：
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```
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SYSCALL_DEFINE3(shmat, int, shmid, char __user *, shmaddr, int, shmflg)
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{
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unsigned long ret;
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long err;
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err = do_shmat(shmid, shmaddr, shmflg, &ret, SHMLBA);
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force_successful_syscall_return();
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return (long)ret;
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}
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long do_shmat(int shmid, char __user *shmaddr, int shmflg,
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ulong *raddr, unsigned long shmlba)
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{
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struct shmid_kernel *shp;
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unsigned long addr = (unsigned long)shmaddr;
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unsigned long size;
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struct file *file;
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int err;
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unsigned long flags = MAP_SHARED;
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unsigned long prot;
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int acc_mode;
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struct ipc_namespace *ns;
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struct shm_file_data *sfd;
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struct path path;
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fmode_t f_mode;
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unsigned long populate = 0;
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......
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prot = PROT_READ | PROT_WRITE;
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acc_mode = S_IRUGO | S_IWUGO;
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f_mode = FMODE_READ | FMODE_WRITE;
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......
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ns = current->nsproxy->ipc_ns;
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shp = shm_obtain_object_check(ns, shmid);
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......
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path = shp->shm_file->f_path;
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path_get(&path);
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shp->shm_nattch++;
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size = i_size_read(d_inode(path.dentry));
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......
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sfd = kzalloc(sizeof(*sfd), GFP_KERNEL);
|
||
......
|
||
file = alloc_file(&path, f_mode,
|
||
is_file_hugepages(shp->shm_file) ?
|
||
&shm_file_operations_huge :
|
||
&shm_file_operations);
|
||
......
|
||
file->private_data = sfd;
|
||
file->f_mapping = shp->shm_file->f_mapping;
|
||
sfd->id = shp->shm_perm.id;
|
||
sfd->ns = get_ipc_ns(ns);
|
||
sfd->file = shp->shm_file;
|
||
sfd->vm_ops = NULL;
|
||
......
|
||
addr = do_mmap_pgoff(file, addr, size, prot, flags, 0, &populate, NULL);
|
||
*raddr = addr;
|
||
err = 0;
|
||
......
|
||
return err;
|
||
}
|
||
|
||
```
|
||
|
||
在这个函数里面，shm_obtain_object_check会通过共享内存的id，在基数树中找到对应的struct shmid_kernel结构，通过它找到shmem上的内存文件。
|
||
|
||
接下来，我们要分配一个struct shm_file_data，来表示这个内存文件。将shmem中指向内存文件的shm_file赋值给struct shm_file_data中的file成员。
|
||
|
||
然后，我们创建了一个struct file，指向的也是shmem中的内存文件。
|
||
|
||
为什么要再创建一个呢？这两个的功能不同，shmem中shm_file用于管理内存文件，是一个中立的，独立于任何一个进程的角色。而新创建的struct file是专门用于做内存映射的，就像咱们在讲内存映射那一节讲过的，一个硬盘上的文件要映射到虚拟地址空间中的时候，需要在vm_area_struct里面有一个struct file *vm_file指向硬盘上的文件，现在变成内存文件了，但是这个结构还是不能少。
|
||
|
||
新创建的struct file的private_data，指向struct shm_file_data，这样内存映射那部分的数据结构，就能够通过它来访问内存文件了。
|
||
|
||
新创建的struct file的file_operations也发生了变化，变成了shm_file_operations。
|
||
|
||
```
|
||
static const struct file_operations shm_file_operations = {
|
||
.mmap = shm_mmap,
|
||
.fsync = shm_fsync,
|
||
.release = shm_release,
|
||
.get_unmapped_area = shm_get_unmapped_area,
|
||
.llseek = noop_llseek,
|
||
.fallocate = shm_fallocate,
|
||
};
|
||
|
||
```
|
||
|
||
接下来，do_mmap_pgoff函数我们遇到过，原来映射硬盘上的文件的时候，也是调用它。这里我们不再详细解析了。它会分配一个vm_area_struct指向虚拟地址空间中没有分配的区域，它的vm_file指向这个内存文件，然后它会调用shm_file_operations的mmap函数，也即shm_mmap进行映射。
|
||
|
||
```
|
||
static int shm_mmap(struct file *file, struct vm_area_struct *vma)
|
||
{
|
||
struct shm_file_data *sfd = shm_file_data(file);
|
||
int ret;
|
||
ret = __shm_open(vma);
|
||
ret = call_mmap(sfd->file, vma);
|
||
sfd->vm_ops = vma->vm_ops;
|
||
vma->vm_ops = &shm_vm_ops;
|
||
return 0;
|
||
}
|
||
|
||
```
|
||
|
||
shm_mmap中调用了shm_file_data中的file的mmap函数，这次调用的是shmem_file_operations的mmap，也即shmem_mmap。
|
||
|
||
```
|
||
static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
|
||
{
|
||
file_accessed(file);
|
||
vma->vm_ops = &shmem_vm_ops;
|
||
return 0;
|
||
}
|
||
|
||
```
|
||
|
||
这里面，vm_area_struct的vm_ops指向shmem_vm_ops。等从call_mmap中返回之后，shm_file_data的vm_ops指向了shmem_vm_ops，而vm_area_struct的vm_ops改为指向shm_vm_ops。
|
||
|
||
我们来看一下，shm_vm_ops和shmem_vm_ops的定义。
|
||
|
||
```
|
||
static const struct vm_operations_struct shm_vm_ops = {
|
||
.open = shm_open, /* callback for a new vm-area open */
|
||
.close = shm_close, /* callback for when the vm-area is released */
|
||
.fault = shm_fault,
|
||
};
|
||
|
||
static const struct vm_operations_struct shmem_vm_ops = {
|
||
.fault = shmem_fault,
|
||
.map_pages = filemap_map_pages,
|
||
};
|
||
|
||
```
|
||
|
||
它们里面最关键的就是fault函数，也即访问虚拟内存的时候，访问不到应该怎么办。
|
||
|
||
当访问不到的时候，先调用vm_area_struct的vm_ops，也即shm_vm_ops的fault函数shm_fault。然后它会转而调用shm_file_data的vm_ops，也即shmem_vm_ops的fault函数shmem_fault。
|
||
|
||
```
|
||
static int shm_fault(struct vm_fault *vmf)
|
||
{
|
||
struct file *file = vmf->vma->vm_file;
|
||
struct shm_file_data *sfd = shm_file_data(file);
|
||
return sfd->vm_ops->fault(vmf);
|
||
}
|
||
|
||
```
|
||
|
||
虽然基于内存的文件系统，已经为这个内存文件分配了inode，但是内存也却是一点儿都没分配，只有在发生缺页异常的时候才进行分配。
|
||
|
||
```
|
||
static int shmem_fault(struct vm_fault *vmf)
|
||
{
|
||
struct vm_area_struct *vma = vmf->vma;
|
||
struct inode *inode = file_inode(vma->vm_file);
|
||
gfp_t gfp = mapping_gfp_mask(inode->i_mapping);
|
||
......
|
||
error = shmem_getpage_gfp(inode, vmf->pgoff, &vmf->page, sgp,
|
||
gfp, vma, vmf, &ret);
|
||
......
|
||
}
|
||
|
||
/*
|
||
* shmem_getpage_gfp - find page in cache, or get from swap, or allocate
|
||
*
|
||
* If we allocate a new one we do not mark it dirty. That's up to the
|
||
* vm. If we swap it in we mark it dirty since we also free the swap
|
||
* entry since a page cannot live in both the swap and page cache.
|
||
*
|
||
* fault_mm and fault_type are only supplied by shmem_fault:
|
||
* otherwise they are NULL.
|
||
*/
|
||
static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
|
||
struct page **pagep, enum sgp_type sgp, gfp_t gfp,
|
||
struct vm_area_struct *vma, struct vm_fault *vmf, int *fault_type)
|
||
{
|
||
......
|
||
page = shmem_alloc_and_acct_page(gfp, info, sbinfo,
|
||
index, false);
|
||
......
|
||
}
|
||
|
||
```
|
||
|
||
shmem_fault会调用shmem_getpage_gfp在page cache和swap中找一个空闲页，如果找不到就通过shmem_alloc_and_acct_page分配一个新的页，他最终会调用内存管理系统的alloc_page_vma在物理内存中分配一个页。
|
||
|
||
至此，共享内存才真的映射到了虚拟地址空间中，进程可以像访问本地内存一样访问共享内存。
|
||
|
||
## 总结时刻
|
||
|
||
我们来总结一下共享内存的创建和映射过程。
|
||
|
||
1. 调用shmget创建共享内存。
|
||
1. 先通过ipc_findkey在基数树中查找key对应的共享内存对象shmid_kernel是否已经被创建过，如果已经被创建，就会被查询出来，例如producer创建过，在consumer中就会查询出来。
|
||
1. 如果共享内存没有被创建过，则调用shm_ops的newseg方法，创建一个共享内存对象shmid_kernel。例如，在producer中就会新建。
|
||
1. 在shmem文件系统里面创建一个文件，共享内存对象shmid_kernel指向这个文件，这个文件用struct file表示，我们姑且称它为file1。
|
||
1. 调用shmat，将共享内存映射到虚拟地址空间。
|
||
1. shm_obtain_object_check先从基数树里面找到shmid_kernel对象。
|
||
1. 创建用于内存映射到文件的file和shm_file_data，这里的struct file我们姑且称为file2。
|
||
1. 关联内存区域vm_area_struct和用于内存映射到文件的file，也即file2，调用file2的mmap函数。
|
||
1. file2的mmap函数shm_mmap，会调用file1的mmap函数shmem_mmap，设置shm_file_data和vm_area_struct的vm_ops。
|
||
1. 内存映射完毕之后，其实并没有真的分配物理内存，当访问内存的时候，会触发缺页异常do_page_fault。
|
||
1. vm_area_struct的vm_ops的shm_fault会调用shm_file_data的vm_ops的shmem_fault。
|
||
1. 在page cache中找一个空闲页，或者创建一个空闲页。
|
||
|
||
<img src="https://static001.geekbang.org/resource/image/20/51/20e8f4e69d47b7469f374bc9fbcf7251.png" alt="">
|
||
|
||
## 课堂练习
|
||
|
||
在这里，我们只分析了shm_ids的结构，消息队列的程序我们写过了，但是msg_ids的结构没有解析，你可以试着解析一下。
|
||
|
||
欢迎留言和我分享你的疑惑和见解 ，也欢迎可以收藏本节内容，反复研读。你也可以把今天的内容分享给你的朋友，和他一起学习和进步。
|
||
|
||
|