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@ -1,6 +1,335 @@
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# mmap_munmap_clone
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## mmap实现
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参考了1.0源码，定义了新的结构体vmarea_struct，在task_struct添加了对应的变量mmap，并进行了初始化
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在memmory.c中实现了这个函数
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由于测试文件只传了两个参数，剩余的自己进行了定义
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### mmap函数实验
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#### 函数的参数介绍：
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~~~
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void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset);
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long mmap()
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prot 页面的权限标志
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flags 页面私有/共享的标志
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off :文件的偏移量，从文件的哪一部分开始映射到页面中
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~~~
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#### vmarea_struct 结构
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因为涉及虚拟地址的管理，所以定义了如下结构：
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~~~c
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struct vmarea_struct{
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unsigned long vm_start;//虚拟地址开始的地址
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unsigned long vm_end;//虚拟地址结束的地址
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int mode;//页面权限
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int flag;//页面是否共享
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int size;//大小 需要与PAGE_SIZE对齐
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off_t off;//映射文件的偏移量
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struct vmarea_struct *next;//下一个vmarea_struct,链表结构比较简便,会牺牲一些速度
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//struct file *dir;//要映射文件的结构
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int fd;//映射文件的文件描述符
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};
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~~~
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在task_struct加入该结构
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~~~c
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struct task_struct {
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..............
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struct vmarea_struct *mmap;//映射虚拟地址的链表
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};
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~~~
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在INIT_TASK需要进行进行初始化
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~~~c
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#define INIT_TASK \
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......
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/*mmap*/ NULL,\
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}
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~~~
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mmap是将文件的一部分内容映射到进程地址空间之中，原理如下图：
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step1:在当前进程的虚拟地址空间中，寻找一段空闲的连续的地址空间(寻址方式)
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step2:寻找并将空闲页映射到对应的虚拟地址空间之中(get_free_page和put_page），并根据flags判断是否将该页面映射到其他进程对应的虚拟内存中(参考了try_to_page函数的实现)
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step3:将文件内容写到空闲页之中(参考了sys_read和find_entry的实现)
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mmap内存映射的实现过程，总的来说可以分为三个阶段：
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***\*(一)进程启动映射过程，并在虚拟地址空间中为映射创建虚拟映射区域\****
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1. 进程在用户空间调用库函数mmap，原型：void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset);
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2. 在当前进程的虚拟地址空间中，寻找一段空闲的满足要求的连续的虚拟地址。
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3. 为此虚拟区分配一个vm_area_struct结构，接着对这个结构的各个域进行了初始化。
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4. 将新建的虚拟区结构（vm_area_struct）插入进程的虚拟地址区域链表。这里采用的链表是最简单的单向链表，会牺牲一些速度，但胜在简便
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注意：寻址方式
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为简便期间并防止进程调用malloc导致的堆生长与 映射的虚拟地址产生冲突，所以从0x2000000开始映射一直到0x4000000
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分配及初始化的代码如下：
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~~~c
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if(start==NULL){
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//需要由程序分配合适的地址空间
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//由于是借助链表实现 所以需要搜索来找到合适的地址进行使用
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if(current->mmap==NULL) buf = 0x2000000;//如果初始化为NULL
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if(!buf){
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struct vmarea_struct *m2 = current->mmap;
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struct vmarea_struct *m3 = current->mmap->next;
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for(m3;m3->next;m3 = m3->next){
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if(m3->vm_start-m2->vm_end>=size){
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buf = m2->vm_end;
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break;
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}
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m2 = m3;
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}
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if(!buf)
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buf = m3->vm_end;
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}
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}
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//初始化
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struct vmarea_struct *m1 = (struct vmarea_struct *)malloc(sizeof(struct vmarea_struct));
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m1->mode = prot;
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m1->flag = flags;
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m1->next = NULL;
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m1->size = size;
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m1->vm_start = buf;
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m1->vm_end = (unsigned long)buf+size;
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m1->fd = fd;
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m1->off = off;
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if(current->mmap==NULL) current->mmap = m1;
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else{//插到最后一个位置
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struct vmarea_struct *m2=current->mmap;
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for(m2;m2->next;m2=m2->next)
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;
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m2->next = m1;
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}
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~~~
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***\*(二)实现文件物理地址和进程虚拟地址的一一映射关系\****
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5. 用get_free_page获得空闲页
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6. 修改页目录项和页表项的内容使其能够指向空闲页，这片虚拟地址并没有任何数据关联到主存中
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7.
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~~~c
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//接下来进行映射等操作
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//参考了try_to_share的实现
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unsigned long from;
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unsigned long to;
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unsigned long from_page;
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unsigned long to_page;
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unsigned long phys_addr;
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unsigned long page;
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unsigned long tmp;
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unsigned long vm_address=m1->vm_start;
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while(vm_address<m1->vm_end){
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from_page = to_page = ((vm_address>>20)&0xffc);//逻辑的页目录项偏移
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from_page +=((current->start_code>>20)&0xffc);//该进程目录项地址
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//from处是否存在页目录项 否则进行申请
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from = *(unsigned long *) from_page;
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from &=0xfffff000;
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from_page = from +((vm_address>>10)&0xffc);//页表项指针
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phys_addr = *(unsigned long *) from_page;//页表项内容
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if(!(page = get_free_page())){
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printk("no free page\n");
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return -1;
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}
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/* phys_addr = page |0x7;
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mem_map[(page-LOW_MEM)>>12]++;*/
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/*if(!(*(unsigned long *) from_page & 0x1)){
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if(page = get_free_page())
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phys_addr = page |0x7,mem_map[(page-LOW_MEM)>>12]++;
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else
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oom();
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}*/
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put_page(page,vm_address);
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//对页面分配权限
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*(unsigned long *)from_page &= (prot&PROT_WRITE)?0xffffffff:0xfffffffd;
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//对flags进行处理,重点处理的是MAP_SHARE,私有的话不需要进行多余的处理
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//需要将所有进程共享同一页面
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if(flags&MAP_SHARED){
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struct task_struct ** task;
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for (task = &LAST_TASK ; task>&FIRST_TASK;--task){
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if (!*task)
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continue;
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else if (current == *task)
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continue;
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else if(!try_to_share(vm_address,*task))
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printk("current pid %d cann't share memory\n",(*task)->pid);
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}
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}
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vm_address += PAGE_SIZE;
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}
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~~~
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***\*(三)进程实现文件内容到物理内存的拷贝\****
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这里采用的实现是在函数内就将文件内容拷贝到物理页面上
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为映射分配了新的虚拟地址区域后，通过待映射的文件指针，在文件描述符表中找到对应的文件描述符，通过文件描述符，链接到内核“已打开文件集”中该文件的文件结构体（struct file），每个文件结构体维护着和这个已打开文件相关各项信息。
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9.参考find_entry和sys_read函数，读取文件内容，并写到内存空间中
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~~~c
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if (!(block = dir->i_zone[0]))
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return NULL;
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if (!(bh = bread(dir->i_dev,block)))
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return NULL;
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char *s = (char *) bh->b_data;
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char *p1=(char *)s;
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char *p2=(char *)buf;
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while(len--)
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put_fs_byte(*(p1++),p2++);
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~~~
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### munmap实验
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munmap函数的功能是解除内存映射
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#### 实现原理及步骤
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***\*(一)在当前进程的链表中能够寻找到对应的vmarea_struct,并将之从链表中删除\****
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1.从当前进程链表开头寻找，找到为止
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***\*(二)根据寻找到的vmarea_struct结构判断页面的权限，如果是有写权限的，需要将页面内容写到对应的文件之中\****
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将buf内容写到fd之中的函数，参照了sys_write的实现
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~~~c
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int mmap_wirte(int fd,off_t pos,char *buf,int count){
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struct file * file;
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struct m_inode * inode;
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if (fd>=NR_OPEN || count <0 || !(file=current->filp[fd]))
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return -EINVAL;
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if (!count)
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return 0;
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inode=file->f_inode;
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file->f_pos = pos;
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if (inode->i_pipe)
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return (file->f_mode&2)?write_pipe(inode,buf,count):-EIO;
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if (S_ISCHR(inode->i_mode))
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return rw_char(WRITE,inode->i_zone[0],buf,count,&file->f_pos);
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if (S_ISBLK(inode->i_mode))
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return block_write(inode->i_zone[0],&file->f_pos,buf,count);
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if (S_ISREG(inode->i_mode))
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return file_write(inode,file,buf,count);
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printk("(Write)inode->i_mode=%06o\n\r",inode->i_mode);
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return -EINVAL;
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}
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~~~
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***\*(三)解除虚拟地址对物理页面的映射，需要判断该页面是否是共享的，如果是共享的需要进行改动\****
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与mmap的映射相似，只是对页表项的最后一位进行了改动
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~~~c
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//取消进程p的共享 与try_share很类似 只是少了几步
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static int cancle_share(unsigned long address, struct task_struct * p)
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{
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unsigned long from;
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unsigned long from_page;
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unsigned long phys_addr;
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from_page = ((address>>20) & 0xffc);
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from_page += ((p->start_code>>20) & 0xffc);
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/* is there a page-directory at from? */
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from = *(unsigned long *) from_page;
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from &= 0xfffff000;
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from_page = from + ((address>>10) & 0xffc);
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phys_addr = *(unsigned long *) from_page;
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/* is the page clean and present? */
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phys_addr &= 0xfffff000;
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if (phys_addr >= HIGH_MEMORY || phys_addr < LOW_MEM)
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return 0;
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/* share them: write-protect */
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*(unsigned long *) from_page &= ~1;
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phys_addr -= LOW_MEM;
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phys_addr >>= 12;
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mem_map[phys_addr]--;
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return 1;
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}
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~~~
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### 函数验证
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#### mmap
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测试文件的函数
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测试的思路是，打开一个文件并将该文件映射到内存中，输出对应位置的字符串
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~~~c
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void test_mmap(void){
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char *array;
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const char *str = " Hello, mmap success.";
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int fd;
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/*打开文件并将文件内容写入到test_mmap.txt中*/
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fd = open("test_mmap.txt", O_RDWR | O_CREAT, S_IRUSR|S_IWUSR);
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write(fd, str, strlen(str));
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fstat(fd, &kst);
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printf("file len: %d\n", (int)kst.st_size);
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//将test_mmap.txt文件映射到内存中，虚拟地址是array
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array = mmap(NULL, kst.st_size, PROT_WRITE | PROT_READ, MAP_FILE | MAP_SHARED, fd, 0);
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printf("mmap addr: %x\n", (unsigned int)array);
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if (array == MAP_FAILED) {
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printf("mmap error.\n");
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}else{
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printf("mmap content: %s\n", array);
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munmap(array, kst.st_size);
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}
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close(fd);
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}
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~~~
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测试结果如下：可以看到与str对应的字符串相同
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<img src="C:\Users\admin\AppData\Local\Temp\WeChat Files\1e4436d61190e2b3451b91b14af78bd.png" alt="1e4436d61190e2b3451b91b14af78bd" style="zoom:150%;" />
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#### munmap
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#### clone
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### 问题与建议
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1.初始化问题
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2.
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