Linux-0.11内核源码分析系列：内存管理copy_page_tables()函数分析

/*  *Author  : DavidLin  *Date    : 2014-11-22pm  *Email   : [e-mail] or [e-mail]  *world   : the city of SZ, in China  *Ver     : 000.000.001  *history :     editor      time            do  *          1)LinPeng       2014-11-22      created this file!  *          2)  */ /* *  Well, here is one of the most complicated functions in mm. It * copies a range of linerar addresses by copying only the pages. * Let's hope this is bug-free, 'cause this one I don't want to debug :-) * * Note! We don't copy just any chunks of memory - addresses have to * be divisible by 4Mb (one page-directory entry), as this makes the * function easier. It's used only by fork anyway. * * NOTE 2!! When from==0 we are copying kernel space for the first * fork(). Then we DONT want to copy a full page-directory entry, as * that would lead to some serious memory waste - we just copy the * first 160 pages - 640kB. Even that is more than we need, but it * doesn't take any more memory - we don't copy-on-write in the low * 1 Mb-range, so the pages can be shared with the kernel. Thus the * special case for nr=xxxx. *//* Linus认为下面copy_page_tables()函数是内存管理部分最难的之一 * copy_page_tables()函数只被fork函数调用 * 拷贝只是拷贝了一个页表，页表是管理4M地址的，所以按照4M对齐 * 不拷贝物理页内容，当发生写时拷贝才会拷贝页表所管理的物理页内容 * 对于进程0和1，只拷贝前160页共640Kb，出于效率考虑 * 0-1M作为内核驻留地址区域，禁止写覆盖 * 参数from，to是0-4G线性地址，size是字节为单位 */int copy_page_tables(unsigned long from,unsigned long to,long size){unsigned long * from_page_table;        //用于管理源页表    unsigned long * to_page_table;          //用于管理目的页表unsigned long this_page;                //用于保存页表unsigned long * from_dir, * to_dir;     //用于管理源页目录项，目的页目录项unsigned long nr;                       //用于保存页表项个数if ((from&0x3fffff) || (to&0x3fffff))    //4M对齐检测，否则diepanic("copy_page_tables called with wrong alignment");from_dir = (unsigned long *) ((from>>20) & 0xffc); /* _pg_dir = 0 */                                                          //源页目录项to_dir = (unsigned long *) ((to>>20) & 0xffc);    //目的页目录项size = ((unsigned) (size+0x3fffff)) >> 22;        //页表项个数是字节数除以4Mfor( ; size-->0 ; from_dir++,to_dir++) {                                if (1 & *to_dir)    //如果目的页目录项已经被使用，diepanic("copy_page_tables: already exist");if (!(1 & *from_dir))    continue;    //如果源页目录项未使用，跳过，不拷贝from_page_table = (unsigned long *) (0xfffff000 & *from_dir);//取源页表        if (!(to_page_table = (unsigned long *) get_free_page()))                return -1;  /* Out of memory, see freeing */  //取空闲物理页为to_page_table赋值                                                                                              //如果没有空闲物理页，die        *to_dir = ((unsigned long) to_page_table) | 7;    //将页表存进相应页目录项，                                                          //7表示可读写                                                          //想一下常用的chmod 777 anyfile        nr = (from==0)?0xA0:1024;    //如果是0地址，只拷贝160页，否则拷贝1024页                                     //一个页目录表管理1024个页目录项                                     //一个页表管理1024个页表项                                     //一个页表项管理有4K物理地址                                                                                       for ( ; nr-- > 0 ; from_page_table++,to_page_table++) {            this_page = *from_page_table;    //从源页表中取源页表项            if (!(1 & this_page))            //如果源页表项未被使用，跳过                continue;            this_page &= ~2;                 //目的页表项读写位,                                             //设置为只读                                           *to_page_table = this_page;      //将源页表项存进目的页表项            if (this_page > LOW_MEM) {       //如果是主内存区                *from_page_table = this_page;//源页表项也要设置为只读                this_page -= LOW_MEM;        //取相对主内存的偏移地址                this_page >>= 12;            //取主内存管理数组索引                mem_map[this_page]++;        //物理页引用次数加1            }        }    }    invalidate();    //刷新高速缓存    return 0;        //返回0表示成功}