Linux-0.11核心源碼分析系列：記憶體管理copy_page_tables()函數分析，

Linux-0.11核心源碼分析系列：記憶體管理copy_page_tables()函數分析，

/*  *Author  : DavidLin  *Date    : 2014-11-22pm  *Email   : linpeng1577@163.com or linpeng1577@gmail.com  *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認為下面copy_page_tables()函數是記憶體管理部分最難的之一 * copy_page_tables()函數只被fork函數調用 * 拷貝只是拷貝了一個頁表，頁表是管理4M地址的，所以按照4M對齊 * 不拷貝物理頁內容，當發生寫時拷貝才會拷貝頁表所管理的物理頁內容 * 對於進程0和1，只拷貝前160頁共640Kb，出於效率考慮 * 0-1M作為核心駐留地址地區，禁止寫覆蓋 * 參數from，to是0-4G線性地址，size是位元組為單位 */int copy_page_tables(unsigned long from,unsigned long to,long size){unsigned long * from_page_table;        //用於管理源頁表    unsigned long * to_page_table;          //用於管理目的頁表unsigned long this_page;                //用於儲存頁表unsigned long * from_dir, * to_dir;     //用於管理源頁目錄項，目的頁目錄項unsigned long nr;                       //用於儲存頁表項個數if ((from&0x3fffff) || (to&0x3fffff))    //4M對齊檢測，否則diepanic("copy_page_tables called with wrong alignment");from_dir = (unsigned long *) ((from>>20) & 0xffc); /* _pg_dir = 0 */                                                          //源頁目錄項to_dir = (unsigned long *) ((to>>20) & 0xffc);    //目的頁目錄項size = ((unsigned) (size+0x3fffff)) >> 22;        //頁表項個數是位元組數除以4Mfor( ; size-->0 ; from_dir++,to_dir++) {                                if (1 & *to_dir)    //如果目的頁目錄項已經被使用，diepanic("copy_page_tables: already exist");if (!(1 & *from_dir))    continue;    //如果源頁目錄項未使用，跳過，不拷貝from_page_table = (unsigned long *) (0xfffff000 & *from_dir);//取源頁表        if (!(to_page_table = (unsigned long *) get_free_page()))                return -1;  /* Out of memory, see freeing */  //取空閑物理頁為to_page_table賦值                                                                                              //如果沒有空閑物理頁，die        *to_dir = ((unsigned long) to_page_table) | 7;    //將頁表存進相應頁目錄項，                                                          //7表示可讀寫                                                          //想一下常用的chmod 777 anyfile        nr = (from==0)?0xA0:1024;    //如果是0地址，只拷貝160頁，否則拷貝1024頁                                     //一個頁目錄表管理1024個頁目錄項                                     //一個頁表管理1024個頁表項                                     //一個頁表項管理有4K物理地址                                                                                       for ( ; nr-- > 0 ; from_page_table++,to_page_table++) {            this_page = *from_page_table;    //從源頁表中取源頁表項            if (!(1 & this_page))            //如果源頁表項未被使用，跳過                continue;            this_page &= ~2;                 //目的頁表項讀寫位,                                             //設定為唯讀                                           *to_page_table = this_page;      //將源頁表項存進目的頁表項            if (this_page > LOW_MEM) {       //如果是主記憶體區                *from_page_table = this_page;//源頁表項也要設定為唯讀                this_page -= LOW_MEM;        //取相對主記憶體的位移地址                this_page >>= 12;            //取主記憶體管理數組索引                mem_map[this_page]++;        //物理頁引用次數加1            }        }    }    invalidate();    //重新整理快取    return 0;        //返回0表示成功}