linux0.11内存管理——try_to_share()

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00292 static int try_to_share(unsigned long address, struct task_struct * p)
00293 {
00294     unsigned long from;
00295     unsigned long to;
00296     unsigned long from_page;
00297     unsigned long to_page;
00298     unsigned long phys_addr;
00299
00300     from_page = to_page = ((address>>20) & 0xffc);
00301     from_page += ((p->start_code>>20) & 0xffc);                   //找到ｐ进程address所对应的页目录项
00302     to_page += ((current->start_code>>20) & 0xffc);　　//找到当前进程address所对应的页目录项　
00303 /* is there a page-directory at from? */
00304     from = *(unsigned long *) from_page;
00305     if (!(from & 1))
00306         return 0;
00307     from &= 0xfffff000;
00308     from_page = from + ((address>>10) & 0xffc);
00309     phys_addr = *(unsigned long *) from_page;
00310 /* is the page clean and present? */
00311     if ((phys_addr & 0x41) != 0x01)
00312         return 0;
00313     phys_addr &= 0xfffff000;
00314     if (phys_addr >= HIGH_MEMORY || phys_addr < LOW_MEM)
00315         return 0;
00316     to = *(unsigned long *) to_page;
00317     if (!(to & 1)) {
00318         if ((to = get_free_page()))
00319             *(unsigned long *) to_page = to | 7;
00320         else
00321             oom();
00322     }
00323     to &= 0xfffff000;
00324     to_page = to + ((address>>10) & 0xffc);
00325     if (1 & *(unsigned long *) to_page)
00326         panic("try_to_share: to_page already exists");
00327 /* share them: write-protect */
00328     *(unsigned long *) from_page &= ~2;
00329     *(unsigned long *) to_page = *(unsigned long *) from_page;
00330     invalidate();
00331     phys_addr -= LOW_MEM;
00332     phys_addr >>= 12;
00333     mem_map[phys_addr]++;
00334     return 1;
00335 }