Linux0.11 Kernel Source Analysis series: Memory management Copy_page_tables () function analysis

/*  *author  : davidlin  *date    : 2014-11-22pm   *Email   : [email protected] or [email protected]   *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 thought the following copy_page_tables () The function is one of the hardest parts of memory management. The  * copy_page_tables () function is only called by the fork function call  *  copy just copy a page table, the page table is to manage the 4M address, so follow the 4M alignment  *   does not copy the physical page content, when a write-time copy will copy the page table to manage the physical page content  *  for processes 0 and 1, only copy the first 160 pages total 640Kb, for efficiency consideration  * 0-1m as the kernel resident address area, Prohibit write overlay  *  parameter from,to is a 0-4g linear address, size is the byte unit  */int copy_page_tables (Unsigned long from, Unsigned long to,long size) {unsigned long * from_page_table;         //for managing source page tables &Nbsp;   unsigned long * to_page_table;           //for Management purposes page Table unsigned long this_page;                 //for saving page tables unsigned long * from_dir, *  to_dir;     //for managing source page catalog entries, destination page catalog entries unsigned long nr;                         //to save page table Entries if  ((from&0x3fffff)  | |   (TO&0X3FFFFF))     //4m alignment detection, otherwise diepanic ("Copy_page_tables called with  wrong alignment ");from_dir =  (unsigned long *)   (from>>20)   & 0XFFC); /* _pg_dir = 0 */                                                            // SOURCE Page Catalog Entry to_dir =  (unsigned long *)   ((to>>20)  & 0XFFC);     //Destination Page Catalog item size =  ((unsigned)   (SIZE+0X3FFFFF))  >> 22;   The number of       //page table entries is the number of bytes divided by 4Mfor ( ; size-->0 ; from_dir++,to_dir+ +)  {                                 if  (1  & *to_dir)     //If the destination page directory entry is already in use, Diepanic ("Copy_page_tables: already  exist ");if  (! ( 1 & *from_dir)     continue;    //If the source page catalog entry is unused, skip, do not copy From_ Page_table =  (unsigned long *)   (0XFFFFF000 & *FROM_DIR);//source page table          if  (!) ( to_page_table =  (unsigned long *)  get_free_page ())                  return -1;  /* Out  of memory, see freeing */  //take free physical pages for to_page_table assignment                                                                                                    //If there is no free physical page,die        *to_dir =  (unsigned  long)  to_page_table)  | 7;    //Save the page table in the corresponding page directory entry,                                                             // 7 = Read                   write                                            //think about the usual chmod 777 anyfile       .  nr =  (from==0) 0xa0:1024;    //If it is a 0 address, copy only 160 pages, otherwise copy 1024 pages                                        //a page Catalog Table management 1024 page Catalog Items                                        // One page Table management 1024 page table Items                                        //one page table Item management has 4 K physical Address                                                                                            for  ( ;  nr-- > 0 ; from_page_table++,to_page_table++)  {             this_page = *from_page_table;    // From the source page table, take the source page table entry             if  (! 1 & this_page))             // If the source page table entry is not being used, skip                  Continue;            this_page &= ~2 ;        &nbsp         //Purpose Page Table entry read-write bit,                                                //set to read-only                                              *to_page_table = this_page;       //to save the source page table entry in the destination page table entry              if  (THIS_PAGE > LOW_MEM)  {       //if it is the main memory area                  *from_page_table  = this_page;//source Page TableItems are also set to read-only                 this_ page -= low_mem;        //offset to relative main memory address                  this_page >>= 12             //Master Memory Management array index                  mem_map[this_page]++;         //physical page reference count plus 1             }        }    }     invalidate ();     //Refresh Cache     return 0;         //return 0 = success}

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Linux0.11 Kernel Source Analysis series: Memory management Copy_page_tables () function analysis