c - 如何创建物理和虚拟页面？

Question

我正在尝试使用一些示例创建一个物理和虚拟页面。我试图了解物理页面地址是如何确定的。我在这里有几个问题。

1. 我们如何知道页面信息存储在哪里？我猜如果我们将虚拟地址除以PAGE_SIZE我们将得到页码。即 val = ((size_t)*virtAddr)/PAGE_SIZE; ，但我不明白为什么我们要乘以 8？

2. 当我这样做cat /proc/self/pagemap时，它就挂在那里。似乎它正在阻止呼叫。当我的应用程序获取文件描述符时，它如何能够从中脱颖而出？

3. 为什么要乘以PAGE_SIZE * pageInfo得到物理地址？这种关系是怎么产生的？背后的场景是什么？

这是我的代码。

#include <sys/mman.h> //for mmap and mlock 
#include <unistd.h> //for NULL, lseek
#include <stdlib.h> //for exit and for valloc
#include <stdio.h> //for printf
#include <fcntl.h> //for file opening
#include <stdint.h> //for uint32_t
#include <string.h> //for memset

#define PAGE_SIZE 4096 //mmap maps pages of memory, so we must give it multiples of this size

void makeVirtPhysPage(void **virtAddr, void **physAddr)
{
 *virtAddr = valloc(PAGE_SIZE); // allocate one page of RAM
 
  //force page into RAM
 ((int *)*virtAddr)[0] = 1; 
  //lock it there.
  mlock(*virtAddr, PAGE_SIZE);
  memset(*virtAddr, 0, PAGE_SIZE); // zero-fill the page for convenience 

  uint64_t pageInfo;
  size_t val;
  int file = open("/proc/self/pagemap", 'r');
  printf("The virtAddr is %lu\r\n", *virtAddr);
  printf("The Page size is %lu\r\n", PAGE_SIZE);
  val = ((size_t)*virtAddr)/PAGE_SIZE; // probably getting page number.
  val = val * 8;
  printf("The val is %lu\r\n", val);
  lseek(file, val, SEEK_SET);
  read(file, &pageInfo,8);
  printf("The pageInfo says %p\r\n", pageInfo);
  *physAddr = (void*)(size_t)(pageInfo*PAGE_SIZE); // why multiply by page size?
  printf("makeVirtualPhysPage virtual to phys: %p -> %p\r\n", *virtAddr, *physAddr);
}

void freeVirtPhysPage(void *virtAddr)
{
  munlock(virtAddr, PAGE_SIZE);
  free(virtAddr);
}

int main()
{
  void *virtSrcPage, *PhysSrcPage;
  makeVirtPhysPage(&virtSrcPage, &PhysSrcPage);
  char *srcPage = (char *)virtSrcPage;
  srcPage[0] = 'S';
  srcPage[1] = 'U';
  srcPage[1] = 'P';
  srcPage[1] = 'E';
  srcPage[1] = 'R';

  freeVirtPhysPage(virtSrcPage);
  printf("The value of virtSrcPage[0] is %c\r\n", srcPage[0]);

  return 0;
}

标签： cmemory-managementlinux-kernel