MMAP，则msync和Linux进程终止 [英] mmap, msync and linux process termination

问题描述

我想使用mmap来实现的，通过使用与MAP_SHARED标志设置mmap（）的一个固定大小的结构与一个众所周知的文件名相关联的Linux下运行的C程序程序状态的某些部分的持久性。出于性能的考虑，我想preFER没有在所有调用则msync（），并没有其他程序要访问此文件。当我的程序终止并重新启动时，它会再次映射相同的文件，做它的一些处理，以恢复，这是在终止前的状态。我的问题是：如果我从来没有对文件描述符调用则msync（），将内核保证到内存中的所有更新将被写入磁盘，并随后即使我的进程终止SIGKILL可恢复？另外，会有来自内核定期编写页面盘一般系统开销，即使我的程序永远不会调用则msync（）？

I want to use mmap to implement persistence of certain portions of program state in a C program running under Linux by associating a fixed-size struct with a well known file name using mmap() with the MAP_SHARED flag set. For performance reasons, I would prefer not to call msync() at all, and no other programs will be accessing this file. When my program terminates and is restarted, it will map the same file again and do some processing on it to recover the state that it was in before the termination. My question is this: if I never call msync() on the file descriptor, will the kernel guarantee that all updates to the memory will get written to disk and be subsequently recoverable even if my process is terminated with SIGKILL? Also, will there be general system overhead from the kernel periodically writing the pages to disk even if my program never calls msync()?

编辑：我已经入驻的数据是否被写入这个问题，但我仍然不能确定这是否会造成一些意想不到的系统负载过试图用开放来处理这个问题（ ）/写（）/ FSYNC（）和服用，如果这个过程得到由KILL / SEGV / ABRT的/ etc击出了一些可能造成数据丢失的风险。在希望增加了一个Linux内核的标签，一些懂行的人可能会附和

I've settled the problem of whether the data is written, but I'm still not sure about whether this will cause some unexpected system loading over trying to handle this problem with open()/write()/fsync() and taking the risk that some data might be lost if the process gets hit by KILL/SEGV/ABRT/etc. Added a 'linux-kernel' tag in hopes that some knowledgeable person might chime in.

推荐答案

我决定不那么懒惰，明确地写了一些code回答的数据是否被写入到磁盘的问题。答案是，它会被写入。

I decided to be less lazy and answer the question of whether the data is written to disk definitively by writing some code. The answer is that it will be written.

下面是一些数据写入一个文件mmap'd后突然杀死自己的程序：

Here is a program that kills itself abruptly after writing some data to an mmap'd file:

#include <stdint.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>

typedef struct {
  char data[100];
  uint16_t count;
} state_data;

const char *test_data = "test";

int main(int argc, const char *argv[]) {
  int fd = open("test.mm", O_RDWR|O_CREAT|O_TRUNC, (mode_t)0700);
  if (fd < 0) {
    perror("Unable to open file 'test.mm'");
    exit(1);
  }
  size_t data_length = sizeof(state_data);
  if (ftruncate(fd, data_length) < 0) {
    perror("Unable to truncate file 'test.mm'");
    exit(1);
  }
  state_data *data = (state_data *)mmap(NULL, data_length, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE, fd, 0);
  if (MAP_FAILED == data) {
    perror("Unable to mmap file 'test.mm'");
    close(fd);
    exit(1);
  }
  memset(data, 0, data_length);
  for (data->count = 0; data->count < 5; ++data->count) {
    data->data[data->count] = test_data[data->count];
  }
  kill(getpid(), 9);
}

下面是验证previous程序后生成的文件的程序是死的：

Here is a program that validates the resulting file after the previous program is dead:

#include <stdint.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <assert.h>

typedef struct {
  char data[100];
  uint16_t count;
} state_data;

const char *test_data = "test";

int main(int argc, const char *argv[]) {
  int fd = open("test.mm", O_RDONLY);
  if (fd < 0) {
    perror("Unable to open file 'test.mm'");
    exit(1);
  }
  size_t data_length = sizeof(state_data);
  state_data *data = (state_data *)mmap(NULL, data_length, PROT_READ, MAP_SHARED|MAP_POPULATE, fd, 0);
  if (MAP_FAILED == data) {
    perror("Unable to mmap file 'test.mm'");
    close(fd);
    exit(1);
  }
  assert(5 == data->count);
  unsigned index;
  for (index = 0; index < 4; ++index) {
    assert(test_data[index] == data->data[index]);
  }
  printf("Validated\n");
}

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