madvise_dontneed.cc

#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>

typedef unsigned long long uint64_t;

int presence(int pagemap_fd, uint64_t address) {
  uint64_t index = (address / getpagesize()) * 8;
  uint64_t offset = lseek64(pagemap_fd, index, SEEK_SET);
  uint64_t pagemap_value;
  int result = read(pagemap_fd, &pagemap_value, 8);

  return !!(pagemap_value & (((uint64_t)1) << 63));
}

void dump_pagemap_presence(char* pagemap_filename, void* address, size_t size) {
  uint64_t address_uint64 = reinterpret_cast<uint64_t>(address);
  int pagemap_fd = open(pagemap_filename, O_RDONLY);
  uint64_t total = 0;
  for (uint64_t offset = 0; offset < size; offset += 4096) {
    if (presence(pagemap_fd, address_uint64 + offset))
      total += 4096;
  }
  close(pagemap_fd);
  printf("\nPresence in pagemap: %llu\n", total);
}

void dump_smaps(char* smaps_filename) {
  FILE *fp = fopen(smaps_filename, "r");
  char buf[1024];
  while (1) {
    char *ret;
    ret = fgets(buf, sizeof(buf), fp);
    if (ret == NULL) { break; }
    fputs(buf, stdout);
  }
  fclose(fp);
}

int main() {
  char pagemap_filename[1024];
  char smaps_filename[1024];

  snprintf(pagemap_filename, sizeof(pagemap_filename), "/proc/%d/pagemap",
           static_cast<int>(getpid()));
  snprintf(smaps_filename, sizeof(smaps_filename), "/proc/%d/smaps",
           static_cast<int>(getpid()));

  char tmp[1024];
  void* addr;

  printf("<< 1. Initialized >>\n");
  dump_smaps(smaps_filename);

  addr = mmap(0, 1024 * 1024 * 100, PROT_READ|PROT_WRITE,
              MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
  printf("\n\n<< 2. After just mmap() >>\n");
  dump_smaps(smaps_filename);
  dump_pagemap_presence(pagemap_filename, addr, 1024 * 1024 * 100);

  memset(addr, 0, 1024 * 1024 * 99);
  printf("\n\n<< 3. After memset() >>\n");
  dump_smaps(smaps_filename);
  dump_pagemap_presence(pagemap_filename, addr, 1024 * 1024 * 100);

  madvise(addr, 1024 * 1024 * 50, MADV_DONTNEED);
  printf("\n\n<< 4. After madvise(MADV_DONTNEED) >>\n");
  dump_smaps(smaps_filename);
  dump_pagemap_presence(pagemap_filename, addr, 1024 * 1024 * 100);

  return 0;
}