test.c

#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/user.h>
#include <unistd.h>

#define MAP_SIZE 20LL*1024*1024*1024*1024 /* 20TB */
#define BUF_PAGE_COUNT 1024*1024

int main(void) {
  void* p = mmap(0, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE, -1, 0);
  int fd = open("/proc/self/pagemap", O_RDONLY);

  uint64_t* buf = (uint64_t*)malloc(BUF_PAGE_COUNT*sizeof(uint64_t));
  uint64_t pages_present = 0;
  for (uintptr_t offset = 0; offset < MAP_SIZE; offset += PAGE_SIZE*BUF_PAGE_COUNT) {
    pread(fd, buf, BUF_PAGE_COUNT*sizeof(uint64_t), 8*(((uintptr_t)p + offset)/PAGE_SIZE));
    for (size_t i = 0; i < BUF_PAGE_COUNT; ++i) {
      if (buf[i] & (1LL << 63)) {
        ++pages_present;
      }
    }
  }
  printf("Pages present: %lld\n", (long long)pages_present);
  return 0;
}