dump-pagemap.c

/*
  dump-pagemap.c - Dump pagemap entries associated with mappings of a given process

  Copyright 2021 Hilko Bengen <[email protected]>
  License: GPL3 or later

  Based on information from:
  - linux/Documentation/filesystems/proc.txt
  - linux/Documentation/admin-guide/mm/pagemap.rst
 */

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <inttypes.h>

int main(int argc, char** argv) {
  if (argc != 2) {
    fprintf(stderr, "usage: %s <pid>\n  pid = numeric PID or \"self\"\n", argv[0]);
    exit(1);
  }
  char filebuf[256];
  snprintf(filebuf, sizeof(filebuf), "/proc/%s/maps", argv[1]);
  FILE* maps = fopen(filebuf, "r");
  if (maps == NULL) {
    fprintf(stderr, "open: %s: %s\n", filebuf, strerror(errno));
    exit(1);
  }
  snprintf(filebuf, sizeof(filebuf), "/proc/%s/pagemap", argv[1]);
  int pagemap = open(filebuf, O_RDONLY);
  if (pagemap == -1) {
    fprintf(stderr, "open: %s: %s\n", filebuf, strerror(errno));
    exit(1);
  }

  uint64_t pagesize = sysconf(_SC_PAGE_SIZE);

  char line[1024];
  while (fgets(line, sizeof(line), maps)) {
    uint64_t begin, end;
    sscanf(line, "%" SCNx64 "-%" SCNx64, &begin, &end);
    fputs(line, stdout);
    printf("map: %"PRIx64"-%"PRIx64"  (%"PRIx64")\n", begin, end, end-begin);
    for (uint64_t pos = begin; pos < end; pos = pos + pagesize) {
      uint64_t entry;
      int sz = pread(pagemap, &entry, sizeof(entry), (pos / pagesize) * sizeof(entry));
      if (sz != 8) {
	fprintf(stderr, "pread\n");
	exit(1);
      }
      // bit 61: present
      // bit 62: file-backed
      // bit 63: shared
      fputc(".fsx:FSX"[entry >> 61], stdout);
    }
    fputc('\n', stdout);
  }
  fclose(maps);
  close(pagemap);
}