Why there are no major page fault on a program that maps a file to memory?

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I am trying to demonstrate paging in Linux. I wrote a small program that maps a file to virtual memory with mmap(), waits for input, access the allocated memory, and waits for input again.

I check the process minor and major faults after mapping the file to memory and after accessing this memory.

I’ve expected to see a major fault, but I only see a minor fault. The number of major faults remains 0 until the program ends. This is very strange, because the kernel must perform an I/O in order to bring the content of the file to memory.

Can anyone explain this?

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

int main (int argc, char *argv[]) {

  char *mblk;
  int fd, fsize, n;
  struct stat fs;

  fd = open ( argv[1], O_RDONLY );
  if ( fd == -1 ) {
    printf ( "Failed to open %s\n", argv[1] );
    return (-1);
  }

  fstat ( fd, &fs );
  fsize = fs.st_size;
  mblk = mmap ( NULL, fsize, PROT_READ, MAP_PRIVATE, fd, 0 );
  close ( fd );
  printf ( "\n %s is maped to virtual memory at %llx\n", argv[1], (unsigned long long) mblk );
  printf ( " Press Enter to continue\n");
  getchar ();
  n = (unsigned) *mblk;
  printf ( "%d\n", n);
  printf ( " %llx accessed\n", (unsigned long long) mblk );
  printf ( " Press any Enter exit\n");
  getchar ();
}
linux-kernel memory-management
2 Answers

You are seeing minor pf, because data is already in page cache.

Try the following (console1):

 $ ./main ./myfile 

 ./myfile is maped to virtual memory at 7f23f3cd6000
 Press Enter to continue

Drop caches after that (console 2)

$ echo 3 > /proc/sys/vm/drop_caches

Press enter && see stat (console 1)

 98
 7f23f3cd6000 accessed
 Press any Enter exit

 $ ps -eo min_flt,maj_flt,cmd | grep main
 77      1 ./main ./myfile
user_11497970

Thanks, but how do the page get to the page cache? I've tried running the program immediately after reboot. If the open in the program loaded the page to memory, I should see at least 1 major fault before accessing the page.

user_122623490
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