Large pages for SHM

Question

Hello,
can you help me with this: I want to create a SHM region that uses 4mb large pages. If possible, I want the OS to choose the virtual address to place the SHM segment. However, I've read that it has to be aligned on 4mb boundaries.
The code to create the SHM segment looks like this:
id = shmget(13235, 16*1024*1024, IPC_CREAT | 0666);
shm = shmat(id, (void *)0, 0);How would I advise the OS to use 4mb pages for this segment?
Thanks,
Nick.

Answer 1

Hi,
in the meantime I tried to code something. It turned out that in my little test program the address returned by shmat is aligned to 4mb boundaries by accident... Therefore I just tried to make the pages 4mb large, but it didn't work: memcntl returned with errno == EINVAL, and I don't know why.
This is my code:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/mman.h>
#define MEGABYTE ((size_t)(1024 * 1024))
#define FOUR_MEGABYTE ((size_t)4 * MEGABYTE)
int main() {
  int id;
  char *shm;
  char c;
  int i;
  struct memcntl_mha mha;
  fprintf(stderr,"memcntl error codes\n");
  fprintf(stderr,"EINVAL : %d\n",EINVAL);
  mha.mha_cmd = MHA_MAPSIZE_VA;
  mha.mha_flags = 0;
  mha.mha_pagesize = FOUR_MEGABYTE;
  if ((id = shmget(13235, 16*1024*1024, IPC_CREAT | 0666)) == -1) {
    fprintf(stderr, "shmget: shmget failed\n");
    return(1);
  } else {
    fprintf(stderr, "shmget successful: shmget returned %d\n", id);
  if((shm = shmat(id, (void *)0, 0)) == (char *)-1) {
    fprintf(stderr, "shmat: shmat failed\n");
    return(1);
  } else {
    fprintf(stderr, "shmat successful: %u\n",shm);
  if ((((uintptr_t)(shm)) & ((uintptr_t)(FOUR_MEGABYTE) - 1)) == 0) {
    fprintf(stderr, "address %u is aligned\n",shm);
  } else {
    fprintf(stderr, "address %u is NOT aligned\n",shm);
  if (memcntl(shm, 16*1024*1024, MC_HAT_ADVISE, (char *)&mha, 0, 0) < 0) {
    fprintf(stderr, "memcntl failed: %d\n",errno);
  } else {
    fprintf(stderr, "memcntl succeeded\n");
  for (i=0; i<16*1024; i++) shm[i*1024] = 'a';
  printf("Hit a key.\n");
  scanf("%c", &c);
  if (shmdt(shm) == -1) {
    fprintf(stderr, "shmdt failed\n");
  } else {
    fprintf(stderr, "shmdt successful\n");
  if (shmctl(id, IPC_RMID, 0) < 0) {
    fprintf(stderr, "shmctl failed\n");
  } else {
    fprintf(stderr, "shmctl successful\n");
  return(0);
}This program produces the following output:
memcntl error codes
EINVAL : 22
shmget successful: shmget returned 2780004
shmat successful: 4261412864
address 4261412864 is aligned
memcntl failed: 22
Hit a key.
shmdt successful
shmctl successfulWith "pmap -sx <pid>" I see that the SHM segment is 16 mb large and consists only of 8kb pages.
Nick.