I am trying to create a memory mapped file using MAP_SHARED. I run into issues when the file size reaches 2gb. The code pasted below is what I am using (as a test).
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <sys/mman.h>
#include <unistd.h>
#include <fcntl.h>
#define MY_FILE "/dev/shm/mmap_test"
#define BLOCK_SIZE (1024*1024)
#define NUM_FILES 1
void mk_file(int f_num)
{
uint64_t len = 0;
int fd, j, k;
char tmp_file[1024], *x, *rr_addr;
// Open file in /dev/shm
sprintf(tmp_file, "%s%d", MY_FILE, f_num);
fd = open(tmp_file, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
if (fd == -1)
{
perror("file open");
exit(-1);
}
// 16Gb file
len = 16UL * 1024 * 1024 * 1024;
printf("len: %ld Gb\n", len/(1024*1024*1024));
printf("Mapping %ld blocks\n", len/BLOCK_SIZE);
for (j = 0; j < len/BLOCK_SIZE; j++) {
// Increase the file size
ftruncate(fd, ((j + 1) * BLOCK_SIZE));
// Just mmap memory... don't have file backing
//rr_addr = mmap(NULL, BLOCK_SIZE, PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE|MAP_NORESERVE, -1, 0);
// MMAP a region to the file at a given offset
rr_addr = mmap(NULL, BLOCK_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_NORESERVE, fd, (j * BLOCK_SIZE));
if (rr_addr == MAP_FAILED) {
perror("mmap error");
printf("INDEX: %d\n", j);
exit(-1);
}
// Write to every byte of allocated memory
x = (char *) rr_addr;
for (k = 0; k < BLOCK_SIZE; k++)
{
*x = '1';
x++;
}
}
return;
}
int main(int argc, char **argv)
{
uint64_t i;
for (i = 0; i < NUM_FILES; i++)
mk_file(i);
return 0;
}
In the code above, I get a bus error when the offset in the file reaches 2gb. These are the things I have tried:
- If I change NUM_FILES to 16 and len to 1GB, I don't have any issues.
- If I remove the for loop which writes to the memory (only mmap), the program does not crash (even for len much greater than 2gb) b/c the linux kernel does not actually map pages to the file until you read/write to the mmap'ed region.
- If I change the mmap call from MAP_SHARED to MAP_ANON (uncomment the first mmap call and comment out the second one) and not linked to a file, there are no issues (even the writes succeed).
- There is enough space on /dev/shm (30gb), I am using only 16gb here.
- I don't have to write to every allocated byte. I only need to write to the last mmap'ed region (move the inner for loop outside) and if the offset + BLOCK_SIZE >= 2gb then I get a bus error.
- I have tried this on Ubuntu 13.10 and CentOS 6.4 and both have the same issue.
I am wondering if this is an issue in the linux kernel? Has anyone tried mmap'ing a single file using MAP_SHARED greater than 2gb and used (read/write to) it successfully?