How to access mmaped /dev/mem without crashing the Linux kernel?

I have a simple program that tries to access the physical memory in user space, where the kernel stores the 1st struct page. On a 64 bit machine this address is:

• kernel virtual address: ffffea0000000000
• physical address: 0000620000000000

I am trying to access this physical address through mmap in user space. But the following code crashes the kernel.

int *addr;
if ((fd = open("/dev/mem", O_RDWR|O_SYNC)) < 0 ) {
    printf("Error opening file. \n");
    close(fd);
    return (-1);
}
/* mmap.  address of first struct page for 64 bit architectures 
 * is 0x0000620000000000.
 */
addr = (int *)mmap(0, num*STRUCT_PAGE_SIZE, PROT_READ, MAP_PRIVATE,
            fd, 0x0000620000000000);
printf("addr: %p \n",addr);
printf("addr: %d \n",*addr); /* CRASH. */