observe physical address content

Question

I compile and run the pagemap_dump.c from here :

/proc/[pid]/pagemaps and /proc/[pid]/maps | linux

and another application attach a exist shared memory and every time I run pagemap_dump , the shared memory's physical address is really the same (virtual memory address is different each time I observe) :

./pagemap_dump.exe 2135  <== 2135 is pid  
./pagemap_dump.exe 5864  <== 5864 is pid 
./pagemap_dump.exe 5110  <== 5110 is pid  

the output :

7fabb37e1000 54edde 0 1 0 1 /dev/shm/memtest.shared
7f0040fa1000 54edde 0 1 0 1 /dev/shm/memtest.shared
7ffbd2869000 54edde 0 1 0 1 /dev/shm/memtest.shared

these 3 application attach memtest.shared and their physical address are all 0x54edde in my linux , then I try to access the contents from /dev/mem in this physical address :

How to access physical addresses from user space in Linux?

int main(int argc, char** argv)
{
    if (argc < 3) {
        printf("Usage: %s <phys_addr> <offset>\n", argv[0]);
        return 0;
    }

    off_t offset = strtoul(argv[1], NULL, 0);
    size_t len = strtoul(argv[2], NULL, 0);

    // Truncate offset to a multiple of the page size, or mmap will fail.
    size_t pagesize = sysconf(_SC_PAGE_SIZE);
    off_t page_base = (offset / pagesize) * pagesize;
    off_t page_offset = offset - page_base;

    int fd = open("/dev/mem", O_RDONLY|O_SYNC);
    unsigned char *mem = (unsigned char *) mmap(NULL, page_offset + len, PROT_READ , MAP_SHARED, fd, page_base);
    if (mem == MAP_FAILED) {
        perror("Can't map memory");
        return -1;
    }

    size_t i;
    for (i = 0; i < len; ++i)
        printf("%02x ", (int)mem[page_offset + i]);

    return 0;
} // main

chown root testz1.exe

chmod 4775 testz1.exe

./testz1.exe  5565918  16    <=== 0x54edde = 5565918

I got Can't map memory: Operation not permitted

./testz1.exe 54edde 16

I got some thing not like "hello world" what I strcpy to /dev/shm/memtest.shared when I create it .

Since I seems to get the physical address of /dev/shm/memtest.shared , how can I get the right way to get the contents from /dev/mem of this physical address ?!

Edit :

After download devmem2.c and run :

./devmem2.exe 0x54edde w   

get the following message :

/dev/mem opened. Error at line 75, file devmem2.c (1) [Operation not permitted]

Look like 0x54edde is not a physical address as I expect , but Why it happened?! pagemap_dump.c really show the physical address of /dev/shm/memtest.shared for all processes which attach it the value 0x54edde , then Why devmem2.c can not access it right ?!