malloc does not allocate physical ram until do read-modify

Why does linux not allocating physical RAM if I try to make pages dirty with only wirte, but does allocates if I do read and write?

Got some code for linux 5.4 and aarch64. Compiler is gcc 6.3.1

int main(int argc, char *argv[])
{
    long size;
    long size_bytes;
    char *ptr;

    size = strtol(argv[1], NULL, 0);
    size_bytes = size * 1024 * 1024;
    ptr = malloc(size_bytes);
    if (!ptr)
        goto exit;

    // Make memory dirty so it is allocated in real RAM
    for (int i = 0; i < size_bytes; i += sysconf(_SC_PAGESIZE))
        ptr[i] += 1; // works ok
        //ptr[i] = 1; // does not allocates physical RAM

    while(1)
        sleep(1);

exit:
    free(ptr);
    return 0;
}

ptr[i] = 1;

You do not use ptr so the compiler is optimizing it out. There is not write operation.

Change the char *ptr to volatile char *ptr and the compiler will not optimize it out.