Question on mmap(2) with kernel alocated memory

[Terry Barnaby <terry@beam.demon.co.uk>]

I am trying to mmap() into user space a kernel buffer  and am having
problems.
I have a simple test example, can someone please tell me what I have got
wrong ?

In a driver I do:
    uint*    kva;

    kva = (uint*)kmalloc(4096, GFP_KERNEL);
    *kva = 0x11223344;
    printk("Address: %p %lx %x\n", kva, virt_to_phys(kva), *kva);

Now in some simple user program I do:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <fcntl.h>

int main(int argc, char** argv){
 int fm;
 char* p;
 uint* pi;
 uint v;
 uint add = 0x74b000;

 if((fm = open("/dev/mem", O_RDWR)) < 0)
  return 1;

 p = mmap(0, 128 * 1024 * 1024, PROT_READ|PROT_WRITE, MAP_SHARED, fm,
0);
 printf("Mapped: %p\n", p);

 lseek(fm, add, SEEK_SET);
 read(fm, &v, sizeof(v));
 printf("V: %x\n", v);

 pi = (uint*)(p + add);
 printf("Vmmap: %p %x\n", pi, *pi);

 close(fm);
 return 0;
}

The value of add is hardcoded to the value printed for the physical
address in the drivers prink routine.
The lseek/read from the /dev/mem device yields the value 0x11223344.
However the mmap method also on /dev/mem yields the value 0.

Whats wrong with my mmap() or kalloc() ?

Terry

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  Dr Terry Barnaby                     BEAM Ltd
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