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SubjectQuestion on mmap(2) with kernel alocated memory
I am trying to mmap() into user space a kernel buffer  and am having
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,
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);

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() ?


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