Re: mmap and MAP_FIXED

[Johnny Billquist <bqt%update.uu.se@localhost>]

On 2016-05-12 17:09, Daniel Jour wrote:
> Regarding the page / map size: At first I tried this with larger sizes
> (around 98 kb), but no difference.
>
>
> Johnny wrote:
> > What is the pagesize of the machine?
>
> pagesize yields 4096 on the system in question, so it shouldn't be of
> relevance, I guess.

OK. Then yeah, that was not a relevant point here.

> Michael wrote:
> > But the start address in the test program is aligned to something much
> > larger, so the first test should succeed if that was the problem. I
> > wouldn't bet on the address range used being available though, but
> > that's just a hunch.
>
> Why shouldn't the range be available, though? /proc/PID/maps does not
> show any mapping in this range. Is there some kernel related mapping
> that doesn't show up in /proc/PID/maps?

I think that is hardware dependent. The OS might be somewhere in the 
address space as well, just not accessible to user mode, for example. I 
would not expect that to show up in the map. There might also be 
limitations to what addresses actually can be mapped by the hardware.

Which are additional reasons why it's probably better to let the OS give 
you an address instead you you trying to pick one. Depending on the 
hardware, there might actually be different ranges of addresses that you 
will have issues with.

	Johnny