(In reply to comment #3)
> I have no idea where to start.
> I know how to compile code and debug program, but i have no clue on the
> mesa's code structure, and how it works.
> I don't know what CB6 is, i'm not sure about page meaning (is it similar to
> "classic" RAM page?) etc..
> can you tell me at least witch bunch of file/operation i have to debug, and
> how?

CB6 is the 6th color buffer and the GPU has a VM page table just like the CPU. 
That info is not really important for you, I mentioned it for other developers.
 If you could bisect mesa using git, that would be great.  There are a lot of
howtos for using git to bisect.  E.g.,
https://wiki.ubuntu.com/X/BisectingMesa

In your case, it would be something like:
git bisect start
git bisect bad mesa-9.1.2
git bisect good mesa-9.1.1