Well.

I have said that I used drm-next-3.18 and had these hangs.
When I applied
http://lists.freedesktop.org/archives/mesa-dev/2014-August/066746.html it did
not help.

Now I am using 3.17-rc7 with that mesa patch and I do not see these hangs
anymore. Or maybe they are these very short stutters.

Sorry if drm-next-3.18 behavior is not relevant here.

As for the num bytes moved: Does the HUD graph only accumulate everything that
happened in the hang? If so, then the hundreds of megabytes still seem more
than normal and the used graphs definitely show change before and after the
hangs. Whatever you make of that...

CONFIG_CMA is not enabled on either kernel.

Indeed, there's less moving of data with the rc kernel I think.
For comparison: https://www.youtube.com/watch?v=mFaqHGle9Hg