I tried nouveau on a 630GT GF108 card (a fermi card) and linux 4.5.
Every time I hibernate/resume, around 100MB of memory was disappearing.
Doing systemctl isolate rescue.target was not recovering it.
I was measuring the memory using 'free' command.
At one point after a few hibernate/resume cycles, the memory leaked was around 500MB.
I am back to the proprietary driver now. This memory leak (or whatever it is) doesn't happen there. I go back to 90MB at rescue.target there even after 10 hibernate cycles.
The compositor used is gnome-shell under Xorg.