Comment 25 for bug 1189850

There was a related bug, fixed with

commit 19bd005056a2083de64753681b96716996e4237d
Author: Chris Wilson <email address hidden>
Date: Fri Feb 22 12:05:04 2013 +0000

    sna: Avoid migrating and making the GPU bo busy prior to mmapping it

    References: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/1131134
    Signed-off-by: Chris Wilson <email address hidden>

that I thought was already in 2.21.3 and so you had tested it. It is actually in master, so can you try compiling from git and checking if that fixes the issue?