Linux graphics drivers aggressively pre-fault device memory. The current interface prevents that from working - at least in terms of expecations implied by existing asserts in the VM code. This change to the device pager's fault interface let's it know how many pages have been passed and let's it communicate how much read ahead it has done.
It turns out that the driver prefault path isn't invoked at X startup. But during glxgears my instrumentation indicates that this change is in fact adding the *rahead mappings.