A VM_GenCollectFull VMOp is always used by GCH::collect_locked, even when the max_generation is YoungGen. This leads to some confusion further down the call chain. The VMOp calls GCH::do_full_collection, which calls GCH::do_collection with a true first (full) argument, even though the max_generation argument is YoungGen.

That mismatch between the full argument and the max_generation argument leads to some confusing code and results in do_collection. (See discussion in JDK-8048556, for example. Some of the confusion was fixed there, but some remains.) Some name changes, or having two VM operations for collect_lock to use, seems called for, with the goal of having do_collection called with consistent arguments.