kernel - VM rework part 2 - Replace backing_object with backing_ba
* Remove the vm_object based backing_object chains and all related
chaining code.
This removes an enormous number of locks from the VM system and
also removes object-to-object dependencies which requires careful
traversal code. A great deal of complex code has been removed
and replaced with far simpler code.
Ultimately the intention will be to support removal of pv_entry
tracking from vm_pages to gain lockless shared faults, but that
is far in the future. It will require hanging vm_map_backing
structures off of a list based in the object.
* Implement the vm_map_backing structure which is embedded in the
vm_map_entry and then links to additional dynamically allocated
vm_map_backing structures via entry->ba.backing_ba. This structure
contains the object and offset and essentially takes over the
functionality that object->backing_object used to have.
backing objects are now handled via vm_map_backing. In this
commit, fork operations create a fan-in tree to shared subsets
of backings via vm_map_backing. In this particular commit,
these subsets are not collapsed in any way.
* Remove all the vm_map_split and collapse code. Every last line
is gone. It will be reimplemented using vm_map_backing in a
later commit.
This means that as-of this commit both recursive forks and
parent-to-multiple-children forks cause an accumulation of
inefficient lists of backing objects to occur in the parent
and children. This will begin to get addressed in part 3.
* The code no longer releases the vm_map lock (typically shared)
across (get_pages) I/O. There are no longer any chaining locks to
get in the way (hopefully). This means that the code does not
have to re-check as carefully as it did before. However, some
complexity will have to be added back in once we begin to address
the accumulation of vm_map_backing structures.
* Paging performance improved by 30-40%
UnifiedSplitRaw
