Add ExternalPageResource and allow discontiguous VM space (#864)

This PR changes `VMSpace` to multiple discontiguous regions as VM space.
This PR is used in mmtk/mmtk-julia#71 to support
system and package images.
* add an `ExternalPageResource` to manage the discontiguous regions.
* implement `VMSpace` with `ExternalPageResource`. `VMSpace` no longer
uses `ImmortalSpace`.
* rename `lazy_init_vm_space` to `set_vm_space`, as we allow setting vm
space multiple times with non overlapping regions.

---------

Co-authored-by: Kunal Sareen <[email protected]>

src/memory_manager.rs

@@ -86,14 +86,13 @@ pub fn mmtk_init<VM: VMBinding>(builder: &MMTKBuilder) -> Box<MMTK<VM>> {
     Box::new(mmtk)
 }
 
+/// Add an externally mmapped region to the VM space. A VM space can be set through MMTk options (`vm_space_start` and `vm_space_size`),
+/// and can also be set through this function call. A VM space can be discontiguous. This function can be called multiple times,
+/// and all the address ranges passed as arguments in the function will be considered as part of the VM space.
+/// Currently we do not allow removing regions from VM space.
 #[cfg(feature = "vm_space")]
-pub fn lazy_init_vm_space<VM: VMBinding>(mmtk: &'static mut MMTK<VM>, start: Address, size: usize) {
-    unsafe {
-        mmtk.get_plan_mut()
-            .base_mut()
-            .vm_space
-            .lazy_initialize(start, size);
-    }
+pub fn set_vm_space<VM: VMBinding>(mmtk: &'static mut MMTK<VM>, start: Address, size: usize) {
+    unsafe { mmtk.get_plan_mut() }.base_mut().vm_space.set_vm_region(start, size);
 }
 
 /// Request MMTk to create a mutator for the given thread. The ownership

src/plan/global.rs

@@ -529,7 +529,11 @@ impl<VM: VMBinding> BasePlan<VM> {
                 VMRequest::discontiguous(),
             )),
             #[cfg(feature = "vm_space")]
-            vm_space: VMSpace::new(&mut args),
+            vm_space: VMSpace::new(args.get_space_args(
+                "vm_space",
+                false,
+                VMRequest::discontiguous(),
+            )),
 
             initialized: AtomicBool::new(false),
             trigger_gc_when_heap_is_full: AtomicBool::new(true),

src/plan/mod.rs

@@ -26,8 +26,6 @@ pub use global::AllocationSemantics;
 pub(crate) use global::GcStatus;
 pub use global::Plan;
 pub(crate) use global::PlanTraceObject;
-#[cfg(feature = "vm_space")] // This is used for creating VM space
-pub(crate) use global::{CreateGeneralPlanArgs, CreateSpecificPlanArgs};
 
 mod mutator_context;
 pub use mutator_context::Mutator;