[Zion] Free paging structures after process exit.

zion/object/address_space.cpp

@@ -18,6 +18,8 @@ AddressSpace::AddressSpace() {
   InitializePml4(cr3_);
 }
 
+AddressSpace::~AddressSpace() { CleanupPml4(cr3_); }
+
 glcr::ErrorOr<uint64_t> AddressSpace::AllocateUserStack() {
   uint64_t base = user_stacks_.NewUserStack();
   auto mem_object = glcr::StaticCastRefPtr<MemoryObject>(

zion/object/address_space.h

@@ -62,6 +62,10 @@ class AddressSpace : public KernelObject {
   AddressSpace(const AddressSpace&) = delete;
   AddressSpace(AddressSpace&&) = delete;
 
+  // Deconstructing an address space will free all paging structures associated
+  // with this address space.
+  ~AddressSpace();
+
   uint64_t cr3() { return cr3_; }
 
   // User Mappings.

zion/object/process.cpp

@@ -105,6 +105,8 @@ void Process::Cleanup() {
   PANIC_ON_ERR(vmas_->FreeAddressRange(0, kUserSpaceMax),
                "Failed to cleanup userspace mappings in process exit.");
 
-  // 4. Release paging structures. TODO
+  // 4. Release paging structures.
+  vmas_ = nullptr;
+
   state_ = FINISHED;
 }