diff --git a/mmtk/Cargo.lock b/mmtk/Cargo.lock
index 1fde435..ef65098 100644
--- a/mmtk/Cargo.lock
+++ b/mmtk/Cargo.lock
@@ -393,8 +393,8 @@ dependencies = [
 
 [[package]]
 name = "mmtk"
-version = "0.19.0"
-source = "git+https://github.com/udesou/mmtk-core.git?rev=a26101f4266b70a8249b59089523e12512a068df#a26101f4266b70a8249b59089523e12512a068df"
+version = "0.20.0"
+source = "git+https://github.com/mmtk/mmtk-core.git?rev=57af17fbfd94ff0df2cd3b1e504abe299ce4f0ab#57af17fbfd94ff0df2cd3b1e504abe299ce4f0ab"
 dependencies = [
  "atomic",
  "atomic-traits",
@@ -427,8 +427,8 @@ dependencies = [
 
 [[package]]
 name = "mmtk-macros"
-version = "0.19.0"
-source = "git+https://github.com/udesou/mmtk-core.git?rev=a26101f4266b70a8249b59089523e12512a068df#a26101f4266b70a8249b59089523e12512a068df"
+version = "0.20.0"
+source = "git+https://github.com/mmtk/mmtk-core.git?rev=57af17fbfd94ff0df2cd3b1e504abe299ce4f0ab#57af17fbfd94ff0df2cd3b1e504abe299ce4f0ab"
 dependencies = [
  "proc-macro-error",
  "proc-macro2",
diff --git a/mmtk/Cargo.toml b/mmtk/Cargo.toml
index 073e548..3ff8500 100644
--- a/mmtk/Cargo.toml
+++ b/mmtk/Cargo.toml
@@ -27,7 +27,7 @@ log = "*"
 # - change branch
 # - change repo name
 # But other changes including adding/removing whitespaces in commented lines may break the CI.
-mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "61d20e2dcd5b4743ef04a8118eb807bcd6f6e2e2" }
+mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "57af17fbfd94ff0df2cd3b1e504abe299ce4f0ab" }
 # Uncomment the following and fix the path to mmtk-core to build locally
 # mmtk = { path = "../repos/mmtk-core" }
 
diff --git a/mmtk/src/active_plan.rs b/mmtk/src/active_plan.rs
index 0afd3bc..64a2d63 100644
--- a/mmtk/src/active_plan.rs
+++ b/mmtk/src/active_plan.rs
@@ -2,18 +2,12 @@ use super::UPCALLS;
 use mmtk::util::opaque_pointer::*;
 use mmtk::vm::ActivePlan;
 use mmtk::Mutator;
-use mmtk::Plan;
 use std::sync::Mutex;
-use SINGLETON;
 use V8;
 
 pub struct VMActivePlan {}
 
 impl ActivePlan<V8> for VMActivePlan {
-    fn global() -> &'static dyn Plan<VM = V8> {
-        SINGLETON.get_plan()
-    }
-
     fn is_mutator(tls: VMThread) -> bool {
         unsafe { ((*UPCALLS).is_mutator)(tls) }
     }
diff --git a/mmtk/src/api.rs b/mmtk/src/api.rs
index bbaf143..6a722dc 100644
--- a/mmtk/src/api.rs
+++ b/mmtk/src/api.rs
@@ -162,11 +162,6 @@ pub extern "C" fn is_mapped_address(address: Address) -> bool {
     memory_manager::is_mapped_address(address)
 }
 
-#[no_mangle]
-pub extern "C" fn modify_check(mmtk: &mut MMTK<V8>, object: ObjectReference) {
-    memory_manager::modify_check(mmtk, object);
-}
-
 #[no_mangle]
 pub extern "C" fn handle_user_collection_request(mmtk: &mut MMTK<V8>, tls: VMMutatorThread) {
     memory_manager::handle_user_collection_request::<V8>(mmtk, tls);
diff --git a/v8/third_party/heap/mmtk/mmtk.h b/v8/third_party/heap/mmtk/mmtk.h
index f1da714..a2b94b9 100644
--- a/v8/third_party/heap/mmtk/mmtk.h
+++ b/v8/third_party/heap/mmtk/mmtk.h
@@ -37,7 +37,6 @@ extern void post_alloc(MMTk_Mutator mutator, void* refer,
 extern bool is_live_object(void* ref);
 extern bool is_in_mmtk_spaces(void* ref);
 extern bool is_mapped_address(void* addr);
-extern void modify_check(void *mmtk, void* ref);
 extern bool is_in_read_only_space(void* addr);
 extern bool is_in_code_space(void* addr);