diff --git a/mmtk/Cargo.lock b/mmtk/Cargo.lock
index dbd7c24..8702c17 100644
--- a/mmtk/Cargo.lock
+++ b/mmtk/Cargo.lock
@@ -383,7 +383,7 @@ dependencies = [
 [[package]]
 name = "mmtk"
 version = "0.20.0"
-source = "git+https://github.com/mmtk/mmtk-core.git#29d482cf7b470ad3b09f88c6993babd6f695d832"
+source = "git+https://github.com/mmtk/mmtk-core.git?rev=57af17fbfd94ff0df2cd3b1e504abe299ce4f0ab#57af17fbfd94ff0df2cd3b1e504abe299ce4f0ab"
 dependencies = [
  "atomic",
  "atomic-traits",
@@ -417,7 +417,7 @@ dependencies = [
 [[package]]
 name = "mmtk-macros"
 version = "0.20.0"
-source = "git+https://github.com/mmtk/mmtk-core.git#29d482cf7b470ad3b09f88c6993babd6f695d832"
+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 73a0554..50f1675 100644
--- a/mmtk/Cargo.toml
+++ b/mmtk/Cargo.toml
@@ -38,6 +38,7 @@ features = ["is_mmtk_object", "object_pinning"]
 # Uncomment one of the following lines to choose where to find mmtk-core.
 git = "https://github.com/mmtk/mmtk-core.git"  # Use mmtk-core from the official repository.
 #path = "../../mmtk-core"  # Use mmtk-core from a local repository.
+rev = "57af17fbfd94ff0df2cd3b1e504abe299ce4f0ab"
 
 [features]
 default = []
diff --git a/mmtk/src/active_plan.rs b/mmtk/src/active_plan.rs
index a0bb879..80372a7 100644
--- a/mmtk/src/active_plan.rs
+++ b/mmtk/src/active_plan.rs
@@ -7,15 +7,10 @@ use crate::Ruby;
 use mmtk::util::opaque_pointer::*;
 use mmtk::vm::ActivePlan;
 use mmtk::Mutator;
-use mmtk::Plan;
 
 pub struct VMActivePlan {}
 
 impl ActivePlan<Ruby> for VMActivePlan {
-    fn global() -> &'static dyn Plan<VM = Ruby> {
-        mmtk().get_plan()
-    }
-
     fn number_of_mutators() -> usize {
         (upcalls().number_of_mutators)()
     }
diff --git a/mmtk/src/api.rs b/mmtk/src/api.rs
index 168e6db..77ddc5f 100644
--- a/mmtk/src/api.rs
+++ b/mmtk/src/api.rs
@@ -208,11 +208,6 @@ pub extern "C" fn mmtk_is_mmtk_object(addr: Address) -> bool {
     memory_manager::is_mmtk_object(addr)
 }
 
-#[no_mangle]
-pub extern "C" fn mmtk_modify_check(object: ObjectReference) {
-    memory_manager::modify_check(mmtk(), object)
-}
-
 #[no_mangle]
 pub extern "C" fn mmtk_handle_user_collection_request(tls: VMMutatorThread) {
     memory_manager::handle_user_collection_request::<Ruby>(mmtk(), tls);