diff --git a/src/libos/src/process/do_clone.rs b/src/libos/src/process/do_clone.rs
index a7fd2eb9..6387e7e1 100644
--- a/src/libos/src/process/do_clone.rs
+++ b/src/libos/src/process/do_clone.rs
@@ -34,7 +34,6 @@ pub fn do_clone(
         let current = current!();
         let vm = current.vm().clone();
         let task = {
-            let vm = vm.lock().unwrap();
             let user_stack_range = guess_user_stack_bound(&vm, user_rsp)?;
             let user_stack_base = user_stack_range.end();
             let user_stack_limit = user_stack_range.start();
@@ -230,16 +229,16 @@ fn check_clone_flags(flags: CloneFlags) -> Result<()> {
     Ok(())
 }
 
-fn guess_user_stack_bound(vm: &ProcessVM, user_rsp: usize) -> Result<&VMRange> {
+fn guess_user_stack_bound(vm: &ProcessVM, user_rsp: usize) -> Result<VMRange> {
     // The first case is most likely
     if let Ok(stack_range) = vm.find_mmap_region(user_rsp) {
         Ok(stack_range)
     }
     // The next three cases are very unlikely, but valid
     else if vm.get_stack_range().contains(user_rsp) {
-        Ok(vm.get_stack_range())
+        Ok(*vm.get_stack_range())
     } else if vm.get_heap_range().contains(user_rsp) {
-        Ok(vm.get_heap_range())
+        Ok(*vm.get_heap_range())
     }
     // Invalid
     else {
diff --git a/src/libos/src/process/do_spawn/mod.rs b/src/libos/src/process/do_spawn/mod.rs
index e8460032..da06487c 100644
--- a/src/libos/src/process/do_spawn/mod.rs
+++ b/src/libos/src/process/do_spawn/mod.rs
@@ -156,7 +156,7 @@ fn new_process(
                 )?
             }
         };
-        let vm_ref = Arc::new(SgxMutex::new(vm));
+        let vm_ref = Arc::new(vm);
         let files_ref = {
             let files = init_files(current_ref, file_actions, host_stdio_fds)?;
             Arc::new(SgxMutex::new(files))
diff --git a/src/libos/src/process/mod.rs b/src/libos/src/process/mod.rs
index 8d198801..a473ddf9 100644
--- a/src/libos/src/process/mod.rs
+++ b/src/libos/src/process/mod.rs
@@ -59,7 +59,7 @@ pub type uid_t = u32;
 pub type ProcessRef = Arc<Process>;
 pub type ThreadRef = Arc<Thread>;
 pub type FileTableRef = Arc<SgxMutex<FileTable>>;
-pub type ProcessVMRef = Arc<SgxMutex<ProcessVM>>;
+pub type ProcessVMRef = Arc<ProcessVM>;
 pub type FsViewRef = Arc<SgxMutex<FsView>>;
 pub type SchedAgentRef = Arc<SgxMutex<SchedAgent>>;
 pub type ResourceLimitsRef = Arc<SgxMutex<ResourceLimits>>;
diff --git a/src/libos/src/process/process/idle.rs b/src/libos/src/process/process/idle.rs
index 543449d4..2dc5c791 100644
--- a/src/libos/src/process/process/idle.rs
+++ b/src/libos/src/process/process/idle.rs
@@ -16,7 +16,7 @@ lazy_static! {
 fn create_idle_thread() -> Result<ThreadRef> {
     // Create dummy values for the mandatory fields
     let dummy_tid = ThreadId::zero();
-    let dummy_vm = Arc::new(SgxMutex::new(ProcessVM::default()));
+    let dummy_vm = Arc::new(ProcessVM::default());
     let dummy_task = Task::default();
 
     // Assemble the idle process
diff --git a/src/libos/src/util/mem_util.rs b/src/libos/src/util/mem_util.rs
index b38f643f..fe54f40b 100644
--- a/src/libos/src/util/mem_util.rs
+++ b/src/libos/src/util/mem_util.rs
@@ -87,8 +87,7 @@ pub mod from_user {
     /// len: the length in byte
     fn is_inside_user_space(addr: *const u8, len: usize) -> bool {
         let current = current!();
-        let current_vm = current.vm().lock().unwrap();
-        let user_range = current_vm.get_process_range();
+        let user_range = current.vm().get_process_range();
         let ur_start = user_range.start();
         let ur_end = user_range.end();
         let addr_start = addr as usize;
diff --git a/src/libos/src/vm/mod.rs b/src/libos/src/vm/mod.rs
index f3d80337..c2e1add5 100644
--- a/src/libos/src/vm/mod.rs
+++ b/src/libos/src/vm/mod.rs
@@ -39,16 +39,13 @@ pub fn do_mmap(
         );
     }
 
-    let current = current!();
-    let mut current_vm = current.vm().lock().unwrap();
-    current_vm.mmap(addr, size, perms, flags, fd, offset)
+    current!().vm().mmap(addr, size, perms, flags, fd, offset)
 }
 
 pub fn do_munmap(addr: usize, size: usize) -> Result<()> {
     debug!("munmap: addr: {:#x}, size: {:#x}", addr, size);
     let current = current!();
-    let mut current_vm = current.vm().lock().unwrap();
-    current_vm.munmap(addr, size)
+    current!().vm().munmap(addr, size)
 }
 
 pub fn do_mremap(
@@ -61,9 +58,7 @@ pub fn do_mremap(
         "mremap: old_addr: {:#x}, old_size: {:#x}, new_size: {:#x}, flags: {:?}",
         old_addr, old_size, new_size, flags
     );
-    let current = current!();
-    let mut current_vm = current.vm().lock().unwrap();
-    current_vm.mremap(old_addr, old_size, new_size, flags)
+    current!().vm().mremap(old_addr, old_size, new_size, flags)
 }
 
 pub fn do_mprotect(addr: usize, size: usize, perms: VMPerms) -> Result<()> {
@@ -71,16 +66,12 @@ pub fn do_mprotect(addr: usize, size: usize, perms: VMPerms) -> Result<()> {
         "mprotect: addr: {:#x}, size: {:#x}, perms: {:?}",
         addr, size, perms
     );
-    let current = current!();
-    let mut current_vm = current.vm().lock().unwrap();
-    current_vm.mprotect(addr, size, perms)
+    current!().vm().mprotect(addr, size, perms)
 }
 
 pub fn do_brk(addr: usize) -> Result<usize> {
     debug!("brk: addr: {:#x}", addr);
-    let current = current!();
-    let mut current_vm = current.vm().lock().unwrap();
-    current_vm.brk(addr)
+    current!().vm().brk(addr)
 }
 
 pub const PAGE_SIZE: usize = 4096;
diff --git a/src/libos/src/vm/process_vm.rs b/src/libos/src/vm/process_vm.rs
index 19840595..e0469f6f 100644
--- a/src/libos/src/vm/process_vm.rs
+++ b/src/libos/src/vm/process_vm.rs
@@ -7,6 +7,7 @@ use super::vm_manager::{
     VMInitializer, VMManager, VMMapAddr, VMMapOptions, VMMapOptionsBuilder, VMRemapOptions,
 };
 use super::vm_perms::VMPerms;
+use std::sync::atomic::{AtomicUsize, Ordering};
 
 #[derive(Debug)]
 pub struct ProcessVMBuilder<'a, 'b> {
@@ -118,13 +119,7 @@ impl<'a, 'b> ProcessVMBuilder<'a, 'b> {
             last_elf_range.end()
         };
         let heap_range = VMRange::new_with_layout(heap_layout, heap_min_start);
-        unsafe {
-            let heap_buf = heap_range.as_slice_mut();
-            for b in heap_buf {
-                *b = 0;
-            }
-        }
-        let brk = heap_range.start();
+        let brk = AtomicUsize::new(heap_range.start());
 
         // Init the stack memory in the process
         let stack_layout = &other_layouts[1];
@@ -147,6 +142,8 @@ impl<'a, 'b> ProcessVMBuilder<'a, 'b> {
         debug_assert!(process_range.range().is_superset_of(&stack_range));
         debug_assert!(process_range.range().is_superset_of(&mmap_range));
 
+        let mmap_manager = SgxMutex::new(mmap_manager);
+
         Ok(ProcessVM {
             process_range,
             elf_ranges,
@@ -203,11 +200,11 @@ impl<'a, 'b> ProcessVMBuilder<'a, 'b> {
 /// The per-process virtual memory
 #[derive(Debug)]
 pub struct ProcessVM {
-    mmap_manager: VMManager,
+    mmap_manager: SgxMutex<VMManager>,
     elf_ranges: Vec<VMRange>,
     heap_range: VMRange,
     stack_range: VMRange,
-    brk: usize,
+    brk: AtomicUsize,
     // Memory safety notes: the process_range field must be the last one.
     //
     // Rust drops fields in the same order as they are declared. So by making
@@ -260,10 +257,10 @@ impl ProcessVM {
     }
 
     pub fn get_brk(&self) -> usize {
-        self.brk
+        self.brk.load(Ordering::SeqCst)
     }
 
-    pub fn brk(&mut self, new_brk: usize) -> Result<usize> {
+    pub fn brk(&self, new_brk: usize) -> Result<usize> {
         let heap_start = self.heap_range.start();
         let heap_end = self.heap_range.end();
 
@@ -275,16 +272,13 @@ impl ProcessVM {
             return_errno!(EINVAL, "New brk address is too high");
         }
 
-        if self.brk < new_brk {
-            unsafe { fill_zeros(self.brk, new_brk - self.brk) };
-        }
-
-        self.brk = new_brk;
-        return Ok(new_brk);
+        self.brk
+            .fetch_update(|old_brk| Some(new_brk), Ordering::SeqCst, Ordering::SeqCst);
+        Ok(new_brk)
     }
 
     pub fn mmap(
-        &mut self,
+        &self,
         addr: usize,
         size: usize,
         perms: VMPerms,
@@ -323,12 +317,12 @@ impl ProcessVM {
             .perms(perms)
             .initializer(initializer)
             .build()?;
-        let mmap_addr = self.mmap_manager.mmap(&mmap_options)?;
+        let mmap_addr = self.mmap_manager.lock().unwrap().mmap(&mmap_options)?;
         Ok(mmap_addr)
     }
 
     pub fn mremap(
-        &mut self,
+        &self,
         old_addr: usize,
         old_size: usize,
         new_size: usize,
@@ -341,29 +335,36 @@ impl ProcessVM {
         }
 
         let mremap_option = VMRemapOptions::new(old_addr, old_size, new_size, flags)?;
-        self.mmap_manager.mremap(&mremap_option)
+        self.mmap_manager.lock().unwrap().mremap(&mremap_option)
     }
 
-    pub fn munmap(&mut self, addr: usize, size: usize) -> Result<()> {
-        self.mmap_manager.munmap(addr, size)
+    pub fn munmap(&self, addr: usize, size: usize) -> Result<()> {
+        self.mmap_manager.lock().unwrap().munmap(addr, size)
     }
 
-    pub fn mprotect(&mut self, addr: usize, size: usize, perms: VMPerms) -> Result<()> {
+    pub fn mprotect(&self, addr: usize, size: usize, perms: VMPerms) -> Result<()> {
         let protect_range = VMRange::new_with_size(addr, size)?;
         if !self.process_range.range().is_superset_of(&protect_range) {
             return_errno!(ENOMEM, "invalid range");
         }
+        let mut mmap_manager = self.mmap_manager.lock().unwrap();
+
         // TODO: support mprotect vm regions in addition to mmap
-        if !self.mmap_manager.range().is_superset_of(&protect_range) {
+        if !mmap_manager.range().is_superset_of(&protect_range) {
             warn!("Do not support mprotect memory outside the mmap region yet");
             return Ok(());
         }
 
-        self.mmap_manager.mprotect(addr, size, perms)
+        mmap_manager.mprotect(addr, size, perms)
     }
 
-    pub fn find_mmap_region(&self, addr: usize) -> Result<&VMRange> {
-        self.mmap_manager.find_mmap_region(addr)
+    // Return: a copy of the found region
+    pub fn find_mmap_region(&self, addr: usize) -> Result<VMRange> {
+        self.mmap_manager
+            .lock()
+            .unwrap()
+            .find_mmap_region(addr)
+            .map(|range_ref| *range_ref)
     }
 }
 
@@ -433,11 +434,3 @@ impl Default for MRemapFlags {
         MRemapFlags::None
     }
 }
-
-unsafe fn fill_zeros(addr: usize, size: usize) {
-    let ptr = addr as *mut u8;
-    let buf = std::slice::from_raw_parts_mut(ptr, size);
-    for b in buf {
-        *b = 0;
-    }
-}