diff --git a/src/sys/fs/block_device.rs b/src/sys/fs/block_device.rs
index 20f1a84c..184eb86a 100644
--- a/src/sys/fs/block_device.rs
+++ b/src/sys/fs/block_device.rs
@@ -86,7 +86,7 @@ impl BlockDeviceIO for MemBlockDevice {
 }
 
 pub fn mount_mem() {
-    let mem = sys::mem::memory_size() / 2; // Half the allocatable memory
+    let mem = sys::mem::memory_free() / 2;
     let len = mem / super::BLOCK_SIZE; // TODO: take a size argument
     let dev = MemBlockDevice::new(len);
     *BLOCK_DEVICE.lock() = Some(BlockDevice::Mem(dev));
diff --git a/src/sys/mem/heap.rs b/src/sys/mem/heap.rs
index eba0a42c..ba380c17 100644
--- a/src/sys/mem/heap.rs
+++ b/src/sys/mem/heap.rs
@@ -140,6 +140,8 @@ fn many_boxes() {
 
 #[test_case]
 fn large_vec() {
+    use alloc::vec::Vec;
+
     let n = 1000;
     let mut vec = Vec::new();
     for i in 0..n {
diff --git a/src/sys/mem/mod.rs b/src/sys/mem/mod.rs
index 4641604b..58b95cbf 100644
--- a/src/sys/mem/mod.rs
+++ b/src/sys/mem/mod.rs
@@ -83,7 +83,7 @@ pub fn memory_used() -> usize {
 }
 
 pub fn memory_free() -> usize {
-    (memory_size() - heap::heap_size()) + heap::heap_free()
+    heap::heap_free()
 }
 
 pub fn phys_to_virt(addr: PhysAddr) -> VirtAddr {