diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index 188b0453df8..7517f1284ed 100644
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -397,16 +397,22 @@ func (h *mheap) sysAlloc(n uintptr) unsafe.Pointer {
 				return nil
 			}
 			if p == h.arena_end {
+				// The new reservation is contiguous
+				// with the old reservation.
 				h.arena_end = new_end
 				h.arena_reserved = reserved
 			} else if h.arena_start <= p && p+p_size-h.arena_start-1 <= _MaxMem {
+				// We were able to reserve more memory
+				// within the arena space, but it's
+				// not contiguous with our previous
+				// reservation. Skip over the unused
+				// address space.
+				//
 				// Keep everything page-aligned.
 				// Our pages are bigger than hardware pages.
 				h.arena_end = p + p_size
 				used := p + (-p & (_PageSize - 1))
-				h.mapBits(used)
-				h.mapSpans(used)
-				h.arena_used = used
+				h.setArenaUsed(used, false)
 				h.arena_reserved = reserved
 			} else {
 				// We haven't added this allocation to
@@ -422,12 +428,7 @@ func (h *mheap) sysAlloc(n uintptr) unsafe.Pointer {
 		// Keep taking from our reservation.
 		p := h.arena_used
 		sysMap(unsafe.Pointer(p), n, h.arena_reserved, &memstats.heap_sys)
-		h.mapBits(p + n)
-		h.mapSpans(p + n)
-		h.arena_used = p + n
-		if raceenabled {
-			racemapshadow(unsafe.Pointer(p), n)
-		}
+		h.setArenaUsed(p+n, true)
 
 		if p&(_PageSize-1) != 0 {
 			throw("misrounded allocation in MHeap_SysAlloc")
@@ -460,15 +461,10 @@ func (h *mheap) sysAlloc(n uintptr) unsafe.Pointer {
 	p_end := p + p_size
 	p += -p & (_PageSize - 1)
 	if p+n > h.arena_used {
-		h.mapBits(p + n)
-		h.mapSpans(p + n)
-		h.arena_used = p + n
+		h.setArenaUsed(p+n, true)
 		if p_end > h.arena_end {
 			h.arena_end = p_end
 		}
-		if raceenabled {
-			racemapshadow(unsafe.Pointer(p), n)
-		}
 	}
 
 	if p&(_PageSize-1) != 0 {
diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go
index b48dbff7f6e..e2add26b06c 100644
--- a/src/runtime/mbitmap.go
+++ b/src/runtime/mbitmap.go
@@ -134,13 +134,9 @@ func subtract1(p *byte) *byte {
 	return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) - 1))
 }
 
-// mHeap_MapBits is called each time arena_used is extended.
-// It maps any additional bitmap memory needed for the new arena memory.
-// It must be called with the expected new value of arena_used,
-// *before* h.arena_used has been updated.
-// Waiting to update arena_used until after the memory has been mapped
-// avoids faults when other threads try access the bitmap immediately
-// after observing the change to arena_used.
+// mapBits maps any additional bitmap memory needed for the new arena memory.
+//
+// Don't call this directly. Call mheap.setArenaUsed.
 //
 //go:nowritebarrier
 func (h *mheap) mapBits(arena_used uintptr) {
diff --git a/src/runtime/mheap.go b/src/runtime/mheap.go
index d4a939830bc..28d2846722b 100644
--- a/src/runtime/mheap.go
+++ b/src/runtime/mheap.go
@@ -93,7 +93,7 @@ type mheap struct {
 	bitmap         uintptr // Points to one byte past the end of the bitmap
 	bitmap_mapped  uintptr
 	arena_start    uintptr
-	arena_used     uintptr // always mHeap_Map{Bits,Spans} before updating
+	arena_used     uintptr // One byte past usable heap arena. Set with setArenaUsed.
 	arena_end      uintptr
 	arena_reserved bool
 
@@ -435,14 +435,35 @@ func (h *mheap) init(spansStart, spansBytes uintptr) {
 	sp.cap = int(spansBytes / sys.PtrSize)
 }
 
-// mHeap_MapSpans makes sure that the spans are mapped
+// setArenaUsed extends the usable arena to address arena_used and
+// maps auxiliary VM regions for any newly usable arena space.
+//
+// racemap indicates that this memory should be managed by the race
+// detector. racemap should be true unless this is covering a VM hole.
+func (h *mheap) setArenaUsed(arena_used uintptr, racemap bool) {
+	// Map auxiliary structures *before* h.arena_used is updated.
+	// Waiting to update arena_used until after the memory has been mapped
+	// avoids faults when other threads try access these regions immediately
+	// after observing the change to arena_used.
+
+	// Map the bitmap.
+	h.mapBits(arena_used)
+
+	// Map spans array.
+	h.mapSpans(arena_used)
+
+	// Tell the race detector about the new heap memory.
+	if racemap && raceenabled {
+		racemapshadow(unsafe.Pointer(h.arena_used), arena_used-h.arena_used)
+	}
+
+	h.arena_used = arena_used
+}
+
+// mapSpans makes sure that the spans are mapped
 // up to the new value of arena_used.
 //
-// It must be called with the expected new value of arena_used,
-// *before* h.arena_used has been updated.
-// Waiting to update arena_used until after the memory has been mapped
-// avoids faults when other threads try access the bitmap immediately
-// after observing the change to arena_used.
+// Don't call this directly. Call mheap.setArenaUsed.
 func (h *mheap) mapSpans(arena_used uintptr) {
 	// Map spans array, PageSize at a time.
 	n := arena_used