diff --git a/mmap_span/mmap_span.go b/mmap_span/mmap_span.go
index bbaa7fca..21f6bd26 100644
--- a/mmap_span/mmap_span.go
+++ b/mmap_span/mmap_span.go
@@ -3,6 +3,7 @@ package mmap_span
 import (
 	"io"
 	"log"
+	"sync"
 
 	"github.com/edsrzf/mmap-go"
 )
@@ -16,6 +17,7 @@ func (s segment) Size() int64 {
 }
 
 type MMapSpan struct {
+	mu sync.RWMutex
 	span
 }
 
@@ -23,7 +25,9 @@ func (ms *MMapSpan) Append(mmap mmap.MMap) {
 	ms.span = append(ms.span, segment{&mmap})
 }
 
-func (ms MMapSpan) Close() error {
+func (ms *MMapSpan) Close() error {
+	ms.mu.Lock()
+	defer ms.mu.Unlock()
 	for _, mMap := range ms.span {
 		err := mMap.(segment).Unmap()
 		if err != nil {
@@ -33,14 +37,18 @@ func (ms MMapSpan) Close() error {
 	return nil
 }
 
-func (ms MMapSpan) Size() (ret int64) {
+func (ms *MMapSpan) Size() (ret int64) {
+	ms.mu.RLock()
+	defer ms.mu.RUnlock()
 	for _, seg := range ms.span {
 		ret += seg.Size()
 	}
 	return
 }
 
-func (ms MMapSpan) ReadAt(p []byte, off int64) (n int, err error) {
+func (ms *MMapSpan) ReadAt(p []byte, off int64) (n int, err error) {
+	ms.mu.RLock()
+	defer ms.mu.RUnlock()
 	ms.ApplyTo(off, func(intervalOffset int64, interval sizer) (stop bool) {
 		_n := copy(p, (*interval.(segment).MMap)[intervalOffset:])
 		p = p[_n:]
@@ -53,7 +61,9 @@ func (ms MMapSpan) ReadAt(p []byte, off int64) (n int, err error) {
 	return
 }
 
-func (ms MMapSpan) WriteAt(p []byte, off int64) (n int, err error) {
+func (ms *MMapSpan) WriteAt(p []byte, off int64) (n int, err error) {
+	ms.mu.RLock()
+	defer ms.mu.RUnlock()
 	ms.ApplyTo(off, func(iOff int64, i sizer) (stop bool) {
 		mMap := i.(segment)
 		_n := copy((*mMap.MMap)[iOff:], p)
diff --git a/storage/mmap.go b/storage/mmap.go
index f48a2bcc..d859c2e5 100644
--- a/storage/mmap.go
+++ b/storage/mmap.go
@@ -55,8 +55,8 @@ func (ts *mmapTorrentStorage) Piece(p metainfo.Piece) PieceImpl {
 		pc:       ts.pc,
 		p:        p,
 		ih:       ts.infoHash,
-		ReaderAt: io.NewSectionReader(ts.span, p.Offset(), p.Length()),
-		WriterAt: missinggo.NewSectionWriter(ts.span, p.Offset(), p.Length()),
+		ReaderAt: io.NewSectionReader(&ts.span, p.Offset(), p.Length()),
+		WriterAt: missinggo.NewSectionWriter(&ts.span, p.Offset(), p.Length()),
 	}
 }