diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_common.h b/compiler-rt/lib/sanitizer_common/sanitizer_common.h
index 52958de6de0e..267903ebca32 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_common.h
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_common.h
@@ -458,7 +458,7 @@ class InternalMmapVectorNoCtor {
     CHECK_LE(size_, capacity_);
     if (size_ == capacity_) {
       uptr new_capacity = RoundUpToPowerOfTwo(size_ + 1);
-      Resize(new_capacity);
+      Realloc(new_capacity);
     }
     internal_memcpy(&data_[size_++], &element, sizeof(T));
   }
@@ -483,12 +483,13 @@ class InternalMmapVectorNoCtor {
     return capacity_;
   }
   void reserve(uptr new_size) {
-    if (new_size >= size()) return;
-    Resize(new_size);
+    // Never downsize internal buffer.
+    if (new_size > capacity())
+      Realloc(new_size);
   }
   void resize(uptr new_size) {
-    Resize(new_size);
     if (new_size > size_) {
+      reserve(new_size);
       internal_memset(&data_[size_], 0, sizeof(T) * (new_size - size_));
     }
     size_ = new_size;
@@ -517,7 +518,7 @@ class InternalMmapVectorNoCtor {
   }
 
  private:
-  void Resize(uptr new_capacity) {
+  void Realloc(uptr new_capacity) {
     CHECK_GT(new_capacity, 0);
     CHECK_LE(size_, new_capacity);
     T *new_data = (T *)MmapOrDie(new_capacity * sizeof(T),
diff --git a/compiler-rt/lib/sanitizer_common/tests/sanitizer_common_test.cc b/compiler-rt/lib/sanitizer_common/tests/sanitizer_common_test.cc
index da761bef689c..02fecddbcc60 100644
--- a/compiler-rt/lib/sanitizer_common/tests/sanitizer_common_test.cc
+++ b/compiler-rt/lib/sanitizer_common/tests/sanitizer_common_test.cc
@@ -88,6 +88,29 @@ TEST(SanitizerCommon, MmapAlignedOrDieOnFatalError) {
   }
 }
 
+TEST(SanitizerCommon, InternalMmapVectorReize) {
+  InternalMmapVector<uptr> v;
+  CHECK_EQ(0U, v.size());
+  CHECK_GE(v.capacity(), v.size());
+
+  v.reserve(1000);
+  CHECK_EQ(0U, v.size());
+  CHECK_GE(v.capacity(), 1000U);
+
+  v.resize(10000);
+  CHECK_EQ(10000U, v.size());
+  CHECK_GE(v.capacity(), v.size());
+  uptr cap = v.capacity();
+
+  v.resize(100);
+  CHECK_EQ(100U, v.size());
+  CHECK_EQ(v.capacity(), cap);
+
+  v.reserve(10);
+  CHECK_EQ(100U, v.size());
+  CHECK_EQ(v.capacity(), cap);
+}
+
 TEST(SanitizerCommon, InternalMmapVector) {
   InternalMmapVector<uptr> vector;
   for (uptr i = 0; i < 100; i++) {