diff -r c4bcca326c0a Lib/test/test_dict.py
--- a/Lib/test/test_dict.py	Thu Dec 15 17:05:04 2016 +0800
+++ b/Lib/test/test_dict.py	Thu Dec 15 18:34:55 2016 +0900
@@ -933,6 +933,36 @@ class DictTest(unittest.TestCase):
         self.assertEqual(list(a), ['x', 'y'])
         self.assertEqual(list(b), ['x', 'y', 'z'])
 
+    @support.cpython_only
+    def test_splittable_setattr_after_pop(self):
+        """setattr must not convert combined table into split table"""
+        # Issue 28147
+        import _testcapi
+
+        class C:
+            pass
+        a = C()
+
+        a.a = 1
+        self.assertTrue(_testcapi.dict_hassplittable(a.__dict__))
+
+        # dict.pop() convert it to combined table
+        a.__dict__.pop('a')
+        self.assertFalse(_testcapi.dict_hassplittable(a.__dict__))
+
+        # But C should not convert a.__dict__ to split table again.
+        a.a = 1
+        self.assertFalse(_testcapi.dict_hassplittable(a.__dict__))
+
+        # Same for popitem()
+        a = C()
+        a.a = 2
+        self.assertTrue(_testcapi.dict_hassplittable(a.__dict__))
+        a.__dict__.popitem()
+        self.assertFalse(_testcapi.dict_hassplittable(a.__dict__))
+        a.a = 3
+        self.assertFalse(_testcapi.dict_hassplittable(a.__dict__))
+
     def test_iterator_pickling(self):
         for proto in range(pickle.HIGHEST_PROTOCOL + 1):
             data = {1:"a", 2:"b", 3:"c"}
diff -r c4bcca326c0a Modules/_testcapimodule.c
--- a/Modules/_testcapimodule.c	Thu Dec 15 17:05:04 2016 +0800
+++ b/Modules/_testcapimodule.c	Thu Dec 15 18:34:55 2016 +0900
@@ -259,6 +259,19 @@ dict_getitem_knownhash(PyObject *self, P
     return result;
 }
 
+static PyObject*
+dict_hassplittable(PyObject *self, PyObject *arg)
+{
+    if (!PyDict_Check(arg)) {
+        PyErr_Format(PyExc_TypeError,
+                     "dict_hassplittable() argument must be dict, not '%s'",
+                     arg->ob_type->tp_name);
+        return NULL;
+    }
+
+    return PyBool_FromLong(_PyDict_HasSplitTable((PyDictObject*)arg));
+}
+
 /* Issue #4701: Check that PyObject_Hash implicitly calls
  *   PyType_Ready if it hasn't already been called
  */
@@ -4016,6 +4029,7 @@ static PyMethodDef TestMethods[] = {
     {"test_list_api",           (PyCFunction)test_list_api,      METH_NOARGS},
     {"test_dict_iteration",     (PyCFunction)test_dict_iteration,METH_NOARGS},
     {"dict_getitem_knownhash",  dict_getitem_knownhash,          METH_VARARGS},
+    {"dict_hassplittable",      dict_hassplittable,              METH_O},
     {"test_lazy_hash_inheritance",      (PyCFunction)test_lazy_hash_inheritance,METH_NOARGS},
     {"test_long_api",           (PyCFunction)test_long_api,      METH_NOARGS},
     {"test_xincref_doesnt_leak",(PyCFunction)test_xincref_doesnt_leak,      METH_NOARGS},
diff -r c4bcca326c0a Objects/dictobject.c
--- a/Objects/dictobject.c	Thu Dec 15 17:05:04 2016 +0800
+++ b/Objects/dictobject.c	Thu Dec 15 18:34:55 2016 +0900
@@ -1214,7 +1214,7 @@ After resizing a table is always combine
 but can be resplit by make_keys_shared().
 */
 static int
-dictresize(PyDictObject *mp, Py_ssize_t minused)
+dictresize(PyDictObject *mp, Py_ssize_t minsize)
 {
     Py_ssize_t newsize, numentries;
     PyDictKeysObject *oldkeys;
@@ -1223,7 +1223,7 @@ dictresize(PyDictObject *mp, Py_ssize_t 
 
     /* Find the smallest table size > minused. */
     for (newsize = PyDict_MINSIZE;
-         newsize <= minused && newsize > 0;
+         newsize < minsize && newsize > 0;
          newsize <<= 1)
         ;
     if (newsize <= 0) {
@@ -1244,6 +1244,8 @@ dictresize(PyDictObject *mp, Py_ssize_t 
         mp->ma_keys = oldkeys;
         return -1;
     }
+    // New table must be large enough.
+    assert(mp->ma_keys->dk_usable >= mp->ma_used);
     if (oldkeys->dk_lookup == lookdict)
         mp->ma_keys->dk_lookup = lookdict;
 
@@ -4270,9 +4272,11 @@ int
                 CACHED_KEYS(tp) = NULL;
                 DK_DECREF(cached);
             }
-        } else {
+        }
+        else {
+            int shared = cached == ((PyDictObject *)dict)->ma_keys;
             res = PyDict_SetItem(dict, key, value);
-            if (cached != ((PyDictObject *)dict)->ma_keys) {
+            if (shared && cached != ((PyDictObject *)dict)->ma_keys) {
                 /* Either update tp->ht_cached_keys or delete it */
                 if (cached->dk_refcnt == 1) {
                     CACHED_KEYS(tp) = make_keys_shared(dict);