diff --git a/py/builtinimport.c b/py/builtinimport.c
index c725f075b6..1ef7be8702 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -149,7 +149,7 @@ mp_obj_t mp_builtin___import__(int n_args, mp_obj_t *args) {
     const char *mod_str = (const char*)mp_obj_str_get_data(args[0], &mod_len);
 
     uint last = 0;
-    vstr_t *path = vstr_new();
+    VSTR_FIXED(path, MICROPY_PATH_MAX)
     module_obj = MP_OBJ_NULL;
     uint i;
     for (i = 1; i <= mod_len; i++) {
@@ -159,14 +159,14 @@ mp_obj_t mp_builtin___import__(int n_args, mp_obj_t *args) {
 
             // find the file corresponding to the module name
             mp_import_stat_t stat;
-            if (vstr_len(path) == 0) {
+            if (vstr_len(&path) == 0) {
                 // first module in the dotted-name; search for a directory or file
-                stat = find_file(mod_str, i, path);
+                stat = find_file(mod_str, i, &path);
             } else {
                 // latter module in the dotted-name; append to path
-                vstr_add_char(path, PATH_SEP_CHAR);
-                vstr_add_strn(path, mod_str + last, i - last);
-                stat = stat_dir_or_file(path);
+                vstr_add_char(&path, PATH_SEP_CHAR);
+                vstr_add_strn(&path, mod_str + last, i - last);
+                stat = stat_dir_or_file(&path);
             }
             last = i + 1;
 
@@ -182,14 +182,14 @@ mp_obj_t mp_builtin___import__(int n_args, mp_obj_t *args) {
                 module_obj = mp_obj_new_module(mod_name);
 
                 if (stat == MP_IMPORT_STAT_DIR) {
-                    vstr_add_char(path, PATH_SEP_CHAR);
-                    vstr_add_str(path, "__init__.py");
-                    if (mp_import_stat(vstr_str(path)) == MP_IMPORT_STAT_FILE) {
-                        do_load(module_obj, path);
+                    vstr_add_char(&path, PATH_SEP_CHAR);
+                    vstr_add_str(&path, "__init__.py");
+                    if (mp_import_stat(vstr_str(&path)) == MP_IMPORT_STAT_FILE) {
+                        do_load(module_obj, &path);
                     }
-                    vstr_cut_tail(path, 12); // cut off /__init__.py
+                    vstr_cut_tail(&path, 12); // cut off /__init__.py
                 } else { // MP_IMPORT_STAT_FILE
-                    do_load(module_obj, path);
+                    do_load(module_obj, &path);
                     break;
                 }
             }
@@ -202,8 +202,6 @@ mp_obj_t mp_builtin___import__(int n_args, mp_obj_t *args) {
         assert(0);
     }
 
-    vstr_free(path);
-
     return module_obj;
 }
 
diff --git a/py/misc.h b/py/misc.h
index 679a14b5b9..065b070892 100644
--- a/py/misc.h
+++ b/py/misc.h
@@ -68,10 +68,17 @@ typedef struct _vstr_t {
     int alloc;
     int len;
     char *buf;
-    bool had_error;
+    struct {
+        bool had_error : 1;
+        bool fixed_buf : 1;
+    };
 } vstr_t;
 
+// convenience macro to declare a vstr with a fixed size buffer on the stack
+#define VSTR_FIXED(vstr, alloc) vstr_t vstr; char vstr##_buf[(alloc)]; vstr_init_fixed_buf(&vstr, (alloc), vstr##_buf);
+
 void vstr_init(vstr_t *vstr, int alloc);
+void vstr_init_fixed_buf(vstr_t *vstr, int alloc, char *buf);
 void vstr_clear(vstr_t *vstr);
 vstr_t *vstr_new(void);
 vstr_t *vstr_new_size(int alloc);
@@ -81,7 +88,7 @@ bool vstr_had_error(vstr_t *vstr);
 char *vstr_str(vstr_t *vstr);
 int vstr_len(vstr_t *vstr);
 void vstr_hint_size(vstr_t *vstr, int size);
-char  *vstr_extend(vstr_t *vstr, int size);
+char *vstr_extend(vstr_t *vstr, int size);
 bool vstr_set_size(vstr_t *vstr, int size);
 bool vstr_shrink(vstr_t *vstr);
 char *vstr_add_len(vstr_t *vstr, int len);
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 38b74b733a..799c9d8e71 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -101,6 +101,12 @@ typedef long long mp_longint_impl_t;
 #define MICROPY_CPYTHON_COMPAT (1)
 #endif
 
+// Maximum length of a path in the filesystem
+// So we can allocate a buffer on the stack for path manipulation in import
+#ifndef MICROPY_PATH_MAX
+#define MICROPY_PATH_MAX (512)
+#endif
+
 /*****************************************************************************/
 /* Miscellaneous settings                                                    */
 
diff --git a/py/vstr.c b/py/vstr.c
index b2e46c89f9..4d10dfba82 100644
--- a/py/vstr.c
+++ b/py/vstr.c
@@ -1,6 +1,7 @@
 #include <stdio.h>
 #include <stdarg.h>
 #include <string.h>
+#include <assert.h>
 #include "misc.h"
 
 // returned value is always at least 1 greater than argument
@@ -16,10 +17,23 @@ void vstr_init(vstr_t *vstr, int alloc) {
     }
     vstr->buf[0] = 0;
     vstr->had_error = false;
+    vstr->fixed_buf = false;
+}
+
+void vstr_init_fixed_buf(vstr_t *vstr, int alloc, char *buf) {
+    assert(alloc > 0); // need at least room for the null byte
+    vstr->alloc = alloc;
+    vstr->len = 0;
+    vstr->buf = buf;
+    vstr->buf[0] = 0;
+    vstr->had_error = false;
+    vstr->fixed_buf = true;
 }
 
 void vstr_clear(vstr_t *vstr) {
-    m_del(char, vstr->buf, vstr->alloc);
+    if (!vstr->fixed_buf) {
+        m_del(char, vstr->buf, vstr->alloc);
+    }
     vstr->buf = NULL;
 }
 
@@ -43,7 +57,9 @@ vstr_t *vstr_new_size(int alloc) {
 
 void vstr_free(vstr_t *vstr) {
     if (vstr != NULL) {
-        m_del(char, vstr->buf, vstr->alloc);
+        if (!vstr->fixed_buf) {
+            m_del(char, vstr->buf, vstr->alloc);
+        }
         m_del_obj(vstr_t, vstr);
     }
 }
@@ -73,7 +89,10 @@ int vstr_len(vstr_t *vstr) {
 }
 
 // Extend vstr strictly to by requested size, return pointer to newly added chunk
-char  *vstr_extend(vstr_t *vstr, int size) {
+char *vstr_extend(vstr_t *vstr, int size) {
+    if (vstr->fixed_buf) {
+        return NULL;
+    }
     char *new_buf = m_renew(char, vstr->buf, vstr->alloc, vstr->alloc + size);
     if (new_buf == NULL) {
         vstr->had_error = true;
@@ -87,6 +106,9 @@ char  *vstr_extend(vstr_t *vstr, int size) {
 
 // Shrink vstr to be given size
 bool vstr_set_size(vstr_t *vstr, int size) {
+    if (vstr->fixed_buf) {
+        return false;
+    }
     char *new_buf = m_renew(char, vstr->buf, vstr->alloc, size);
     if (new_buf == NULL) {
         vstr->had_error = true;
@@ -102,8 +124,11 @@ bool vstr_shrink(vstr_t *vstr) {
     return vstr_set_size(vstr, vstr->len);
 }
 
-bool vstr_ensure_extra(vstr_t *vstr, int size) {
+static bool vstr_ensure_extra(vstr_t *vstr, int size) {
     if (vstr->len + size + 1 > vstr->alloc) {
+        if (vstr->fixed_buf) {
+            return false;
+        }
         int new_alloc = ROUND_ALLOC((vstr->len + size + 1) * 2);
         char *new_buf = m_renew(char, vstr->buf, vstr->alloc, new_alloc);
         if (new_buf == NULL) {
@@ -156,8 +181,15 @@ void vstr_add_str(vstr_t *vstr, const char *str) {
 
 void vstr_add_strn(vstr_t *vstr, const char *str, int len) {
     if (vstr->had_error || !vstr_ensure_extra(vstr, len)) {
+        // if buf is fixed, we got here because there isn't enough room left
+        // so just try to copy as much as we can, with room for null byte
+        if (vstr->fixed_buf && vstr->len + 1 < vstr->alloc) {
+            len = vstr->alloc - vstr->len - 1;
+            goto copy;
+        }
         return;
     }
+copy:
     memmove(vstr->buf + vstr->len, str, len);
     vstr->len += len;
     vstr->buf[vstr->len] = 0;
diff --git a/stm/mpconfigport.h b/stm/mpconfigport.h
index d6a7f697b2..fd7af29d27 100644
--- a/stm/mpconfigport.h
+++ b/stm/mpconfigport.h
@@ -7,6 +7,7 @@
 #define MICROPY_ENABLE_GC           (1)
 #define MICROPY_ENABLE_REPL_HELPERS (1)
 #define MICROPY_ENABLE_FLOAT        (1)
+#define MICROPY_PATH_MAX            (128)
 
 // type definitions for the specific machine
 
diff --git a/unix/mpconfigport.h b/unix/mpconfigport.h
index 5eba0df613..dc92970104 100644
--- a/unix/mpconfigport.h
+++ b/unix/mpconfigport.h
@@ -15,6 +15,7 @@
 #define MICROPY_ENABLE_SOURCE_LINE  (1)
 #define MICROPY_ENABLE_FLOAT        (1)
 #define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_LONGLONG)
+#define MICROPY_PATH_MAX            (PATH_MAX)
 
 // type definitions for the specific machine