diff --git a/updater/install.c b/updater/install.c
index 656c607..6a79964 100644
--- a/updater/install.c
+++ b/updater/install.c
@@ -177,19 +177,23 @@
 }
 
 
-// format(fs_type, partition_type, location)
+// format(fs_type, partition_type, location, fs_size)
 //
-//    fs_type="yaffs2" partition_type="MTD"     location=partition
-//    fs_type="ext4"   partition_type="EMMC"    location=device
+//    fs_type="yaffs2" partition_type="MTD"     location=partition fs_size=<bytes>
+//    fs_type="ext4"   partition_type="EMMC"    location=device    fs_size=<bytes>
+//    if fs_size == 0, then make_ext4fs uses the entire partition.
+//    if fs_size > 0, that is the size to use
+//    if fs_size < 0, then reserve that many bytes at the end of the partition
 Value* FormatFn(const char* name, State* state, int argc, Expr* argv[]) {
     char* result = NULL;
-    if (argc != 3) {
-        return ErrorAbort(state, "%s() expects 3 args, got %d", name, argc);
+    if (argc != 4) {
+        return ErrorAbort(state, "%s() expects 4 args, got %d", name, argc);
     }
     char* fs_type;
     char* partition_type;
     char* location;
-    if (ReadArgs(state, argv, 3, &fs_type, &partition_type, &location) < 0) {
+    char* fs_size;
+    if (ReadArgs(state, argv, 4, &fs_type, &partition_type, &location, &fs_size) < 0) {
         return NULL;
     }
 
@@ -236,8 +240,7 @@
         result = location;
 #ifdef USE_EXT4
     } else if (strcmp(fs_type, "ext4") == 0) {
-        reset_ext4fs_info();
-        int status = make_ext4fs(location, NULL, NULL, 0, 0, 0, 0);
+        int status = make_ext4fs(location, atoll(fs_size));
         if (status != 0) {
             fprintf(stderr, "%s: make_ext4fs failed (%d) on %s",
                     name, status, location);