diff --git a/lib/Slic3r/Print/Object.pm b/lib/Slic3r/Print/Object.pm
index 3e9f38d22..368a23406 100644
--- a/lib/Slic3r/Print/Object.pm
+++ b/lib/Slic3r/Print/Object.pm
@@ -155,12 +155,18 @@ sub detect_surfaces_type {
 sub prepare_infill {
     my ($self) = @_;
     
-    # prerequisites
-    $self->make_perimeters;  # do we need them? TODO: check
-    $self->detect_surfaces_type;
-    
     return if $self->step_done(STEP_PREPARE_INFILL);
     $self->set_step_started(STEP_PREPARE_INFILL);
+    
+    # This prepare_infill() is not really idempotent.
+    # TODO: It should clear and regenerate fill_surfaces at every run 
+    # instead of modifying it in place.
+    $self->invalidate_step(STEP_PERIMETERS);
+    $self->make_perimeters;
+    
+    # prerequisites
+    $self->detect_surfaces_type;
+    
     $self->print->status_cb->(30, "Preparing infill");
     
     # decide what surfaces are to be filled
diff --git a/xs/src/libslic3r/LayerRegion.cpp b/xs/src/libslic3r/LayerRegion.cpp
index 148dd843c..5b4a6cc5c 100644
--- a/xs/src/libslic3r/LayerRegion.cpp
+++ b/xs/src/libslic3r/LayerRegion.cpp
@@ -66,7 +66,7 @@ LayerRegion::make_perimeters(const SurfaceCollection &slices, SurfaceCollection*
     g.process();
 }
 
-// This function reads layer->slices andlower_layer->slices
+// This function reads layer->slices and lower_layer->slices
 // and writes this->bridged and this->fill_surfaces, so it's thread-safe.
 void
 LayerRegion::process_external_surfaces()
@@ -106,7 +106,6 @@ LayerRegion::process_external_surfaces()
     }
     
     SurfaceCollection bottom;
-    Polygons removed_holes;
     for (const Surface &surface : surfaces) {
         if (!surface.is_bottom()) continue;
         
diff --git a/xs/src/libslic3r/PrintRegion.cpp b/xs/src/libslic3r/PrintRegion.cpp
index cb4e7fa2e..c5a1c948e 100644
--- a/xs/src/libslic3r/PrintRegion.cpp
+++ b/xs/src/libslic3r/PrintRegion.cpp
@@ -85,11 +85,19 @@ PrintRegion::invalidate_state_by_config(const PrintConfigBase &config)
             steps.insert(posPerimeters);
         } else if (opt_key == "first_layer_extrusion_width") {
             steps.insert(posSupportMaterial);
+        } else if (opt_key == "solid_infill_below_area") {
+            const float &cur_value = config.opt<ConfigOptionFloat>(opt_key)->value;
+            const float &new_value = this->config.solid_infill_below_area.value;
+            if (new_value >= cur_value) {
+                steps.insert(posPrepareInfill);
+            } else {
+                // prepare_infill is not idempotent when solid_infill_below_area is reduced
+                steps.insert(posPerimeters);
+            }
         } else if (opt_key == "infill_every_layers"
             || opt_key == "solid_infill_every_layers"
             || opt_key == "bottom_solid_layers"
             || opt_key == "top_solid_layers"
-            || opt_key == "solid_infill_below_area"
             || opt_key == "infill_extruder"
             || opt_key == "solid_infill_extruder"
             || opt_key == "infill_extrusion_width") {
@@ -108,10 +116,10 @@ PrintRegion::invalidate_state_by_config(const PrintConfigBase &config)
         } else if (opt_key == "fill_density") {
             const float &cur_value = config.opt<ConfigOptionFloat>("fill_density")->value;
             const float &new_value = this->config.fill_density.value;
-            if ((cur_value == 0) != (new_value == 0))
+            if ((cur_value == 0) != (new_value == 0) || (cur_value == 100) != (new_value == 100))
                 steps.insert(posPerimeters);
             
-            steps.insert(posPrepareInfill);
+            steps.insert(posInfill);
         } else if (opt_key == "external_perimeter_extrusion_width"
             || opt_key == "perimeter_extruder") {
             steps.insert(posPerimeters);