diff --git a/gsk/gpu/gskglbuffer.c b/gsk/gpu/gskglbuffer.c
index 7f7b567419..c5a98fcd4e 100644
--- a/gsk/gpu/gskglbuffer.c
+++ b/gsk/gpu/gskglbuffer.c
@@ -34,13 +34,17 @@ gsk_gl_buffer_map (GskGpuBuffer *buffer)
 }
 
 static void
-gsk_gl_buffer_unmap (GskGpuBuffer *buffer)
+gsk_gl_buffer_unmap (GskGpuBuffer *buffer,
+                     gsize         used)
 {
   GskGLBuffer *self = GSK_GL_BUFFER (buffer);
 
+  if (used == 0)
+    return;
+
   gsk_gl_buffer_bind (self);
 
-  glBufferSubData (self->target, 0, gsk_gpu_buffer_get_size (buffer), self->data);
+  glBufferSubData (self->target, 0, used, self->data);
 }
 
 static void
diff --git a/gsk/gpu/gskgpubuffer.c b/gsk/gpu/gskgpubuffer.c
index 218f5a448a..de143ff5a4 100644
--- a/gsk/gpu/gskgpubuffer.c
+++ b/gsk/gpu/gskgpubuffer.c
@@ -45,8 +45,9 @@ gsk_gpu_buffer_map (GskGpuBuffer *self)
 }
 
 void
-gsk_gpu_buffer_unmap (GskGpuBuffer *self)
+gsk_gpu_buffer_unmap (GskGpuBuffer *self,
+                      gsize         size)
 {
-  GSK_GPU_BUFFER_GET_CLASS (self)->unmap (self);
+  GSK_GPU_BUFFER_GET_CLASS (self)->unmap (self, size);
 }
 
diff --git a/gsk/gpu/gskgpubufferprivate.h b/gsk/gpu/gskgpubufferprivate.h
index 143e5a37d0..bbe47a9df0 100644
--- a/gsk/gpu/gskgpubufferprivate.h
+++ b/gsk/gpu/gskgpubufferprivate.h
@@ -23,7 +23,8 @@ struct _GskGpuBufferClass
   GObjectClass parent_class;
 
   guchar *              (* map)                                         (GskGpuBuffer           *self);
-  void                  (* unmap)                                       (GskGpuBuffer           *self);
+  void                  (* unmap)                                       (GskGpuBuffer           *self,
+                                                                         gsize                   used);
 };
 
 GType                   gsk_gpu_buffer_get_type                         (void) G_GNUC_CONST;
@@ -34,7 +35,8 @@ void                    gsk_gpu_buffer_setup                            (GskGpuB
 gsize                   gsk_gpu_buffer_get_size                         (GskGpuBuffer           *self);
 
 guchar *                gsk_gpu_buffer_map                              (GskGpuBuffer           *self);
-void                    gsk_gpu_buffer_unmap                            (GskGpuBuffer           *self);
+void                    gsk_gpu_buffer_unmap                            (GskGpuBuffer           *self,
+                                                                         gsize                   used);
 
 
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GskGpuBuffer, g_object_unref)
diff --git a/gsk/gpu/gskgpudownloadop.c b/gsk/gpu/gskgpudownloadop.c
index a9c3ec6148..5251f3aa21 100644
--- a/gsk/gpu/gskgpudownloadop.c
+++ b/gsk/gpu/gskgpudownloadop.c
@@ -126,7 +126,7 @@ gsk_gpu_download_op_vk_create (GskGpuDownloadOp *self)
                                           bytes,
                                           stride);
   g_bytes_unref (bytes);
-  gsk_gpu_buffer_unmap (self->buffer);
+  gsk_gpu_buffer_unmap (self->buffer, 0);
 }
 
 static GskGpuOp *
diff --git a/gsk/gpu/gskgpuframe.c b/gsk/gpu/gskgpuframe.c
index f28ded21fc..957a2095ae 100644
--- a/gsk/gpu/gskgpuframe.c
+++ b/gsk/gpu/gskgpuframe.c
@@ -426,7 +426,7 @@ gsk_gpu_frame_reserve_vertex_data (GskGpuFrame *self,
       if (priv->vertex_buffer_data)
         {
           memcpy (new_data, priv->vertex_buffer_data, old_size);
-          gsk_gpu_buffer_unmap (priv->vertex_buffer);
+          gsk_gpu_buffer_unmap (priv->vertex_buffer, old_size);
         }
       g_object_unref (priv->vertex_buffer);
       priv->vertex_buffer = new_buffer;
@@ -434,7 +434,7 @@ gsk_gpu_frame_reserve_vertex_data (GskGpuFrame *self,
     }
 
   priv->vertex_buffer_used = size_needed;
-  
+
   return size_needed - size;
 }
 
@@ -480,7 +480,7 @@ gsk_gpu_frame_write_storage_buffer (GskGpuFrame  *self,
     {
       g_assert (offset > 0);
 
-      gsk_gpu_buffer_unmap (priv->storage_buffer);
+      gsk_gpu_buffer_unmap (priv->storage_buffer, 0);
       g_clear_object (&priv->storage_buffer);
       priv->storage_buffer_data = 0;
       priv->storage_buffer_used = 0;
@@ -591,14 +591,14 @@ gsk_gpu_frame_submit (GskGpuFrame *self)
 
   if (priv->vertex_buffer)
     {
-      gsk_gpu_buffer_unmap (priv->vertex_buffer);
+      gsk_gpu_buffer_unmap (priv->vertex_buffer, priv->vertex_buffer_used);
       priv->vertex_buffer_data = NULL;
       priv->vertex_buffer_used = 0;
     }
 
   if (priv->storage_buffer_data)
     {
-      gsk_gpu_buffer_unmap (priv->storage_buffer);
+      gsk_gpu_buffer_unmap (priv->storage_buffer, priv->storage_buffer_used);
       priv->storage_buffer_data = NULL;
       priv->storage_buffer_used = 0;
     }
diff --git a/gsk/gpu/gskgpuuploadop.c b/gsk/gpu/gskgpuuploadop.c
index a4eeead875..4d48fa9ab9 100644
--- a/gsk/gpu/gskgpuuploadop.c
+++ b/gsk/gpu/gskgpuuploadop.c
@@ -110,8 +110,8 @@ gsk_gpu_upload_op_vk_command_with_area (GskGpuOp                    *op,
   data = gsk_gpu_buffer_map (*buffer);
 
   draw_func (op, data, stride);
-  
-  gsk_gpu_buffer_unmap (*buffer);
+
+  gsk_gpu_buffer_unmap (*buffer, area->height * stride);
 
   vkCmdPipelineBarrier (state->vk_command_buffer,
                         VK_PIPELINE_STAGE_HOST_BIT,
diff --git a/gsk/gpu/gskvulkanbuffer.c b/gsk/gpu/gskvulkanbuffer.c
index f181666a24..93dbba0b0f 100644
--- a/gsk/gpu/gskvulkanbuffer.c
+++ b/gsk/gpu/gskvulkanbuffer.c
@@ -45,7 +45,8 @@ gsk_vulkan_buffer_map (GskGpuBuffer *buffer)
 }
 
 static void
-gsk_vulkan_buffer_unmap (GskGpuBuffer *buffer)
+gsk_vulkan_buffer_unmap (GskGpuBuffer *buffer,
+                         gsize         size)
 {
 }