drm/vram-helper: Reuse existing page mappings in vmap

For performance, BO page mappings can stay in place even if the
map counter has returned to 0. In these cases, the existing page
mapping has to be reused by the next vmap operation. Otherwise
a new mapping would be installed and the old mapping's pages leak.

Fix the issue by reusing existing page mappings for vmap operations.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 1086db71a1 ("drm/vram-helper: Remove invariant parameters from internal kmap function")
Acked-by: Christian König <christian.koenig@amd.com>
Tested-by: Eli Cohen <elic@nvidia.com>
Reported-by: Eli Cohen <elic@nvidia.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210118144639.27307-1-tzimmermann@suse.de

drivers/gpu/drm/drm_gem_vram_helper.c

@@ -387,9 +387,16 @@ static int drm_gem_vram_kmap_locked(struct drm_gem_vram_object *gbo,
 	if (gbo->vmap_use_count > 0)
 		goto out;
 
-	ret = ttm_bo_vmap(&gbo->bo, &gbo->map);
-	if (ret)
-		return ret;
+	/*
+	 * VRAM helpers unmap the BO only on demand. So the previous
+	 * page mapping might still be around. Only vmap if the there's
+	 * no mapping present.
+	 */
+	if (dma_buf_map_is_null(&gbo->map)) {
+		ret = ttm_bo_vmap(&gbo->bo, &gbo->map);
+		if (ret)
+			return ret;
+	}
 
 out:
 	++gbo->vmap_use_count;
@@ -577,6 +584,7 @@ static void drm_gem_vram_bo_driver_move_notify(struct drm_gem_vram_object *gbo,
 		return;
 
 	ttm_bo_vunmap(bo, &gbo->map);
+	dma_buf_map_clear(&gbo->map); /* explicitly clear mapping for next vmap call */
 }
 
 static int drm_gem_vram_bo_driver_move(struct drm_gem_vram_object *gbo,