drm/i915: Pin backing pages whilst exporting through a dmabuf vmap

We need to refcount our pages in order to prevent reaping them at
inopportune times, such as when they currently vmapped or exported to
another driver. However, we also wish to keep the lazy deallocation of
our pages so we need to take a pin/unpinned approach rather than a
simple refcount.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -1008,6 +1008,7 @@ struct drm_i915_gem_object {
 	unsigned int has_global_gtt_mapping:1;
 
 	struct page **pages;
+	int pages_pin_count;
 
 	/**
 	 * DMAR support
@@ -1341,6 +1342,17 @@ void i915_gem_release_mmap(struct drm_i915_gem_object *obj);
 void i915_gem_lastclose(struct drm_device *dev);
 
 int __must_check i915_gem_object_get_pages(struct drm_i915_gem_object *obj);
+static inline void i915_gem_object_pin_pages(struct drm_i915_gem_object *obj)
+{
+	BUG_ON(obj->pages == NULL);
+	obj->pages_pin_count++;
+}
+static inline void i915_gem_object_unpin_pages(struct drm_i915_gem_object *obj)
+{
+	BUG_ON(obj->pages_pin_count == 0);
+	obj->pages_pin_count--;
+}
+
 int __must_check i915_mutex_lock_interruptible(struct drm_device *dev);
 int i915_gem_object_sync(struct drm_i915_gem_object *obj,
 			 struct intel_ring_buffer *to);

drivers/gpu/drm/i915/i915_gem.c

@@ -1699,6 +1699,9 @@ i915_gem_object_put_pages(struct drm_i915_gem_object *obj)
 
 	BUG_ON(obj->gtt_space);
 
+	if (obj->pages_pin_count)
+		return -EBUSY;
+
 	ops->put_pages(obj);
 
 	list_del(&obj->gtt_list);
@@ -1837,6 +1840,8 @@ i915_gem_object_get_pages(struct drm_i915_gem_object *obj)
 	if (obj->sg_table || obj->pages)
 		return 0;
 
+	BUG_ON(obj->pages_pin_count);
+
 	ret = ops->get_pages(obj);
 	if (ret)
 		return ret;
@@ -3743,6 +3748,7 @@ void i915_gem_free_object(struct drm_gem_object *gem_obj)
 		dev_priv->mm.interruptible = was_interruptible;
 	}
 
+	obj->pages_pin_count = 0;
 	i915_gem_object_put_pages(obj);
 	i915_gem_object_free_mmap_offset(obj);
 
@@ -4402,9 +4408,10 @@ i915_gem_inactive_shrink(struct shrinker *shrinker, struct shrink_control *sc)
 
 	cnt = 0;
 	list_for_each_entry(obj, &dev_priv->mm.unbound_list, gtt_list)
-		cnt += obj->base.size >> PAGE_SHIFT;
+		if (obj->pages_pin_count == 0)
+			cnt += obj->base.size >> PAGE_SHIFT;
 	list_for_each_entry(obj, &dev_priv->mm.bound_list, gtt_list)
-		if (obj->pin_count == 0)
+		if (obj->pin_count == 0 && obj->pages_pin_count == 0)
 			cnt += obj->base.size >> PAGE_SHIFT;
 
 	mutex_unlock(&dev->struct_mutex);

drivers/gpu/drm/i915/i915_gem_dmabuf.c

@@ -50,6 +50,8 @@ static struct sg_table *i915_gem_map_dma_buf(struct dma_buf_attachment *attachme
 	/* link the pages into an SG then map the sg */
 	sg = drm_prime_pages_to_sg(obj->pages, npages);
 	nents = dma_map_sg(attachment->dev, sg->sgl, sg->nents, dir);
+	i915_gem_object_pin_pages(obj);
+
 out:
 	mutex_unlock(&dev->struct_mutex);
 	return sg;
@@ -102,6 +104,7 @@ static void *i915_gem_dmabuf_vmap(struct dma_buf *dma_buf)
 	}
 
 	obj->vmapping_count = 1;
+	i915_gem_object_pin_pages(obj);
 out_unlock:
 	mutex_unlock(&dev->struct_mutex);
 	return obj->dma_buf_vmapping;
@@ -117,10 +120,11 @@ static void i915_gem_dmabuf_vunmap(struct dma_buf *dma_buf, void *vaddr)
 	if (ret)
 		return;
 
-	--obj->vmapping_count;
-	if (obj->vmapping_count == 0) {
+	if (--obj->vmapping_count == 0) {
 		vunmap(obj->dma_buf_vmapping);
 		obj->dma_buf_vmapping = NULL;
+
+		i915_gem_object_unpin_pages(obj);
 	}
 	mutex_unlock(&dev->struct_mutex);
 }