shmem: keep pages mapped

TODO: This patch is unfinished. Calls to drm_gem_shmem_{vunmap,unpin} should
only decrement shmem->pages_use_count, but never release any pages. Releasing
pages would be done be the purge or free functions.

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -162,6 +162,8 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
 			sg_free_table(shmem->sgt);
 			kfree(shmem->sgt);
 		}
+		if (shmem->vaddr)
+			vunmap(shmem->vaddr);
 		if (shmem->pages)
 			drm_gem_shmem_put_pages(shmem);
 	}
@@ -336,6 +338,8 @@ static int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem,
 			ret = -ENOMEM;
 		else
 			iosys_map_set_vaddr(map, shmem->vaddr);
+
+		drm_gem_never_purge(obj);
 	}
 
 	if (ret) {
@@ -395,11 +399,17 @@ static void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
 	if (--shmem->vmap_use_count > 0)
 		return;
 
+	/*
+	 * For dma-buf imported objects we do the vunmap directly here. For
+	 * natively allocated objects, we keep the pages around until the
+	 * object gets purged or freed. Caching the page mappings improves
+	 * performance for frequently vmapped objects.
+	 */
 	if (obj->import_attach) {
 		dma_buf_vunmap(obj->import_attach->dmabuf, map);
+		shmem->vaddr = NULL;
 	} else {
-		vunmap(shmem->vaddr);
-		drm_gem_shmem_put_pages(shmem);
+		drm_gem_may_purge(obj);
 	}
 
 	shmem->vaddr = NULL;
@@ -469,7 +479,7 @@ int drm_gem_shmem_madvise(struct drm_gem_shmem_object *shmem, int madv)
 }
 EXPORT_SYMBOL(drm_gem_shmem_madvise);
 
-static bool drm_gem_shmem_is_purgeable(struct drm_gem_shmem_object *shmem)
+static bool drm_gem_shmem_is_purgeable_locked(struct drm_gem_shmem_object *shmem)
 {
 	return (shmem->madv > 0) &&
 		!shmem->vmap_use_count && shmem->sgt &&
@@ -479,11 +489,21 @@ static bool drm_gem_shmem_is_purgeable(struct drm_gem_shmem_object *shmem)
 unsigned long drm_gem_shmem_purgeable_size(struct drm_gem_shmem_object *shmem)
 {
 	struct drm_gem_object *obj = &shmem->base;
+	unsigned long count = 0;
 
-	if (!drm_gem_shmem_is_purgeable(shmem))
+	if (!mutex_trylock(&shmem->pages_lock))
 		return 0;
+	if (!mutex_trylock(&shmem->vmap_lock))
+		goto out_mutex_unlock;
 
-	return obj->size >> PAGE_SHIFT;
+	if (drm_gem_shmem_is_purgeable_locked(shmem))
+		count = obj->size >> PAGE_SHIFT;
+
+	mutex_unlock(&shmem->vmap_lock);
+out_mutex_unlock:
+	mutex_unlock(&shmem->pages_lock);
+
+	return count;
 }
 EXPORT_SYMBOL(drm_gem_shmem_purgeable_size);
 
@@ -492,7 +512,12 @@ unsigned long drm_gem_shmem_purge_locked(struct drm_gem_shmem_object *shmem)
 	struct drm_gem_object *obj = &shmem->base;
 	struct drm_device *dev = obj->dev;
 
-	WARN_ON(!drm_gem_shmem_is_purgeable(shmem));
+	WARN_ON(!drm_gem_shmem_is_purgeable_locked(shmem));
+
+	if (shmem->vaddr) {
+		vunmap(shmem->vaddr);
+		shmem->vaddr = NULL;
+	}
 
 	dma_unmap_sgtable(dev->dev, shmem->sgt, DMA_BIDIRECTIONAL, 0);
 	sg_free_table(shmem->sgt);