linux-kernel - Re: [PATCH v16 06/20] drm/virtio: Replace drm_gem_shmem_free() with drm_gem_object

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Message-ID: <20230905092028.182f8ed8@collabora.com>
Date:   Tue, 5 Sep 2023 09:20:28 +0200
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc:     David Airlie <airlied@...il.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Christian König <christian.koenig@....com>,
        Qiang Yu <yuq825@...il.com>,
        Steven Price <steven.price@....com>,
        Emma Anholt <emma@...olt.net>, Melissa Wen <mwen@...lia.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v16 06/20] drm/virtio: Replace drm_gem_shmem_free() with
 drm_gem_object_put()

On Sun,  3 Sep 2023 20:07:22 +0300
Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:

> Prepare virtio_gpu_object_create() to addition of memory shrinker support
> by replacing open-coded drm_gem_shmem_free() with drm_gem_object_put() that
> decrements GEM refcount to 0, which becomes important for drm-shmem because
> it will start to use GEM's refcount during the shmem's BO freeing time in
> order to prevent spurious lockdep warning about resv lock ordering vs
> fs_reclaim code paths.

I think I'm okay with the change (assuming virtio_gpu_free_object()
can deal with partially initialized objects), not with the explanation
:-). I don't really see why we need to take the resv lock in
drm_gem_shmem_free(). As said in my v15 review, I think we should
replace the drm_gem_shmem_put_pages() call we have in
drm_gem_shmem_free() by a call to a new drm_gem_shmem_free_pages()
helper that does exactly what drm_gem_shmem_put_pages() does without
the refcounting/locking, because all that should remain at the time
drm_gem_shmem_free() is called is the implicit pages ref owned by
shmem->sgt, and there's no risk of other threads accessing the GEM
object at that point.

> 
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
> ---
>  drivers/gpu/drm/virtio/virtgpu_object.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/virtio/virtgpu_object.c b/drivers/gpu/drm/virtio/virtgpu_object.c
> index c7e74cf13022..343b13428125 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_object.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_object.c
> @@ -244,6 +244,6 @@ int virtio_gpu_object_create(struct virtio_gpu_device *vgdev,
>  err_put_id:
>  	virtio_gpu_resource_id_put(vgdev, bo->hw_res_handle);
>  err_free_gem:
> -	drm_gem_shmem_free(shmem_obj);
> +	drm_gem_object_put(&bo->base.base);
>  	return ret;
>  }