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

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Message-ID: <b653f475-d1f7-d1b8-e4b4-3a6611e9d70b@collabora.com>
Date:   Tue, 12 Sep 2023 02:32:14 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Boris Brezillon <boris.brezillon@...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 9/5/23 10:20, Boris Brezillon wrote:
> 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.

Apparently I forgot to drop these drm_gem_object_put() patches by
accident in v16, the drm_gem_shmem_free() doesn't touch resv lock
anymore. Will re-check for v17.

-- 
Best regards,
Dmitry