linux-kernel - Re: [PATCH v18 19/26] drm/shmem-helper: Add common memory shrinker

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Date:   Fri, 10 Nov 2023 15:58:58 +0100
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 v18 19/26] drm/shmem-helper: Add common memory shrinker

On Mon, 30 Oct 2023 02:01:58 +0300
Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:

> @@ -238,6 +308,20 @@ void drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem)
>  	if (refcount_dec_not_one(&shmem->pages_use_count))
>  		return;
>  
> +	/*
> +	 * Destroying the object is a special case because acquiring
> +	 * the obj lock can cause a locking order inversion between
> +	 * reservation_ww_class_mutex and fs_reclaim.
> +	 *
> +	 * This deadlock is not actually possible, because no one should
> +	 * be already holding the lock when GEM is released.  Unfortunately
> +	 * lockdep is not aware of this detail.  So when the refcount drops
> +	 * to zero, we pretend it is already locked.
> +	 */
> +	if (!kref_read(&shmem->base.refcount) &&
> +	    refcount_dec_and_test(&shmem->pages_use_count))
> +		return drm_gem_shmem_free_pages(shmem);

Uh, with get/put_pages() being moved to the create/free_gem()
hooks, we're back to a situation where pages_use_count > 0 when we
reach gem->refcount == 0, which is not nice. We really need to patch
drivers so they dissociate GEM creation from the backing storage
allocation/reservation + mapping of the BO in GPU VM space.