linux-kernel - Re: [PATCH v14 02/12] drm/shmem-helper: Add pages_pin

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Date:   Tue, 25 Jul 2023 09:27:09 +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 v14 02/12] drm/shmem-helper: Add pages_pin_count field

On Sun, 23 Jul 2023 02:47:36 +0300
Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:

> And new pages_pin_count field to struct drm_gem_shmem_object that will
> determine whether pages are evictable by memory shrinker. The pages will
> be evictable only when pages_pin_count=0. This patch prepares code for
> addition of the memory shrinker that will utilize the new field.
> 
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 9 +++++++++
>  include/drm/drm_gem_shmem_helper.h     | 9 +++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 267153853e2c..42ba201dda50 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -274,15 +274,24 @@ static int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem)
>  	dma_resv_assert_held(shmem->base.resv);
>  
>  	ret = drm_gem_shmem_get_pages(shmem);
> +	if (!ret)
> +		shmem->pages_pin_count++;
>  
>  	return ret;
>  }
>  
>  static void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem)
>  {
> +	struct drm_gem_object *obj = &shmem->base;
> +
>  	dma_resv_assert_held(shmem->base.resv);
>  
> +	if (drm_WARN_ON_ONCE(obj->dev, !shmem->pages_pin_count))
> +		return;
> +
>  	drm_gem_shmem_put_pages(shmem);
> +
> +	shmem->pages_pin_count--;
>  }
>  
>  /**
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index bf0c31aa8fbe..7111f5743006 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -39,6 +39,15 @@ struct drm_gem_shmem_object {
>  	 */
>  	unsigned int pages_use_count;
>  
> +	/**
> +	 * @pages_pin_count:
> +	 *
> +	 * Reference count on the pinned pages table.
> +	 * The pages allowed to be evicted by memory shrinker
> +	 * only when the count is zero.
> +	 */
> +	unsigned int pages_pin_count;

Can we make it an atomic_t, so we can avoid taking the lock when the
GEM has already been pinned. That's something I need to be able to grab
a pin-ref in a path where the GEM resv lock is already held[1]. We could
of course expose the locked version, but in my case, I want to enforce
the fact the GEM has been pinned before the drm_gem_shmem_pin() call in
the section protected by the resv lock, so catching a "refcount 0 -> 1"
situation would be useful. Beside, using an atomic to avoid the
lock/unlock dance when refcount > 1 might be beneficial to everyone.

[1]https://gitlab.freedesktop.org/bbrezillon/linux/-/commit/4420fa0d5768ebdc35b34d58d4ae5fad9fbb93f9

> +
>  	/**
>  	 * @madv: State for madvise
>  	 *