Re: [PATCH v15 12/23] drm/shmem-helper: Add and use pages_pin_count

[Boris Brezillon <boris.brezillon@collabora.com>]

On Sun, 27 Aug 2023 20:54:38 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> Add separate pages_pin_count for tracking of whether drm-shmem pages are
> moveable or not. With the addition of memory shrinker support to drm-shmem,
> the pages_use_count will no longer determine whether pages are hard-pinned
> in memory, but whether pages exit and are soft-pinned (and could be swapped
> out). The pages_pin_count > 1 will hard-pin pages in memory.
> 
> Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 22 +++++++++++++++++-----
>  include/drm/drm_gem_shmem_helper.h     | 10 ++++++++++
>  2 files changed, 27 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index d545d3d227d7..1a7e5c332fd8 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -234,14 +234,22 @@ static int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem)
>  
>  	dma_resv_assert_held(shmem->base.resv);
>  
> +	if (kref_get_unless_zero(&shmem->pages_pin_count))
> +		return 0;
> +
>  	ret = drm_gem_shmem_get_pages_locked(shmem);
> +	if (!ret)
> +		kref_init(&shmem->pages_pin_count);
>  
>  	return ret;
>  }
>  
> -static void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem)
> +static void drm_gem_shmem_kref_unpin_pages(struct kref *kref)
>  {
> -	dma_resv_assert_held(shmem->base.resv);
> +	struct drm_gem_shmem_object *shmem;
> +
> +	shmem = container_of(kref, struct drm_gem_shmem_object,
> +			     pages_pin_count);
>  
>  	drm_gem_shmem_put_pages_locked(shmem);
>  }
> @@ -263,6 +271,9 @@ int drm_gem_shmem_pin(struct drm_gem_shmem_object *shmem)
>  
>  	drm_WARN_ON(obj->dev, obj->import_attach);
>  
> +	if (kref_get_unless_zero(&shmem->pages_pin_count))
> +		return 0;
> +
>  	ret = dma_resv_lock_interruptible(shmem->base.resv, NULL);
>  	if (ret)
>  		return ret;
> @@ -286,9 +297,10 @@ void drm_gem_shmem_unpin(struct drm_gem_shmem_object *shmem)
>  
>  	drm_WARN_ON(obj->dev, obj->import_attach);
>  
> -	dma_resv_lock(shmem->base.resv, NULL);
> -	drm_gem_shmem_unpin_locked(shmem);
> -	dma_resv_unlock(shmem->base.resv);
> +	if (kref_put_dma_resv(&shmem->pages_pin_count,
> +			      drm_gem_shmem_kref_unpin_pages,
> +			      obj->resv, NULL))
> +		dma_resv_unlock(obj->resv);
>  }
>  EXPORT_SYMBOL_GPL(drm_gem_shmem_unpin);
>  
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index ec2d8b24e3cf..afb7cd671e2a 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -39,6 +39,16 @@ 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 and purged by memory
> +	 * shrinker only when the count is zero, otherwise pages
> +	 * are hard-pinned in memory.
> +	 */
> +	struct kref pages_pin_count;

I know it's tempting to use kref for the pages use/pin count, but I'm
wondering if we wouldn't be better using a refcount_t, which provides
overflow/underflow protection while still letting us control how we
want to handle the locking for 0 <-> 1 transitions. By doing that, we
avoid introducing core locking changes that might be more
controversial/longer to get accepted. Besides, I suspect the resulting
code (the one using a refcount_t) won't be more verbose/complicated (no
release functions needed if you don't use kref_put(), which makes
things closer to what we have right now).

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