Re: [PATCH 8/8] drm/panfrost: Make sure the shrinker does not reclaim referenced BOs

[Robin Murphy <robin.murphy@xxxxxxx>]

On 29/11/2019 1:59 pm, Boris Brezillon wrote:
> Userspace might tag a BO purgeable while it's still referenced by GPU
> jobs. We need to make sure the shrinker does not purge such BOs until
> all jobs referencing it are finished.

Nit: for extra robustness, perhaps it's worth using the refcount_t API 
rather than bare atomic_t?

Robin.

> Fixes: 013b65101315 ("drm/panfrost: Add madvise and shrinker support")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
> ---
>   drivers/gpu/drm/panfrost/panfrost_drv.c          | 1 +
>   drivers/gpu/drm/panfrost/panfrost_gem.h          | 6 ++++++
>   drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c | 2 ++
>   drivers/gpu/drm/panfrost/panfrost_job.c          | 7 ++++++-
>   4 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c b/drivers/gpu/drm/panfrost/panfrost_drv.c
> index b406b5243b40..297c0e7304d2 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_drv.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
> @@ -166,6 +166,7 @@ panfrost_lookup_bos(struct drm_device *dev,
>   			break;
>   		}
>   
> +		atomic_inc(&bo->gpu_usecount);
>   		job->mappings[i] = mapping;
>   	}
>   
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.h b/drivers/gpu/drm/panfrost/panfrost_gem.h
> index ca1bc9019600..b3517ff9630c 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem.h
> @@ -30,6 +30,12 @@ struct panfrost_gem_object {
>   		struct mutex lock;
>   	} mappings;
>   
> +	/*
> +	 * Count the number of jobs referencing this BO so we don't let the
> +	 * shrinker reclaim this object prematurely.
> +	 */
> +	atomic_t gpu_usecount;
> +
>   	bool noexec		:1;
>   	bool is_heap		:1;
>   };
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> index b36df326c860..288e46c40673 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> @@ -41,6 +41,8 @@ static bool panfrost_gem_purge(struct drm_gem_object *obj)
>   	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
>   	struct panfrost_gem_object *bo = to_panfrost_bo(obj);
>   
> +	if (atomic_read(&bo->gpu_usecount))
> +		return false;
>   
>   	if (!mutex_trylock(&shmem->pages_lock))
>   		return false;
> diff --git a/drivers/gpu/drm/panfrost/panfrost_job.c b/drivers/gpu/drm/panfrost/panfrost_job.c
> index c85d45be3b5e..2b12aa87ff32 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_job.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_job.c
> @@ -270,8 +270,13 @@ static void panfrost_job_cleanup(struct kref *ref)
>   	dma_fence_put(job->render_done_fence);
>   
>   	if (job->mappings) {
> -		for (i = 0; i < job->bo_count; i++)
> +		for (i = 0; i < job->bo_count; i++) {
> +			if (!job->mappings[i])
> +				break;
> +
> +			atomic_dec(&job->mappings[i]->obj->gpu_usecount);
>   			panfrost_gem_mapping_put(job->mappings[i]);
> +		}
>   		kvfree(job->mappings);
>   	}
>