Re: [PATCH v19 18/30] drm/panfrost: Explicitly get and put drm-shmem pages

[Steven Price <steven.price@arm.com>]

On 05/01/2024 18:46, Dmitry Osipenko wrote:
> To simplify the drm-shmem refcnt handling, we're moving away from
> the implicit get_pages() that is used by get_pages_sgt(). From now on
> drivers will have to pin pages while they use sgt. Panfrost's shrinker
> doesn't support swapping out BOs, hence pages are pinned and sgt is valid
> as long as pages' use-count > 0.
> 
> In Panfrost, panfrost_gem_mapping, which is the object representing a
> GPU mapping of a BO, owns a pages ref. This guarantees that any BO being
> mapped GPU side has its pages retained till the mapping is destroyed.
> 
> Since pages are no longer guaranteed to stay pinned for the BO lifetime,
> and MADVISE(DONT_NEED) flagging remains after the GEM handle has been
> destroyed, we need to add an extra 'is_purgeable' check in
> panfrost_gem_purge(), to make sure we're not trying to purge a BO that
> already had its pages released.
> 
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>

Reviewed-by: Steven Price <steven.price@arm.com>

Although I don't like the condition in panfrost_gem_mapping_release()
for drm_gem_shmem_put_pages() and assigning NULL to bo->sgts - it feels
very fragile. See below.

> ---
>  drivers/gpu/drm/panfrost/panfrost_gem.c       | 63 ++++++++++++++-----
>  .../gpu/drm/panfrost/panfrost_gem_shrinker.c  |  6 ++
>  2 files changed, 52 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.c b/drivers/gpu/drm/panfrost/panfrost_gem.c
> index f268bd5c2884..7edfc12f7c1f 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
> @@ -35,20 +35,6 @@ static void panfrost_gem_free_object(struct drm_gem_object *obj)
>  	 */
>  	WARN_ON_ONCE(!list_empty(&bo->mappings.list));
>  
> -	if (bo->sgts) {
> -		int i;
> -		int n_sgt = bo->base.base.size / SZ_2M;
> -
> -		for (i = 0; i < n_sgt; i++) {
> -			if (bo->sgts[i].sgl) {
> -				dma_unmap_sgtable(pfdev->dev, &bo->sgts[i],
> -						  DMA_BIDIRECTIONAL, 0);
> -				sg_free_table(&bo->sgts[i]);
> -			}
> -		}
> -		kvfree(bo->sgts);
> -	}
> -
>  	drm_gem_shmem_free(&bo->base);
>  }
>  
> @@ -85,11 +71,40 @@ panfrost_gem_teardown_mapping(struct panfrost_gem_mapping *mapping)
>  
>  static void panfrost_gem_mapping_release(struct kref *kref)
>  {
> -	struct panfrost_gem_mapping *mapping;
> -
> -	mapping = container_of(kref, struct panfrost_gem_mapping, refcount);
> +	struct panfrost_gem_mapping *mapping =
> +		container_of(kref, struct panfrost_gem_mapping, refcount);
> +	struct panfrost_gem_object *bo = mapping->obj;
> +	struct panfrost_device *pfdev = bo->base.base.dev->dev_private;
>  
>  	panfrost_gem_teardown_mapping(mapping);
> +
> +	/* On heap BOs, release the sgts created in the fault handler path. */
> +	if (bo->sgts) {
> +		int i, n_sgt = bo->base.base.size / SZ_2M;
> +
> +		for (i = 0; i < n_sgt; i++) {
> +			if (bo->sgts[i].sgl) {
> +				dma_unmap_sgtable(pfdev->dev, &bo->sgts[i],
> +						  DMA_BIDIRECTIONAL, 0);
> +				sg_free_table(&bo->sgts[i]);
> +			}
> +		}
> +		kvfree(bo->sgts);
> +	}
> +
> +	/* Pages ref is owned by the panfrost_gem_mapping object. We must
> +	 * release our pages ref (if any), before releasing the object
> +	 * ref.
> +	 * Non-heap BOs acquired the pages at panfrost_gem_mapping creation
> +	 * time, and heap BOs may have acquired pages if the fault handler
> +	 * was called, in which case bo->sgts should be non-NULL.
> +	 */
> +	if (!bo->base.base.import_attach && (!bo->is_heap || bo->sgts) &&
> +	    bo->base.madv >= 0) {
> +		drm_gem_shmem_put_pages(&bo->base);
> +		bo->sgts = NULL;

The assignment of NULL here really ought to be unconditional - it isn't
a valid pointer because of the kvfree() above.

I also feel that the big condition above suggests there's a need for a
better state machine to keep track of what's going on.

But having said that I do think this series as a whole is an
improvement, it's nice to get the shrinker code generic. And sadly I
don't have an immediate idea for cleaning this up, hence my R-b.

Steve

> +	}
> +
>  	drm_gem_object_put(&mapping->obj->base.base);
>  	panfrost_mmu_ctx_put(mapping->mmu);
>  	kfree(mapping);
> @@ -125,6 +140,20 @@ int panfrost_gem_open(struct drm_gem_object *obj, struct drm_file *file_priv)
>  	if (!mapping)
>  		return -ENOMEM;
>  
> +	if (!bo->is_heap && !bo->base.base.import_attach) {
> +		/* Pages ref is owned by the panfrost_gem_mapping object.
> +		 * For non-heap BOs, we request pages at mapping creation
> +		 * time, such that the panfrost_mmu_map() call, further down in
> +		 * this function, is guaranteed to have pages_use_count > 0
> +		 * when drm_gem_shmem_get_pages_sgt() is called.
> +		 */
> +		ret = drm_gem_shmem_get_pages(&bo->base);
> +		if (ret) {
> +			kfree(mapping);
> +			return ret;
> +		}
> +	}
> +
>  	INIT_LIST_HEAD(&mapping->node);
>  	kref_init(&mapping->refcount);
>  	drm_gem_object_get(obj);
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> index 02b60ea1433a..d4fb0854cf2f 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c
> @@ -50,6 +50,12 @@ static bool panfrost_gem_purge(struct drm_gem_object *obj)
>  	if (!dma_resv_trylock(shmem->base.resv))
>  		goto unlock_mappings;
>  
> +	/* BO might have become unpurgeable if the last pages_use_count ref
> +	 * was dropped, but the BO hasn't been destroyed yet.
> +	 */
> +	if (!drm_gem_shmem_is_purgeable(shmem))
> +		goto unlock_mappings;
> +
>  	panfrost_gem_teardown_mappings_locked(bo);
>  	drm_gem_shmem_purge_locked(&bo->base);
>  	ret = true;