Re: [PATCH v16 02/20] drm/shmem-helper: Use flag for tracking page count bumped by get_pages_sgt()

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

On Sun,  3 Sep 2023 20:07:18 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> Use separate flag for tracking page count bumped by shmem->sgt to avoid
> imbalanced page counter during of drm_gem_shmem_free() time. It's fragile
> to assume that populated shmem->pages at a freeing time means that the
> count was bumped by drm_gem_shmem_get_pages_sgt(), using a flag removes
> the ambiguity.
> 
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 11 ++++++++++-
>  drivers/gpu/drm/lima/lima_gem.c        |  1 +
>  include/drm/drm_gem_shmem_helper.h     |  7 +++++++
>  3 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 6693d4061ca1..848435e08eb2 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -152,8 +152,10 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>  			sg_free_table(shmem->sgt);
>  			kfree(shmem->sgt);
>  		}
> -		if (shmem->pages)
> +		if (shmem->pages) {
>  			drm_gem_shmem_put_pages(shmem);
> +			drm_WARN_ON(obj->dev, !shmem->got_pages_sgt);
> +		}

Already mentioned in v15, but I keep thinking the following:

		if (shmem->sgt) {
			// existing code in the preceding
			// if (shmem->sgt) branch
			...

			/*
			 * Release the implicit pages ref taken in
			 * drm_gem_shmem_get_pages_sgt_locked().
			 */
			drm_gem_shmem_put_pages(shmem);
		}

does exactly the same without requiring the addition of a new field.

>  
>  		drm_WARN_ON(obj->dev, shmem->pages_use_count);
>  
> @@ -693,6 +695,13 @@ static struct sg_table *drm_gem_shmem_get_pages_sgt_locked(struct drm_gem_shmem_
>  	if (ret)
>  		goto err_free_sgt;
>  
> +	/*
> +	 * This flag prevents imbalanced pages_use_count during
> +	 * drm_gem_shmem_free(), where pages_use_count=1 only if
> +	 * drm_gem_shmem_get_pages_sgt() was used by a driver.
> +	 */
> +	shmem->got_pages_sgt = true;
> +
>  	shmem->sgt = sgt;
>  
>  	return sgt;
> diff --git a/drivers/gpu/drm/lima/lima_gem.c b/drivers/gpu/drm/lima/lima_gem.c
> index 4f9736e5f929..67c39b95e30e 100644
> --- a/drivers/gpu/drm/lima/lima_gem.c
> +++ b/drivers/gpu/drm/lima/lima_gem.c
> @@ -48,6 +48,7 @@ int lima_heap_alloc(struct lima_bo *bo, struct lima_vm *vm)
>  
>  		bo->base.pages = pages;
>  		bo->base.pages_use_count = 1;
> +		bo->base.got_pages_sgt = true;
>  
>  		mapping_set_unevictable(mapping);
>  	}
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index ec70a98a8fe1..a53c0874b3c4 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -73,6 +73,13 @@ struct drm_gem_shmem_object {
>  	 */
>  	unsigned int vmap_use_count;
>  
> +	/**
> +	 * @got_pages_sgt:
> +	 *
> +	 * True if SG table was retrieved using drm_gem_shmem_get_pages_sgt()
> +	 */
> +	bool got_pages_sgt : 1;
> +
>  	/**
>  	 * @imported_sgt:
>  	 *