Re: [PATCH v15 02/23] drm/shmem-helper: Use flag for tracking page count bumped by get_pages_sgt()

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

On Sun, 27 Aug 2023 20:54:28 +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 | 3 ++-
>  drivers/gpu/drm/lima/lima_gem.c        | 1 +
>  include/drm/drm_gem_shmem_helper.h     | 7 +++++++
>  3 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 78d9cf2355a5..db20b9123891 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -152,7 +152,7 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>  			sg_free_table(shmem->sgt);
>  			kfree(shmem->sgt);
>  		}
> -		if (shmem->pages)
> +		if (shmem->got_sgt)
>  			drm_gem_shmem_put_pages(shmem);

Can't we just move this drm_gem_shmem_put_pages() call in the
if (shmem->sgt) block?

>  
>  		drm_WARN_ON(obj->dev, shmem->pages_use_count);
> @@ -687,6 +687,7 @@ static struct sg_table *drm_gem_shmem_get_pages_sgt_locked(struct drm_gem_shmem_
>  	if (ret)
>  		goto err_free_sgt;
>  
> +	shmem->got_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..28602302c281 100644
> --- a/drivers/gpu/drm/lima/lima_gem.c
> +++ b/drivers/gpu/drm/lima/lima_gem.c
> @@ -89,6 +89,7 @@ int lima_heap_alloc(struct lima_bo *bo, struct lima_vm *vm)
>  	}
>  
>  	*bo->base.sgt = sgt;
> +	bo->base.got_sgt = true;
>  
>  	if (vm) {
>  		ret = lima_vm_map_bo(vm, bo, old_size >> PAGE_SHIFT);
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index ec70a98a8fe1..f87124629bb5 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_sgt:
> +	 *
> +	 * True if SG table was retrieved using drm_gem_shmem_get_pages_sgt()
> +	 */
> +	bool got_sgt : 1;
> +
>  	/**
>  	 * @imported_sgt:
>  	 *