Re: [PATCH v15 16/23] drm/shmem-helper: Use kref for vmap_use_count

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

On Sun, 27 Aug 2023 20:54:42 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> Use kref helper for vmap_use_count to make refcounting consistent with
> pages_use_count and pages_pin_count that use kref. This will allow to
> optimize unlocked vmappings by skipping reservation locking if refcnt > 1.

The core is taking the resv lock before calling ->v[un]map(), so
switching to a kref sounds a bit premature/useless, unless there are
plans to delegate the locking to the drivers. The only thing it brings
is standard overflow/underflow checks. Not really sure it's worth
transitioning to a kref for this field until we have a real use case.

> 
> 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 | 37 ++++++++++++++------------
>  include/drm/drm_gem_shmem_helper.h     |  2 +-
>  2 files changed, 21 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 17a0177acb5d..d96fee3d6166 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -144,7 +144,7 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>  	} else if (!shmem->imported_sgt) {
>  		dma_resv_lock(shmem->base.resv, NULL);
>  
> -		drm_WARN_ON(obj->dev, shmem->vmap_use_count);
> +		drm_WARN_ON(obj->dev, kref_read(&shmem->vmap_use_count));
>  
>  		if (shmem->sgt) {
>  			dma_unmap_sgtable(obj->dev->dev, shmem->sgt,
> @@ -359,23 +359,25 @@ int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem,
>  
>  		dma_resv_assert_held(shmem->base.resv);
>  
> -		if (shmem->vmap_use_count++ > 0) {
> +		if (kref_get_unless_zero(&shmem->vmap_use_count)) {
>  			iosys_map_set_vaddr(map, shmem->vaddr);
>  			return 0;
>  		}
>  
>  		ret = drm_gem_shmem_pin_locked(shmem);
>  		if (ret)
> -			goto err_zero_use;
> +			return ret;
>  
>  		if (shmem->map_wc)
>  			prot = pgprot_writecombine(prot);
>  		shmem->vaddr = vmap(shmem->pages, obj->size >> PAGE_SHIFT,
>  				    VM_MAP, prot);
> -		if (!shmem->vaddr)
> +		if (!shmem->vaddr) {
>  			ret = -ENOMEM;
> -		else
> +		} else {
>  			iosys_map_set_vaddr(map, shmem->vaddr);
> +			kref_init(&shmem->vmap_use_count);
> +		}
>  	}
>  
>  	if (ret) {
> @@ -388,13 +390,22 @@ int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem,
>  err_put_pages:
>  	if (!obj->import_attach)
>  		drm_gem_shmem_unpin_locked(shmem);
> -err_zero_use:
> -	shmem->vmap_use_count = 0;
>  
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(drm_gem_shmem_vmap_locked);
>  
> +static void drm_gem_shmem_kref_vunmap(struct kref *kref)
> +{
> +	struct drm_gem_shmem_object *shmem;
> +
> +	shmem = container_of(kref, struct drm_gem_shmem_object,
> +			     vmap_use_count);
> +
> +	vunmap(shmem->vaddr);
> +	drm_gem_shmem_unpin_locked(shmem);
> +}
> +
>  /*
>   * drm_gem_shmem_vunmap_locked - Unmap a virtual mapping for a shmem GEM object
>   * @shmem: shmem GEM object
> @@ -416,15 +427,7 @@ void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
>  		dma_buf_vunmap(obj->import_attach->dmabuf, map);
>  	} else {
>  		dma_resv_assert_held(shmem->base.resv);
> -
> -		if (drm_WARN_ON_ONCE(obj->dev, !shmem->vmap_use_count))
> -			return;
> -
> -		if (--shmem->vmap_use_count > 0)
> -			return;
> -
> -		vunmap(shmem->vaddr);
> -		drm_gem_shmem_unpin_locked(shmem);
> +		kref_put(&shmem->vmap_use_count, drm_gem_shmem_kref_vunmap);
>  	}
>  
>  	shmem->vaddr = NULL;
> @@ -663,7 +666,7 @@ void drm_gem_shmem_print_info(const struct drm_gem_shmem_object *shmem,
>  		return;
>  
>  	drm_printf_indent(p, indent, "pages_use_count=%u\n", kref_read(&shmem->pages_use_count));
> -	drm_printf_indent(p, indent, "vmap_use_count=%u\n", shmem->vmap_use_count);
> +	drm_printf_indent(p, indent, "vmap_use_count=%u\n", kref_read(&shmem->vmap_use_count));
>  	drm_printf_indent(p, indent, "vaddr=%p\n", shmem->vaddr);
>  }
>  EXPORT_SYMBOL_GPL(drm_gem_shmem_print_info);
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index 400ecd63f45f..0e0ccd380f66 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -81,7 +81,7 @@ struct drm_gem_shmem_object {
>  	 * Reference count on the virtual address.
>  	 * The address are un-mapped when the count reaches zero.
>  	 */
> -	unsigned int vmap_use_count;
> +	struct kref vmap_use_count;
>  
>  	/**
>  	 * @got_sgt: