Re: [PATCH v16 11/20] drm/shmem-helper: Use refcount_t for pages_use_count

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

On Sun,  3 Sep 2023 20:07:27 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> Use atomic refcount_t helper for pages_use_count to optimize pin/unpin
> functions by skipping reservation locking while GEM's pin refcount > 1.
> 
> 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  | 35 +++++++++++--------------
>  drivers/gpu/drm/lima/lima_gem.c         |  2 +-
>  drivers/gpu/drm/panfrost/panfrost_mmu.c |  2 +-
>  include/drm/drm_gem_shmem_helper.h      |  2 +-
>  4 files changed, 19 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 7e1e674e2c9f..a0faef3e762d 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -152,12 +152,12 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>  			sg_free_table(shmem->sgt);
>  			kfree(shmem->sgt);
>  		}
> -		if (shmem->pages) {
> +		if (refcount_read(&shmem->pages_use_count)) {

As explained in my v15 review, I'm not convinced this is the right
thing to do. We should instead move the
drm_gem_shmem_put_pages_locked() call in the preceding 'if (shmem->sgt)'
block, because the sgt creation logic is where this implicit pages ref
was taken. If shmem->sgt == NULL, there's no reason to call
drm_gem_shmem_put_pages_locked() and we should let the following
drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count)) complain.

Besides, I don't think this change should be part of the
uint -> refcount_t conversion.

>  			drm_gem_shmem_put_pages_locked(shmem);
>  			drm_WARN_ON(obj->dev, !shmem->got_pages_sgt);
>  		}
>  
> -		drm_WARN_ON(obj->dev, shmem->pages_use_count);
> +		drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count));
>  
>  		dma_resv_unlock(shmem->base.resv);
>  	}
> @@ -174,14 +174,13 @@ static int drm_gem_shmem_get_pages_locked(struct drm_gem_shmem_object *shmem)
>  
>  	dma_resv_assert_held(shmem->base.resv);
>  
> -	if (shmem->pages_use_count++ > 0)
> +	if (refcount_inc_not_zero(&shmem->pages_use_count))
>  		return 0;
>  
>  	pages = drm_gem_get_pages(obj);
>  	if (IS_ERR(pages)) {
>  		drm_dbg_kms(obj->dev, "Failed to get pages (%ld)\n",
>  			    PTR_ERR(pages));
> -		shmem->pages_use_count = 0;
>  		return PTR_ERR(pages);
>  	}
>  
> @@ -197,6 +196,8 @@ static int drm_gem_shmem_get_pages_locked(struct drm_gem_shmem_object *shmem)
>  
>  	shmem->pages = pages;
>  
> +	refcount_set(&shmem->pages_use_count, 1);
> +
>  	return 0;
>  }
>  
> @@ -212,21 +213,17 @@ void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
>  
>  	dma_resv_assert_held(shmem->base.resv);
>  
> -	if (drm_WARN_ON_ONCE(obj->dev, !shmem->pages_use_count))
> -		return;
> -
> -	if (--shmem->pages_use_count > 0)
> -		return;
> -
> +	if (refcount_dec_and_test(&shmem->pages_use_count)) {
>  #ifdef CONFIG_X86
> -	if (shmem->map_wc)
> -		set_pages_array_wb(shmem->pages, obj->size >> PAGE_SHIFT);
> +		if (shmem->map_wc)
> +			set_pages_array_wb(shmem->pages, obj->size >> PAGE_SHIFT);
>  #endif
>  
> -	drm_gem_put_pages(obj, shmem->pages,
> -			  shmem->pages_mark_dirty_on_put,
> -			  shmem->pages_mark_accessed_on_put);
> -	shmem->pages = NULL;
> +		drm_gem_put_pages(obj, shmem->pages,
> +				  shmem->pages_mark_dirty_on_put,
> +				  shmem->pages_mark_accessed_on_put);
> +		shmem->pages = NULL;
> +	}
>  }
>  EXPORT_SYMBOL_GPL(drm_gem_shmem_put_pages_locked);
>  
> @@ -553,8 +550,8 @@ static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
>  	 * mmap'd, vm_open() just grabs an additional reference for the new
>  	 * mm the vma is getting copied into (ie. on fork()).
>  	 */
> -	if (!drm_WARN_ON_ONCE(obj->dev, !shmem->pages_use_count))
> -		shmem->pages_use_count++;
> +	drm_WARN_ON_ONCE(obj->dev,
> +			 !refcount_inc_not_zero(&shmem->pages_use_count));
>  
>  	dma_resv_unlock(shmem->base.resv);
>  
> @@ -642,7 +639,7 @@ void drm_gem_shmem_print_info(const struct drm_gem_shmem_object *shmem,
>  		return;
>  
>  	drm_printf_indent(p, indent, "pages_pin_count=%u\n", refcount_read(&shmem->pages_pin_count));
> -	drm_printf_indent(p, indent, "pages_use_count=%u\n", shmem->pages_use_count);
> +	drm_printf_indent(p, indent, "pages_use_count=%u\n", refcount_read(&shmem->pages_use_count));
>  	drm_printf_indent(p, indent, "vmap_use_count=%u\n", shmem->vmap_use_count);
>  	drm_printf_indent(p, indent, "vaddr=%p\n", shmem->vaddr);
>  }
> diff --git a/drivers/gpu/drm/lima/lima_gem.c b/drivers/gpu/drm/lima/lima_gem.c
> index ec8f718aa539..4be2fccbf6d9 100644
> --- a/drivers/gpu/drm/lima/lima_gem.c
> +++ b/drivers/gpu/drm/lima/lima_gem.c
> @@ -47,8 +47,8 @@ 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;
> +		refcount_set(&bo->base.pages_use_count, 1);
>  
>  		mapping_set_unevictable(mapping);
>  	}
> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> index 7771769f0ce0..a91252053aa3 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -487,7 +487,7 @@ static int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as,
>  			goto err_unlock;
>  		}
>  		bo->base.pages = pages;
> -		bo->base.pages_use_count = 1;
> +		refcount_set(&bo->base.pages_use_count, 1);
>  	} else {
>  		pages = bo->base.pages;
>  		if (pages[page_offset]) {
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index 1cd74ae5761a..bd545428a7ee 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -37,7 +37,7 @@ struct drm_gem_shmem_object {
>  	 * Reference count on the pages table.
>  	 * The pages are put when the count reaches zero.
>  	 */
> -	unsigned int pages_use_count;
> +	refcount_t pages_use_count;
>  
>  	/**
>  	 * @pages_pin_count: