Subject: Re: [PATCH net-next v12 1/5] page_pool: unify frag_count handling in page_pool_is_last_frag()

From: Ilias Apalodimas <[email-masked]>
Date: Mon, 23 Oct 2023 14:43:10 +0300

Hi Yunsheng, 

[...]

> +	 * 1. 'n == 1': no need to actually overwrite it.
> +	 * 2. 'n != 1': overwrite it with one, which is the rare case
> +	 *              for pp_frag_count draining.
>  	 *
> -	 * The main advantage to doing this is that an atomic_read is
> -	 * generally a much cheaper operation than an atomic update,
> -	 * especially when dealing with a page that may be partitioned
> -	 * into only 2 or 3 pieces.
> +	 * The main advantage to doing this is that not only we avoid a atomic
> +	 * update, as an atomic_read is generally a much cheaper operation than
> +	 * an atomic update, especially when dealing with a page that may be
> +	 * partitioned into only 2 or 3 pieces; but also unify the pp_frag_count
> +	 * handling by ensuring all pages have partitioned into only 1 piece
> +	 * initially, and only overwrite it when the page is partitioned into
> +	 * more than one piece.
>  	 */
> -	if (atomic_long_read(&page->pp_frag_count) == nr)
> +	if (atomic_long_read(&page->pp_frag_count) == nr) {
> +		/* As we have ensured nr is always one for constant case using
> +		 * the BUILD_BUG_ON(), only need to handle the non-constant case
> +		 * here for pp_frag_count draining, which is a rare case.
> +		 */
> +		BUILD_BUG_ON(__builtin_constant_p(nr) && nr != 1);
> +		if (!__builtin_constant_p(nr))
> +			atomic_long_set(&page->pp_frag_count, 1);

Aren't we changing the behaviour of the current code here? IIRC is
atomic_long_read(&page->pp_frag_count) == nr we never updated the atomic
pp_frag_count and the reasoning was that the next caller can set it
properly. 

> +
>  		return 0;
> +	}
>  
>  	ret = atomic_long_sub_return(nr, &page->pp_frag_count);
>  	WARN_ON(ret < 0);
> +
> +	/* We are the last user here too, reset pp_frag_count back to 1 to
> +	 * ensure all pages have been partitioned into 1 piece initially,
> +	 * this should be the rare case when the last two fragment users call
> +	 * page_pool_defrag_page() currently.
> +	 */
> +	if (unlikely(!ret))
> +		atomic_long_set(&page->pp_frag_count, 1);
> +
>  	return ret;
>  }
>  
 
 [....]

 Thanks
 /Ilias