netdev - Re: [PATCH net-next v4 2/5] page_pool: unify frag_count handling in page_pool_is_last

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Date: Tue, 13 Jun 2023 21:33:17 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Yunsheng Lin <linyunsheng@...wei.com>
Cc: <davem@...emloft.net>, <pabeni@...hat.com>, <netdev@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, Lorenzo Bianconi <lorenzo@...nel.org>,
 Alexander Duyck <alexander.duyck@...il.com>, Jesper Dangaard Brouer
 <hawk@...nel.org>, Ilias Apalodimas <ilias.apalodimas@...aro.org>, Eric
 Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next v4 2/5] page_pool: unify frag_count handling in
 page_pool_is_last_frag()

On Mon, 12 Jun 2023 21:02:53 +0800 Yunsheng Lin wrote:
>  static inline void page_pool_fragment_page(struct page *page, long nr)
>  {
> -	atomic_long_set(&page->pp_frag_count, nr);
> +	if (!PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> +		atomic_long_set(&page->pp_frag_count, nr);

why not let the driver take references on the page count in this case?
I'm not saying it's worth the effort, but a comment may be useful?

> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 9c4118c62997..69e3c5175236 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -352,6 +352,14 @@ static void page_pool_set_pp_info(struct page_pool *pool,
>  {
>  	page->pp = pool;
>  	page->pp_magic |= PP_SIGNATURE;
> +
> +	/* Ensuring all pages have been split into one big frag initially:

Again, I find the "one big frag" slightly confusing.
Maybe we should rename pp_frag_cnt into pp_refcnt?
After this series is looks even more like a page pool reference
count rather than some form of number of fragments.

> +	 * page_pool_set_pp_info() is only called once for every page when it
> +	 * is allocated from the page allocator and page_pool_fragment_page()
> +	 * is dirtying the same cache line as the page->pp_magic above, so
> +	 * the overhead is negligible.
> +	 */
> +	page_pool_fragment_page(page, 1);
>  	if (pool->p.init_callback)
>  		pool->p.init_callback(page, pool->p.init_arg);
>  }