Subject: Re: [PATCH net-next 1/2] page_pool: unify frag page and non-frag page handling

From: Ilias Apalodimas <[email-masked]>
Date: Fri, 26 May 2023 15:03:51 +0300

Hi Yunsheng

Apologies for not replying to the RFC,  I was pretty busy with internal
stuff

On Fri, May 26, 2023 at 05:26:14PM +0800, Yunsheng Lin wrote:
> Currently page_pool_dev_alloc_pages() can not be called
> when PP_FLAG_PAGE_FRAG is set, because it does not use
> the frag reference counting.
>
> As we are already doing a optimization by not updating
> page->pp_frag_count in page_pool_defrag_page() for the
> last frag user, and non-frag page only have one user,
> so we utilize that to unify frag page and non-frag page
> handling, so that page_pool_dev_alloc_pages() can also
> be called with PP_FLAG_PAGE_FRAG set.

What happens here is clear.  But why do we need this?  Do you have a
specific use case in mind where a driver will call
page_pool_dev_alloc_pages() and the PP_FLAG_PAGE_FRAG will be set?
If that's the case isn't it a better idea to unify the functions entirely?

Thanks
/Ilias
>
> Signed-off-by: Yunsheng Lin <[email-masked]>
> CC: Lorenzo Bianconi <[email-masked]>
> CC: Alexander Duyck <[email-masked]>
> ---
>  include/net/page_pool.h | 38 +++++++++++++++++++++++++++++++-------
>  net/core/page_pool.c    |  1 +
>  2 files changed, 32 insertions(+), 7 deletions(-)
>
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index c8ec2f34722b..ea7a0c0592a5 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -50,6 +50,9 @@
>  				 PP_FLAG_DMA_SYNC_DEV |\
>  				 PP_FLAG_PAGE_FRAG)
>
> +#define PAGE_POOL_DMA_USE_PP_FRAG_COUNT \
> +		(sizeof(dma_addr_t) > sizeof(unsigned long))
> +
>  /*
>   * Fast allocation side cache array/stack
>   *
> @@ -295,13 +298,20 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page,
>   */
>  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);
>  }
>
> +/* We need to reset frag_count back to 1 for the last user to allow
> + * only one user in case the page is recycled and allocated as non-frag
> + * page.
> + */
>  static inline long page_pool_defrag_page(struct page *page, long nr)
>  {
>  	long ret;
>
> +	BUILD_BUG_ON(__builtin_constant_p(nr) && nr != 1);
> +
>  	/* If nr == pp_frag_count then we have cleared all remaining
>  	 * references to the page. No need to actually overwrite it, instead
>  	 * we can leave this to be overwritten by the calling function.
> @@ -311,19 +321,36 @@ static inline long page_pool_defrag_page(struct page *page, long nr)
>  	 * especially when dealing with a page that may be partitioned
>  	 * into only 2 or 3 pieces.
>  	 */
> -	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() as above, only need to handle
> +		 * the non-constant case here for frag count draining.
> +		 */
> +		if (!__builtin_constant_p(nr))
> +			atomic_long_set(&page->pp_frag_count, 1);
> +
>  		return 0;
> +	}
>
>  	ret = atomic_long_sub_return(nr, &page->pp_frag_count);
>  	WARN_ON(ret < 0);
> +
> +	/* Reset frag count back to 1, this should be the rare case when
> +	 * two users call page_pool_defrag_page() currently.
> +	 */
> +	if (!ret)
> +		atomic_long_set(&page->pp_frag_count, 1);
> +
>  	return ret;
>  }
>
>  static inline bool page_pool_is_last_frag(struct page_pool *pool,
>  					  struct page *page)
>  {
> -	/* If fragments aren't enabled or count is 0 we were the last user */
> -	return !(pool->p.flags & PP_FLAG_PAGE_FRAG) ||
> +	/* When dma_addr_upper is overlapped with pp_frag_count
> +	 * or we were the last page frag user.
> +	 */
> +	return PAGE_POOL_DMA_USE_PP_FRAG_COUNT ||
>  	       (page_pool_defrag_page(page, 1) == 0);
>  }
>
> @@ -357,9 +384,6 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>  	page_pool_put_full_page(pool, page, true);
>  }
>
> -#define PAGE_POOL_DMA_USE_PP_FRAG_COUNT	\
> -		(sizeof(dma_addr_t) > sizeof(unsigned long))
> -
>  static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>  {
>  	dma_addr_t ret = page->dma_addr;
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index e212e9d7edcb..0868aa8f6323 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -334,6 +334,7 @@ static void page_pool_set_pp_info(struct page_pool *pool,
>  {
>  	page->pp = pool;
>  	page->pp_magic |= PP_SIGNATURE;
> +	page_pool_fragment_page(page, 1);
>  	if (pool->p.init_callback)
>  		pool->p.init_callback(page, pool->p.init_arg);
>  }
> --
> 2.33.0
>