Date: Mon, 14 Aug 2023 15:42:34 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
 kuba@...nel.org, pabeni@...hat.com
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 Lorenzo Bianconi <lorenzo@...nel.org>,
 Alexander Duyck <alexander.duyck@...il.com>,
 Liang Chen <liangchen.linux@...il.com>,
 Alexander Lobakin <aleksander.lobakin@...el.com>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 Ilias Apalodimas <ilias.apalodimas@...aro.org>,
 Eric Dumazet <edumazet@...gle.com>, Jonathan Corbet <corbet@....net>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 John Fastabend <john.fastabend@...il.com>, linux-doc@...r.kernel.org,
 bpf@...r.kernel.org
Subject: Re: [PATCH net-next v6 5/6] page_pool: update document about frag API

Hi--

On 8/14/23 05:56, Yunsheng Lin wrote:
> As more drivers begin to use the frag API, update the
> document about how to decide which API to use for the
> driver author.
> 
> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> CC: Lorenzo Bianconi <lorenzo@...nel.org>
> CC: Alexander Duyck <alexander.duyck@...il.com>
> CC: Liang Chen <liangchen.linux@...il.com>
> CC: Alexander Lobakin <aleksander.lobakin@...el.com>
> ---
>   Documentation/networking/page_pool.rst |  4 +-
>   include/net/page_pool/helpers.h        | 58 +++++++++++++++++++++++---
>   2 files changed, 55 insertions(+), 7 deletions(-)
> 

> diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
> index b920224f6584..0f1eaa2986f9 100644
> --- a/include/net/page_pool/helpers.h
> +++ b/include/net/page_pool/helpers.h
> @@ -8,13 +8,28 @@
>   /**
>    * DOC: page_pool allocator
>    *
> - * The page_pool allocator is optimized for the XDP mode that
> - * uses one frame per-page, but it can fallback on the
> - * regular page allocator APIs.
> + * The page_pool allocator is optimized for recycling page or page frag used by
> + * skb packet and xdp frame.
>    *
> - * Basic use involves replacing alloc_pages() calls with the
> - * page_pool_alloc_pages() call.  Drivers should use
> - * page_pool_dev_alloc_pages() replacing dev_alloc_pages().
> + * Basic use involves replacing napi_alloc_frag() and alloc_pages() calls with
> + * page_pool_cache_alloc() and page_pool_alloc(), which allocate memory with or
> + * without page splitting depending on the requested memory size.
> + *
> + * If the driver knows that it always requires full pages or its allocates are

                                                              allocations

> + * always smaller than half a page, it can use one of the more specific API
> + * calls:
> + *
> + * 1. page_pool_alloc_pages(): allocate memory without page splitting when
> + * driver knows that the memory it need is always bigger than half of the page
> + * allocated from page pool. There is no cache line dirtying for 'struct page'
> + * when a page is recycled back to the page pool.
> + *
> + * 2. page_pool_alloc_frag(): allocate memory with page splitting when driver
> + * knows that the memory it need is always smaller than or equal to half of the
> + * page allocated from page pool. Page splitting enables memory saving and thus
> + * avoid TLB/cache miss for data access, but there also is some cost to

       avoids

> + * implement page splitting, mainly some cache line dirtying/bouncing for
> + * 'struct page' and atomic operation for page->pp_frag_count.
>    *
>    * API keeps track of in-flight pages, in order to let API user know
>    * when it is safe to free a page_pool object.  Thus, API users
> @@ -100,6 +115,14 @@ static inline struct page *page_pool_alloc_frag(struct page_pool *pool,
>   	return __page_pool_alloc_frag(pool, offset, size, gfp);
>   }
>   
> +/**
> + * page_pool_dev_alloc_frag() - allocate a page frag.
> + * @pool[in]	pool from which to allocate
> + * @offset[out]	offset to the allocated page
> + * @size[in]	requested size

Please use kernel-doc syntax/notation here.

> + *
> + * Get a page frag from the page allocator or page_pool caches.
> + */
>   static inline struct page *page_pool_dev_alloc_frag(struct page_pool *pool,
>   						    unsigned int *offset,
>   						    unsigned int size)
> @@ -143,6 +166,14 @@ static inline struct page *page_pool_alloc(struct page_pool *pool,
>   	return page;
>   }
>   
> +/**
> + * page_pool_dev_alloc() - allocate a page or a page frag.
> + * @pool[in]:		pool from which to allocate
> + * @offset[out]:	offset to the allocated page
> + * @size[in, out]:	in as the requested size, out as the allocated size

and here.

> + *
> + * Get a page or a page frag from the page allocator or page_pool caches.
> + */
>   static inline struct page *page_pool_dev_alloc(struct page_pool *pool,
>   					       unsigned int *offset,
>   					       unsigned int *size)
> @@ -165,6 +196,13 @@ static inline void *page_pool_cache_alloc(struct page_pool *pool,
>   	return page_address(page) + offset;
>   }
>   
> +/**
> + * page_pool_dev_cache_alloc() - allocate a cache.
> + * @pool[in]:		pool from which to allocate
> + * @size[in, out]:	in as the requested size, out as the allocated size

and here.

> + *
> + * Get a cache from the page allocator or page_pool caches.
> + */
>   static inline void *page_pool_dev_cache_alloc(struct page_pool *pool,
>   					      unsigned int *size)
>   {
> @@ -316,6 +354,14 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>   	page_pool_put_full_page(pool, page, true);
>   }
>   
> +/**
> + * page_pool_cache_free() - free a cache into the page_pool
> + * @pool[in]:		pool from which cache was allocated
> + * @data[in]:		cache to free
> + * @allow_direct[in]:	freed by the consumer, allow lockless caching

and here.

> + *
> + * Free a cache allocated from page_pool_dev_cache_alloc().
> + */
>   static inline void page_pool_cache_free(struct page_pool *pool, void *data,
>   					bool allow_direct)
>   {

Thanks.

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