linux-kernel - Re: [PATCH v5 RFC 5/6] page_pool: update document about frag API

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Date:   Thu, 29 Jun 2023 13:30:42 -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 v5 RFC 5/6] page_pool: update document about frag API

Hi--

On 6/29/23 05:02, 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.
> 
> Also it seems there is a similar document in page_pool.h,
> so remove it to avoid the duplication.
> 
> 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 | 34 ++++++++++++++++++++++----
>  include/net/page_pool.h                | 22 -----------------
>  2 files changed, 29 insertions(+), 27 deletions(-)
> 
> diff --git a/Documentation/networking/page_pool.rst b/Documentation/networking/page_pool.rst
> index 873efd97f822..18b13d659c98 100644
> --- a/Documentation/networking/page_pool.rst
> +++ b/Documentation/networking/page_pool.rst
> @@ -4,12 +4,27 @@
>  Page Pool API
>  =============
>  
> -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.

That sentence could use some adjectives. Choose singular or plural:

> +The page_pool allocator is optimized for recycling a page or page frag used by an skb
> +packet or xdp frame.

or

> +The page_pool allocator is optimized for recycling pages or page frags used by skb
> +packets or xdp frames.

Now that I have written them, I prefer the latter one (plural). FWIW.

>  
> -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 are

> +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

                                                                     and thus avoids

> +   TLB/cache miss for data access, but there also is some cost to 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
> @@ -93,6 +108,15 @@ a page will cause no race conditions is enough.
>  * page_pool_dev_alloc_pages(): Get a page from the page allocator or page_pool
>    caches.
>  
> +* page_pool_dev_alloc_frag(): Get a page frag from the page allocator or
> +  page_pool caches.
> +
> +* page_pool_dev_alloc(): Get a page or page frag from the page allocator or
> +  page_pool caches.
> +
> +* page_pool_dev_cache_alloc(): Get a cache from the page allocator or page_pool
> +  caches.
> +
>  * page_pool_get_dma_addr(): Retrieve the stored DMA address.
>  
>  * page_pool_get_dma_dir(): Retrieve the stored DMA direction.

Thanks for adding the documentation.

-- 
~Randy