linux-kernel - Re: [PATCH net-next v4 5/5] page_pool: update document about frag API

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Message-ID: <20230614095649.5f7d8d40@kernel.org>
Date:   Wed, 14 Jun 2023 09:56:49 -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>,
        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 v4 5/5] page_pool: update document about frag
 API

On Wed, 14 Jun 2023 20:04:39 +0800 Yunsheng Lin wrote:
> > Seems like the semantics of page_pool_alloc() are always better than
> > page_pool_alloc_frag(). Is there a reason to keep these two separate?  
> 
> I am agree the semantics of page_pool_alloc() is better, I was thinking
> about combining those two too.
> The reason I am keeping it is about the nic hw with fixed buffer size for
> each desc, and that buffer size is always smaller than or equal to half
> of the page allocated from page pool, so it doesn't bother doing the
> checking of 'size << 1 > max_size' and doesn't care about the actual
> truesize.

I see. Let's reorg the documentation, then? Something along the lines
of, maybe:

The page_pool allocator is optimized for recycling page or page frag used by skb
packet and xdp frame.

Basic use involves replacing napi_alloc_frag() and alloc_pages() calls
with page_pool_alloc(). page_pool_alloc() allocates 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 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 implement page
   splitting, mainly some cache line dirtying/bouncing for 'struct page' and
   atomic operation for page->pp_frag_count.