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

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Date:   Tue, 13 Jun 2023 21:40:41 -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 Mon, 12 Jun 2023 21:02:56 +0800 Yunsheng Lin wrote:
> +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.
> +
> +3. page_pool_alloc(): allocate memory with or without page splitting depending
> +   on the requested memory size when driver doesn't know the size of memory it
> +   need beforehand. It is a mix of the above two case, so it is a wrapper of the
> +   above API to simplify driver's interface for memory allocation with least
> +   memory utilization and performance penalty.

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?