netdev - Re: [PATCH net-next v6 3/5] page_pool: Allow drivers to hint on SKB recycling

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Date:   Mon, 7 Jun 2021 07:35:58 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     David Ahern <dsahern@...il.com>
Cc:     Yunsheng Lin <linyunsheng@...wei.com>,
        Matteo Croce <mcroce@...ux.microsoft.com>,
        netdev@...r.kernel.org, linux-mm@...ck.org,
        Ayush Sawal <ayush.sawal@...lsio.com>,
        Vinay Kumar Yadav <vinay.yadav@...lsio.com>,
        Rohit Maheshwari <rohitm@...lsio.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        Russell King <linux@...linux.org.uk>,
        Mirko Lindner <mlindner@...vell.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Boris Pismenny <borisp@...dia.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Yu Zhao <yuzhao@...gle.com>,
        Will Deacon <will@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Roman Gushchin <guro@...com>, Hugh Dickins <hughd@...gle.com>,
        Peter Xu <peterx@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexander Lobakin <alobakin@...me>,
        Cong Wang <cong.wang@...edance.com>, wenxu <wenxu@...oud.cn>,
        Kevin Hao <haokexin@...il.com>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Marco Elver <elver@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Guillaume Nault <gnault@...hat.com>,
        linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
        bpf@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Saeed Mahameed <saeedm@...dia.com>,
        Andrew Lunn <andrew@...n.ch>, Paolo Abeni <pabeni@...hat.com>,
        Sven Auhagen <sven.auhagen@...eatech.de>
Subject: Re: [PATCH net-next v6 3/5] page_pool: Allow drivers to hint on SKB
 recycling

Hi David,

On Sat, Jun 05, 2021 at 10:06:30AM -0600, David Ahern wrote:
> On 6/4/21 2:42 AM, Ilias Apalodimas wrote:
> > [...]
> >>> +	/* Driver set this to memory recycling info. Reset it on recycle.
> >>> +	 * This will *not* work for NIC using a split-page memory model.
> >>> +	 * The page will be returned to the pool here regardless of the
> >>> +	 * 'flipped' fragment being in use or not.
> >>> +	 */
> >>
> >> I am not sure I understand how does the last part of comment related
> >> to the code below, as there is no driver using split-page memory model
> >> will reach here because those driver will not call skb_mark_for_recycle(),
> >> right?
> >>
> > 
> > Yes the comment is there to prohibit people (mlx5 only actually) to add the
> > recycling bit on their driver.  Because if they do it will *probably* work
> > but they might get random corrupted packets which will be hard to debug.
> > 
> 
> What's the complexity for getting it to work with split page model?
> Since 1500 is the default MTU, requiring a page per packet means a lot
> of wasted memory.

It boils down to 'can we re-use the page or is someone using it'.
Yunsheng sent a patch in earlier series that implements this with
ref counters. As Matteo mentions we can also add another page pool type.

In theory none of those sound too hard, but we'll have to code it and see.

/Ilias