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

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Message-ID: <CAFnufp3rWwFgknBUBy9mHB36zpTKRiTeUAFeJXKVvp2DzvG3bw@mail.gmail.com>
Date:   Sat, 5 Jun 2021 18:34:43 +0200
From:   Matteo Croce <mcroce@...ux.microsoft.com>
To:     David Ahern <dsahern@...il.com>
Cc:     Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Yunsheng Lin <linyunsheng@...wei.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

On Sat, Jun 5, 2021 at 6:06 PM David Ahern <dsahern@...il.com> 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.

We could create a new memory model, e.g. MEM_TYPE_PAGE_SPLIT, and
restore the behavior present in the previous versions of this serie,
which is, save xdp_mem_info in struct page.
As this could slightly impact the performances, this can be added in a
future change when the drivers which are doing it want to use this
recycling api.

-- 
per aspera ad upstream