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From:   Matteo Croce <mcroce@linux.microsoft.com>
Date:   Sat, 5 Jun 2021 18:34:43 +0200
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Subject: Re: [PATCH net-next v6 3/5] page_pool: Allow drivers to hint on SKB recycling
To:     David Ahern <dsahern@gmail.com>
Cc:     Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        Yunsheng Lin <linyunsheng@huawei.com>, netdev@vger.kernel.org,
        linux-mm@kvack.org, Ayush Sawal <ayush.sawal@chelsio.com>,
        Vinay Kumar Yadav <vinay.yadav@chelsio.com>,
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        Boris Pismenny <borisp@nvidia.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Saeed Mahameed <saeedm@nvidia.com>,
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        Sven Auhagen <sven.auhagen@voleatech.de>
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On Sat, Jun 5, 2021 at 6:06 PM David Ahern <dsahern@gmail.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