linux-kernel - Re: [PATCH net-next 5/9] page_pool: don't use driver-set flags field directly

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Date:   Thu, 3 Aug 2023 16:56:22 +0200
From:   Alexander Lobakin <aleksander.lobakin@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     Yunsheng Lin <linyunsheng@...wei.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Larysa Zaremba <larysa.zaremba@...el.com>,
        Alexander Duyck <alexanderduyck@...com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        "Ilias Apalodimas" <ilias.apalodimas@...aro.org>,
        Simon Horman <simon.horman@...igine.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 5/9] page_pool: don't use driver-set flags field
 directly

From: Jakub Kicinski <kuba@...nel.org>
Date: Wed, 2 Aug 2023 14:29:20 -0700

> On Tue, 1 Aug 2023 15:36:33 +0200 Alexander Lobakin wrote:
>>>> You would need a separate patch to convert all the page_pool_create()
>>>> users then either way.
>>>> And it doesn't look really natural to me to pass both driver-set params
>>>> and driver-set flags as separate function arguments. Someone may then
>>>> think "why aren't flags just put in the params itself". The fact that
>>>> Page Pool copies the whole params in the page_pool struct after
>>>> allocating it is internals, page_pool_create() prototype however isn't.
>>>> Thoughts?  
>>>
>>> It just seems odd to me that dma_map and page_frag is duplicated as we
>>> seems to have the same info in the page_pool->p.flags.  
>>
>> It's just because we copy the whole &page_pool_params passed by the
>> driver. It doesn't look good to me to define a new structure and copy
>> the values field-by-field just to avoid duplicating 3 bits :s
> 
> FWIW I'm tempted to do something like the patch below (an obvious move,
> I suspect). I want to add another pointer (netdev) to the params and 

Just take napi->dev as I do in libie :)

> I don't want it to eat up bytes in the first cache line.
> The patch is incomplete, we need to stash a one-bit indication in 
> the first cache line to know init_callback is not present without
> having to look at @slow. I'll defer doing that cleanly until your
> patches land.

I would propose to include it in the series, but it has grown a bunch
already and it's better to do that later separately :s

> With this in place we can move flags outside of @fast, and interpret

Oh, really nice. We could avoid copying them at all.

> it manually while copying all the other members in one go.

[...]

Thanks,
Olek