netdev - Re: [net PATCH] skb: Do mix page pool and page referenced frags in GRO

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Date:   Sat, 28 Jan 2023 09:08:44 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Yunsheng Lin <linyunsheng@...wei.com>, nbd@....name,
        davem@...emloft.net, hawk@...nel.org, ilias.apalodimas@...aro.org,
        linux-kernel@...r.kernel.org, lorenzo@...nel.org,
        netdev@...r.kernel.org, pabeni@...hat.com
Subject: Re: [net PATCH] skb: Do mix page pool and page referenced frags in GRO

On Fri, Jan 27, 2023 at 11:16 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Sat, Jan 28, 2023 at 6:26 AM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Sat, 28 Jan 2023 10:37:47 +0800 Yunsheng Lin wrote:
> > > If we are not allowing gro for the above case, setting NAPI_GRO_CB(p)->flush
> > > to 1 in gro_list_prepare() seems to be making more sense so that the above
> > > case has the same handling as skb_has_frag_list() handling?
> > > https://elixir.bootlin.com/linux/v6.2-rc4/source/net/core/gro.c#L503
> > >
> > > As it seems to avoid some unnecessary operation according to comment
> > > in tcp4_gro_receive():
> > > https://elixir.bootlin.com/linux/v6.2-rc4/source/net/ipv4/tcp_offload.c#L322
> >
> > The frag_list case can be determined with just the input skb.
> > For pp_recycle we need to compare input skb's pp_recycle with
> > the pp_recycle of the skb already held by GRO.
> >
> > I'll hold off with applying a bit longer tho, in case Eric
> > wants to chime in with an ack or opinion.
>
> Doing the test only if the final step (once all headers have been
> verified) seems less costly
> for the vast majority of the cases the driver cooks skbs with a
> consistent pp_recycle bit ?
>
> So Alex patch seems less expensive to me than adding the check very early.

That was the general idea. Basically there is no need to look into
this until we are looking at merging the skb and it is very unlikely
that we will see a mix of page pool and non-page pool skbs. I
considered this check to be something equivalent to discovering there
is no space in the skb to store the frags so that is one of the
reasons why I had picked the spot that I did.