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From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 1 Aug 2023 12:16:23 +0200
Message-ID: <CANn89iLghUDUSbNv-QOgyJ4dv5DhXGL60caeuVMnHW4HZQVJmg@mail.gmail.com>
Subject: Re: [PATCH v2] tun: avoid high-order page allocation for packet header
To:     Tahsin Erdogan <trdgn@amazon.com>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
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On Tue, Aug 1, 2023 at 11:37=E2=80=AFAM Eric Dumazet <edumazet@google.com> =
wrote:
>
> On Tue, Aug 1, 2023 at 1:07=E2=80=AFAM Tahsin Erdogan <trdgn@amazon.com> =
wrote:
> >
> > When GSO is not enabled and a packet is transmitted via writev(), all
> > payload is treated as header which requires a contiguous memory allocat=
ion.
> > This allocation request is harder to satisfy, and may even fail if ther=
e is
> > enough fragmentation.
> >
> > Note that sendmsg() code path limits the linear copy length, so this ch=
ange
> > makes writev() and sendmsg() more consistent.
> >
> > Signed-off-by: Tahsin Erdogan <trdgn@amazon.com>
> > ---
>
> I will have to tweak one existing packetdrill test, nothing major.
>
> Tested-by: Eric Dumazet <edumazet@google.com>
> Reviewed-by: Eric Dumazet <edumazet@google.com>

I have to take this back, sorry.

We need to change alloc_skb_with_frags() and tun.c to attempt
high-order allocations,
otherwise tun users sending very large buffers will regress.
(Even if this _could_ fail as you pointed out if memory is tight/fragmented=
)

I am working to make the change in alloc_skb_with_frags() and in tun,
we can apply your patch after this prereq.