netdev - Re: [PATCH bpf] xdp: Fix spurious packet loss in generic XDP TX path

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Message-ID: <73774e57-64c3-e32d-d762-1fcf64d5628c@iogearbox.net>
Date:   Fri, 1 Jul 2022 15:29:21 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Eric Dumazet <edumazet@...gle.com>,
        Johan Almbladh <johan.almbladh@...finetworks.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>, song@...nel.org,
        martin.lau@...ux.dev, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>, haoluo@...gle.com,
        jolsa@...nel.org, bpf <bpf@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] xdp: Fix spurious packet loss in generic XDP TX path

On 7/1/22 11:57 AM, Eric Dumazet wrote:
> On Fri, Jul 1, 2022 at 11:43 AM Johan Almbladh
> <johan.almbladh@...finetworks.com> wrote:
>>
>> The byte queue limits (BQL) mechanism is intended to move queuing from
>> the driver to the network stack in order to reduce latency caused by
>> excessive queuing in hardware. However, when transmitting or redirecting
>> a packet with XDP, the qdisc layer is bypassed and there are no
>> additional queues. Since netif_xmit_stopped() also takes BQL limits into
>> account, but without having any alternative queuing, packets are
>> silently dropped.
>>
>> This patch modifies the drop condition to only consider cases when the
>> driver itself cannot accept any more packets. This is analogous to the
>> condition in __dev_direct_xmit(). Dropped packets are also counted on
>> the device.
> 
> This means XDP packets are able to starve other packets going through a qdisc,
> DDOS attacks will be more effective.
> 
> in-driver-XDP use dedicated TX queues, so they do not have this
> starvation issue.
> 
> This should be mentioned somewhere I guess.

+1, Johan, could you add this as comment and into commit description in a v2
of your fix? Definitely should be clarified that it's limited to generic XDP.

Thanks,
Daniel