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From:   Eric Dumazet <edumazet@google.com>
Date:   Mon, 3 Jul 2023 19:13:19 +0200
Message-ID: <CANn89iKAvrf92Fy8a_M+V9eya6OHokey2_yxQ3JiCT87fKND_w@mail.gmail.com>
Subject: Re: Memory providers multiplexing (Was: [PATCH net-next v4 4/5]
 page_pool: remove PP_FLAG_PAGE_FRAG flag)
To:     David Ahern <dsahern@kernel.org>
Cc:     Mina Almasry <almasrymina@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Jesper Dangaard Brouer <jbrouer@redhat.com>,
        brouer@redhat.com, Alexander Duyck <alexander.duyck@gmail.com>,
        Yunsheng Lin <linyunsheng@huawei.com>, davem@davemloft.net,
        pabeni@redhat.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Lorenzo Bianconi <lorenzo@kernel.org>,
        Yisen Zhuang <yisen.zhuang@huawei.com>,
        Salil Mehta <salil.mehta@huawei.com>,
        Sunil Goutham <sgoutham@marvell.com>,
        Geetha sowjanya <gakula@marvell.com>,
        Subbaraya Sundeep <sbhatta@marvell.com>,
        hariprasad <hkelam@marvell.com>,
        Saeed Mahameed <saeedm@nvidia.com>,
        Leon Romanovsky <leon@kernel.org>,
        Felix Fietkau <nbd@nbd.name>,
        Ryder Lee <ryder.lee@mediatek.com>,
        Shayne Chen <shayne.chen@mediatek.com>,
        Sean Wang <sean.wang@mediatek.com>,
        Kalle Valo <kvalo@kernel.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
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        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org,
        Jonathan Lemon <jonathan.lemon@gmail.com>
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On Mon, Jul 3, 2023 at 4:45=E2=80=AFPM David Ahern <dsahern@kernel.org> wro=
te:
>
> On 7/3/23 12:22 AM, Mina Almasry wrote:
> > tcpdump is able to access the header of these skbs which is in host
> > memory, but not the payload in device memory. Here is an example
> > session with my netcat-like test for device memory TCP:
> > https://pastebin.com/raw/FRjKf0kv
> >
> > tcpdump seems to work, and the length of the packets above is correct.
> > tcpdump -A however isn't able to print the payload of the packets:
> > https://pastebin.com/raw/2PcNxaZV
>
> That is my expectation. The tcpdump is just an easy example of accessing
> the skb page frags. skb_copy_and_csum_bits used by icmp is another
> example that can walk frags wanting access to device memory. You did not
> cause a panic or trip a WARN_ON for example with the tcpdump?
>

Change for af_packet was not too hard :)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index a2dbeb264f260e5b8923ece9aac99fe19ddfeb62..aa4133d1b1e0676e408499ea453=
4b51262394432
100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2152,7 +2152,7 @@ static int packet_rcv(struct sk_buff *skb,
struct net_device *dev,
                }
        }

-       snaplen =3D skb->len;
+       snaplen =3D skb->devmem ? skb_headlen(skb) : skb->len;

        res =3D run_filter(skb, sk, snaplen);
        if (!res)
@@ -2275,7 +2275,7 @@ static int tpacket_rcv(struct sk_buff *skb,
struct net_device *dev,
                }
        }

-       snaplen =3D skb->len;
+       snaplen =3D skb->devmem ? skb_headlen(skb) : skb->len;

        res =3D run_filter(skb, sk, snaplen);
        if (!res)


And a generic change in pskb_may_pull() ( __pskb_pull_tail() more
exactly) was enough to cover most other cases.