netdev - Re: Memory providers multiplexing (Was: [PATCH net-next v4 4/5] page_pool: remove PP_FLAG_PAGE

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Date: Mon, 3 Jul 2023 19:13:19 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: David Ahern <dsahern@...nel.org>
Cc: Mina Almasry <almasrymina@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Jesper Dangaard Brouer <jbrouer@...hat.com>, brouer@...hat.com, 
	Alexander Duyck <alexander.duyck@...il.com>, Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net, 
	pabeni@...hat.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Lorenzo Bianconi <lorenzo@...nel.org>, Yisen Zhuang <yisen.zhuang@...wei.com>, 
	Salil Mehta <salil.mehta@...wei.com>, Sunil Goutham <sgoutham@...vell.com>, 
	Geetha sowjanya <gakula@...vell.com>, Subbaraya Sundeep <sbhatta@...vell.com>, 
	hariprasad <hkelam@...vell.com>, Saeed Mahameed <saeedm@...dia.com>, 
	Leon Romanovsky <leon@...nel.org>, Felix Fietkau <nbd@....name>, Ryder Lee <ryder.lee@...iatek.com>, 
	Shayne Chen <shayne.chen@...iatek.com>, Sean Wang <sean.wang@...iatek.com>, 
	Kalle Valo <kvalo@...nel.org>, Matthias Brugger <matthias.bgg@...il.com>, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, Ilias Apalodimas <ilias.apalodimas@...aro.org>, 
	linux-rdma@...r.kernel.org, linux-wireless@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-mediatek@...ts.infradead.org, 
	Jonathan Lemon <jonathan.lemon@...il.com>
Subject: Re: Memory providers multiplexing (Was: [PATCH net-next v4 4/5]
 page_pool: remove PP_FLAG_PAGE_FRAG flag)

On Mon, Jul 3, 2023 at 4:45 PM David Ahern <dsahern@...nel.org> wrote:
>
> 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..aa4133d1b1e0676e408499ea4534b51262394432
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 = skb->len;
+       snaplen = skb->devmem ? skb_headlen(skb) : skb->len;

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

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

        res = 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.