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 08:45:38 -0600
From: David Ahern <dsahern@...nel.org>
To: Mina Almasry <almasrymina@...gle.com>
Cc: 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>,
 Eric Dumazet <edumazet@...gle.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 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?

Thanks for checking.