netdev - Re: [PATCH bpf 0/6] Address XDP frags having negative tailroom

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Message-ID: <20260204172628.165ac4c8@kernel.org>
Date: Wed, 4 Feb 2026 17:26:28 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Larysa Zaremba <larysa.zaremba@...el.com>
Cc: bpf@...r.kernel.org, Claudiu Manoil <claudiu.manoil@....com>, Vladimir
 Oltean <vladimir.oltean@....com>, Wei Fang <wei.fang@....com>, Clark Wang
 <xiaoning.wang@....com>, Andrew Lunn <andrew+netdev@...n.ch>, "David S.
 Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo
 Abeni <pabeni@...hat.com>, Tony Nguyen <anthony.l.nguyen@...el.com>,
 Przemek Kitszel <przemyslaw.kitszel@...el.com>, Alexei Starovoitov
 <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard
 Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>,
 Stanislav Fomichev <sdf@...ichev.me>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>, Yonghong Song
 <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, Hao Luo
 <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, Simon Horman
 <horms@...nel.org>, Shuah Khan <shuah@...nel.org>, Alexander Lobakin
 <aleksander.lobakin@...el.com>, Maciej Fijalkowski
 <maciej.fijalkowski@...el.com>, "Bastien Curutchet (eBPF Foundation)"
 <bastien.curutchet@...tlin.com>, Tushar Vyavahare
 <tushar.vyavahare@...el.com>, Jason Xing <kernelxing@...cent.com>, "Ricardo
 B. Marlière" <rbm@...e.com>, Eelco Chaudron
 <echaudro@...hat.com>, Lorenzo Bianconi <lorenzo@...nel.org>, Toke
 Hoiland-Jorgensen <toke@...hat.com>, imx@...ts.linux.dev,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 intel-wired-lan@...ts.osuosl.org, linux-kselftest@...r.kernel.org,
 Aleksandr Loktionov <aleksandr.loktionov@...el.com>
Subject: Re: [PATCH bpf 0/6] Address XDP frags having negative tailroom

On Tue,  3 Feb 2026 11:53:28 +0100 Larysa Zaremba wrote:
> Many ethernet drivers report xdp Rx queue frag size as being the same as
> DMA write size. However, the only user of this field, namely
> bpf_xdp_frags_increase_tail(), clearly expects a truesize.

truesize may not be the most fortunate term.
truesize is how much memory we charge to the skb (for memory management)
which may relate to frag size in strange ways for strange HW :S
I don't have a great term, but I have a feeling truesize already
confuses driver developers.

Note two our xdp.py tests covers tail adjust, I wonder if it would have
caught this for you. Looking at Intel's results the XDP tests don't run
due to missing BTF. Could you work with Adrian to address that?