netdev - Re: [RFC bpf-next v1 2/7] bpf: Allow bpf_xdp_shrink_data to shrink a frag from head and tail

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Message-ID: <20250904185241.607552f7@kernel.org>
Date: Thu, 4 Sep 2025 18:52:41 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Amery Hung <ameryhung@...il.com>
Cc: Nimrod Oren <noren@...dia.com>, bpf@...r.kernel.org,
 netdev@...r.kernel.org, alexei.starovoitov@...il.com, andrii@...nel.org,
 daniel@...earbox.net, martin.lau@...nel.org, mohsin.bashr@...il.com,
 saeedm@...dia.com, tariqt@...dia.com, mbloch@...dia.com,
 maciej.fijalkowski@...el.com, kernel-team@...a.com, Dragos Tatulea
 <dtatulea@...dia.com>, Gal Pressman <gal@...dia.com>
Subject: Re: [RFC bpf-next v1 2/7] bpf: Allow bpf_xdp_shrink_data to shrink
 a frag from head and tail

On Thu, 4 Sep 2025 15:19:28 -0700 Amery Hung wrote:
> On Thu, Aug 28, 2025 at 6:44 AM Nimrod Oren <noren@...dia.com> wrote:
> > On 25/08/2025 22:39, Amery Hung wrote:  
> > > Move skb_frag_t adjustment into bpf_xdp_shrink_data() and extend its
> > > functionality to be able to shrink an xdp fragment from both head and
> > > tail. In a later patch, bpf_xdp_pull_data() will reuse it to shrink an
> > > xdp fragment from head.  
> > I had assumed that XDP multi-buffer frags must always be the same size,
> > except for the last one. If that’s the case, shrinking from the head
> > seems to break this rule.  
> 
> I am not aware of the assumption in the code. Is this documented somewhere?

There's no such rule. Perhaps conflating frags with segments after TSO.