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: <CAMB2axOZW_t1y8_wQN=e-vx1LHWLA-CKnYDjVo_g6FcY9NQ5uA@mail.gmail.com>
Date: Thu, 4 Sep 2025 15:19:28 -0700
From: Amery Hung <ameryhung@...il.com>
To: Nimrod Oren <noren@...dia.com>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, alexei.starovoitov@...il.com, 
	andrii@...nel.org, daniel@...earbox.net, kuba@...nel.org, 
	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, 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?

Thanks,
Amery