netdev - Re: [PATCH net-next 00/10] Call skb_metadata_set when skb->data points past metadata

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Message-ID: <ad835a9b-e544-48d3-b6e2-ffe172fcfa6d@linux.dev>
Date: Thu, 22 Jan 2026 12:21:21 -0800
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Jakub Sitnicki <jakub@...udflare.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, Michael Chan <michael.chan@...adcom.com>,
 Pavan Chebbi <pavan.chebbi@...adcom.com>, Andrew Lunn
 <andrew+netdev@...n.ch>, Tony Nguyen <anthony.l.nguyen@...el.com>,
 Przemek Kitszel <przemyslaw.kitszel@...el.com>,
 Saeed Mahameed <saeedm@...dia.com>, Leon Romanovsky <leon@...nel.org>,
 Tariq Toukan <tariqt@...dia.com>, Mark Bloch <mbloch@...dia.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>, intel-wired-lan@...ts.osuosl.org,
 bpf@...r.kernel.org, kernel-team@...udflare.com,
 Jakub Kicinski <kuba@...nel.org>, Amery Hung <ameryhung@...il.com>
Subject: Re: [PATCH net-next 00/10] Call skb_metadata_set when skb->data
 points past metadata

On 1/13/26 4:33 AM, Jakub Sitnicki wrote:
> Good point. I'm hoping we don't have to allocate from
> skb_metadata_set(), which does sound prohibitively expensive. Instead
> we'd allocate the extension together with the skb if we know upfront
> that metadata will be used.

[ Sorry for being late. Have been catching up after holidays. ]

For the sk local storage (which was mentioned in other replies as making 
skb metadata to look more like sk local storage), there is a plan (Amery 
has been looking into it) to allocate the storage together with sk for 
performance reason. This means allocating a larger 'struct sock'. The 
extra space will be at the front of sk instead of the end of sk because 
of how the 'struct sock' is embedded in tcp_sock/udp_sock/... If skb is 
going in the same direction, it should be useful to have a similar 
scheme on: upfront allocation and then shared by multiple BPF progs.

The current thinking is to built upon the existing bpf_sk_local_storage 
usage. A boot param decides how much BPF space should be allocated for 
'struct sock'. When a bpf_sk_storage_map is created (with a new 
use_reserve flag), the space will be allocated permanently from the head 
space of every sk for this map. The read (from a BPF prog) will be at 
one stable offset before a sk. If there is no more head space left, the 
map creation will fail. User can decide if it wants to retry without the 
'use_reserve' flag.