> -----Original Message-----
> From: Zaremba, Larysa <[email protected]>
> Sent: Tuesday, February 17, 2026 2:25 PM
> To: [email protected]
> Cc: Zaremba, Larysa <[email protected]>; Claudiu Manoil
> <[email protected]>; Vladimir Oltean <[email protected]>;
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> Kicinski <[email protected]>; Paolo Abeni <[email protected]>; Nguyen,
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> [email protected]; [email protected]; intel-wired-
> [email protected]; [email protected]; Loktionov,
> Aleksandr <[email protected]>; Dragos Tatulea
> <[email protected]>
> Subject: [PATCH bpf v3 1/9] xdp: use modulo operation to calculate XDP
> frag tailroom
>
> The current formula for calculating XDP tailroom in mbuf packets works
> only if each frag has its own page (if rxq->frag_size is PAGE_SIZE),
> this defeats the purpose of the parameter overall and without any
> indication leads to negative calculated tailroom on at least half of
> frags, if shared pages are used.
>
> There are not many drivers that set rxq->frag_size. Among them:
> * i40e and enetc always split page uniformly between frags, use shared
> pages
> * ice uses page_pool frags via libeth, those are power-of-2 and
> uniformly
> distributed across page
> * idpf has variable frag_size with XDP on, so current API is not
> applicable
> * mlx5, mtk and mvneta use PAGE_SIZE or 0 as frag_size for page_pool
>
> As for AF_XDP ZC, only ice, i40e and idpf declare frag_size for it.
> Modulo operation yields good results for aligned chunks, they are all
> power-of-2, between 2K and PAGE_SIZE. Formula without modulo fails
> when chunk_size is 2K. Buffers in unaligned mode are not distributed
> uniformly, so modulo operation would not work.
>
> To accommodate unaligned buffers, we could define frag_size as data +
> tailroom, and hence do not subtract offset when calculating tailroom,
> but this would necessitate more changes in the drivers.
>
> Define rxq->frag_size as an even portion of a page that fully belongs
> to a single frag. When calculating tailroom, locate the data start
> within such portion by performing a modulo operation on page offset.
>
> Fixes: bf25146a5595 ("bpf: add frags support to the
> bpf_xdp_adjust_tail() API")
> Acked-by: Jakub Kicinski <[email protected]>
> Signed-off-by: Larysa Zaremba <[email protected]>
> ---
> net/core/filter.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/filter.c b/net/core/filter.c index
> ba019ded773d..5f5489665c58 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4156,7 +4156,8 @@ static int bpf_xdp_frags_increase_tail(struct
> xdp_buff *xdp, int offset)
> if (!rxq->frag_size || rxq->frag_size > xdp->frame_sz)
> return -EOPNOTSUPP;
>
> - tailroom = rxq->frag_size - skb_frag_size(frag) -
> skb_frag_off(frag);
> + tailroom = rxq->frag_size - skb_frag_size(frag) -
> + skb_frag_off(frag) % rxq->frag_size;
> if (unlikely(offset > tailroom))
> return -EINVAL;
>
> --
> 2.52.0
Reviewed-by: Aleksandr Loktionov <[email protected]>
