Re: [PATCH net-next 6/7] net: devmem: size TX net_iovs from the dmabuf sg table

[Mina Almasry]

On Mon, Nov 2, 2026 at 11:14 AM Stanislav Fomichev <sdf.kernel@xxxxxxxxx> wrote:
>
> PAGE_SIZE TX net_iovs cap devmem skbs at MAX_SKB_FRAGS pages (~68KB),
> which makes BIG TCP ineffective.
>
> TX net_iovs only need to be DMA contiguous. Use the largest power of 2,
> up to 2MB, which aligns every DMA segment of the dmabuf:
> - 2MB hugepage-backed udmabufs get 2MB net_iovs
> - no UAPI change, TX addresses are dmabuf offsets
> - skbs are still bounded by TCP's size_goal, only frags get larger
>
> Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxxx>
> ---
>  net/core/devmem.c      | 28 ++++++++++++++++++++++++++--
>  net/core/netdev-genl.c |  2 +-
>  2 files changed, 27 insertions(+), 3 deletions(-)
>
> diff --git a/net/core/devmem.c b/net/core/devmem.c
> index 19c039ed2cb2..98c9594dbf99 100644
> --- a/net/core/devmem.c
> +++ b/net/core/devmem.c
> @@ -181,6 +181,20 @@ int net_devmem_bind_dmabuf_to_queue(struct net_device *dev, u32 rxq_idx,
>         return err;
>  }
>
> +static bool net_devmem_sgt_aligned(struct sg_table *sgt, size_t size)
> +{
> +       struct scatterlist *sg;
> +       unsigned int i;
> +
> +       for_each_sgtable_dma_sg(sgt, sg, i) {
> +               if (!IS_ALIGNED(sg_dma_address(sg), size) ||
> +                   !IS_ALIGNED(sg_dma_len(sg), size))
> +                       return false;
> +       }
> +       return true;
> +}
> +
> +/* niov_shift == 0 picks the net_iov size from the dmabuf, TX only */
>  struct net_devmem_dmabuf_binding *
>  net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
>                        struct device *dma_dev,
> @@ -191,11 +205,11 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
>                        struct netlink_ext_ack *extack)
>  {
>         struct net_devmem_dmabuf_binding *binding;
> -       size_t niov_size = 1UL << niov_shift;
>         static u32 id_alloc_next;
>         struct scatterlist *sg;
>         struct dma_buf *dmabuf;
>         unsigned int sg_idx;
> +       size_t niov_size;
>         size_t niov_idx;
>         size_t i;
>         long err;
> @@ -221,7 +235,6 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
>         binding->tx_max_frag_size = dev->netmem_tx_max_frag_size;
>         write_pnet(&binding->net, sock_net(nl_sk));
>         binding->uid = sk_uid(nl_sk);
> -       binding->niov_shift = niov_shift;
>         xa_init_flags(&binding->bound_rxqs, XA_FLAGS_ALLOC);
>
>         err = percpu_ref_init(&binding->ref,
> @@ -266,6 +279,17 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev,
>                 goto err_unmap;
>         }
>
> +       if (!niov_shift) {
> +               niov_size = SZ_2M;
> +               while (niov_size > PAGE_SIZE &&
> +                      (!IS_ALIGNED(dmabuf->size, niov_size) ||
> +                       !net_devmem_sgt_aligned(binding->sgt, niov_size)))
> +                       niov_size /= 2;
> +               niov_shift = ilog2(niov_size);
> +       }
> +       binding->niov_shift = niov_shift;
> +       niov_size = 1UL << niov_shift;
> +

How come we're not checknig the largest supported size of the netdev here?

Also, maybe should clarify in the cl with the largest supported size,
is it actually only powers of 2 up to that power of 2? Or all sizes up
to that (not-necassirly) power of 2 size?

-- 
Thanks,
Mina