Re: [PATCH net-next] net: mana: Linearize SKB if TX SGEs exceeds hardware limit

Eric Dumazet
2025-10-03T16:15:13Z

On Fri, Oct 3, 2025 at 8:47 AM Aditya Garg
<gargaditya@linux.microsoft.com> wrote:
>
> The MANA hardware supports a maximum of 30 scatter-gather entries (SGEs)
> per TX WQE. In rare configurations where MAX_SKB_FRAGS + 2 exceeds this
> limit, the driver drops the skb. Add a check in mana_start_xmit() to
> detect such cases and linearize the SKB before transmission.
>
> Return NETDEV_TX_BUSY only for -ENOSPC from mana_gd_post_work_request(),
> send other errors to free_sgl_ptr to free resources and record the tx
> drop.
>
> Signed-off-by: Aditya Garg <gargaditya@linux.microsoft.com>
> Reviewed-by: Dipayaan Roy <dipayanroy@linux.microsoft.com>
> ---
> drivers/net/ethernet/microsoft/mana/mana_en.c | 26 +++++++++++++++----
> include/net/mana/gdma.h | 8 +++++-
> include/net/mana/mana.h | 1 +
> 3 files changed, 29 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c b/drivers/net/ethernet/microsoft/mana/mana_en.c
> index f4fc86f20213..22605753ca84 100644
> --- a/drivers/net/ethernet/microsoft/mana/mana_en.c
> +++ b/drivers/net/ethernet/microsoft/mana/mana_en.c
> @@ -20,6 +20,7 @@
>
> #include <net/mana/mana.h>
> #include <net/mana/mana_auxiliary.h>
> +#include <linux/skbuff.h>
>
> static DEFINE_IDA(mana_adev_ida);
>
> @@ -289,6 +290,19 @@ netdev_tx_t mana_start_xmit(struct sk_buff *skb, struct net_device *ndev)
> cq = &apc->tx_qp[txq_idx].tx_cq;
> tx_stats = &txq->stats;
>
> + BUILD_BUG_ON(MAX_TX_WQE_SGL_ENTRIES != MANA_MAX_TX_WQE_SGL_ENTRIES);
> + #if (MAX_SKB_FRAGS + 2 > MANA_MAX_TX_WQE_SGL_ENTRIES)
> + if (skb_shinfo(skb)->nr_frags + 2 > MANA_MAX_TX_WQE_SGL_ENTRIES) {
> + netdev_info_once(ndev,
> + "nr_frags %d exceeds max supported sge limit. Attempting skb_linearize\n",
> + skb_shinfo(skb)->nr_frags);
> + if (skb_linearize(skb)) {

This will fail in many cases.

This sort of check is better done in ndo_features_check()

Most probably this would occur for GSO packets, so can ask a software
segmentation
to avoid this big and risky kmalloc() by all means.

Look at idpf_features_check() which has something similar.