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

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Message-ID: <CANn89iJwkbxC5HvSKmk807K-3HY+YR1kt-LhcYwnoFLAaeVVow@mail.gmail.com>
Date: Fri, 3 Oct 2025 09:15:00 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Aditya Garg <gargaditya@...ux.microsoft.com>
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org, 
	decui@...rosoft.com, andrew+netdev@...n.ch, davem@...emloft.net, 
	kuba@...nel.org, pabeni@...hat.com, longli@...rosoft.com, 
	kotaranov@...rosoft.com, horms@...nel.org, shradhagupta@...ux.microsoft.com, 
	ernis@...ux.microsoft.com, dipayanroy@...ux.microsoft.com, 
	shirazsaleem@...rosoft.com, linux-hyperv@...r.kernel.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-rdma@...r.kernel.org, gargaditya@...rosoft.com, 
	ssengar@...ux.microsoft.com
Subject: Re: [PATCH net-next] net: mana: Linearize SKB if TX SGEs exceeds
 hardware limit

On Fri, Oct 3, 2025 at 8:47 AM Aditya Garg
<gargaditya@...ux.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@...ux.microsoft.com>
> Reviewed-by: Dipayaan Roy <dipayanroy@...ux.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.