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

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Message-ID: <20251004093805.GB3060232@horms.kernel.org>
Date: Sat, 4 Oct 2025 10:38:05 +0100
From: Simon Horman <horms@...nel.org>
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,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	longli@...rosoft.com, kotaranov@...rosoft.com,
	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 03, 2025 at 08:47:24AM -0700, Aditya Garg 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)

Hi Aditya,

I see that Eric has made a more substantial review of this patch,
so please follow his advice.

But I wanted to add something to keep in mind for the future: I if the #if
/ #else used here can be replaced by a simple if() statement, then that
would be preferable.  The advantage being that it improves compile
coverage.  And, as these are all constants, I would expect the compiler to
optimise away any unused code.

N.B: I did not check, so please consider this more of a general statement

> +		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)) {
> +				netdev_warn_once(ndev, "Failed to linearize skb\n");
> +				goto tx_drop_count;
> +			}
> +		}
> +	#endif
> +
>  	pkg.tx_oob.s_oob.vcq_num = cq->gdma_id;
>  	pkg.tx_oob.s_oob.vsq_frame = txq->vsq_frame;
>  

...