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

[Aditya Garg <gargaditya@linux.microsoft.com>]

On 03-10-2025 21:45, Eric Dumazet wrote:
> 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.

Hi Eric,
Thank you for your review. I understand your concerns regarding the use 
of skb_linearize() in the xmit path, as it can fail under memory 
pressure and introduces additional overhead in the transmit path. Based 
on your input, I will work on a v2 that will move the SGE limit check to 
the ndo_features_check() path and for GSO skbs exceding the hw limit 
will disable the NETIF_F_GSO_MASK to enforce software segmentation in 
kernel before the call to xmit.
Also for non GSO skb exceeding the SGE hw limit should we go for using 
skb_linearize only then or would you suggest some other approach here?

Regards,
Aditya