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

Aditya Garg
2025-10-08T15:16:20Z

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