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

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

On 08-10-2025 20:58, Aditya Garg wrote:
> On 08-10-2025 20:51, Eric Dumazet wrote:
>> On Wed, Oct 8, 2025 at 8:16 AM Aditya Garg
>> <gargaditya@linux.microsoft.com> wrote:
>>>
>>> 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?
>>
>> I think that for non GSO, the linearization attempt is fine.
>>
>> Note that this is extremely unlikely for non malicious users,
>> and MTU being usually small (9K or less),
>> the allocation will be much smaller than a GSO packet.
> 
> Okay. Will send a v2
Hi Eric,
I tested the code by disabling GSO in ndo_features_check when the number 
of SGEs exceeds the hardware limit, using iperf for a single TCP 
connection with zerocopy enabled. I noticed a significant difference in 
throughput compared to when we linearize the skbs.
For reference, the throughput is 35.6 Gbits/sec when using 
skb_linearize, but drops to 6.75 Gbits/sec when disabling GSO per skb.

Hence, We propose to  linearizing skbs until the first failure occurs. 
After that, we switch to a fail-safe mode by disabling GSO for SKBs with 
  sge > hw limit using the ndo_feature_check implementation, while 
continuing to apply  skb_linearize() for non-GSO packets that exceed the 
hardware limit. This ensures we remain on the optimal performance path 
initially, and only transition to the fail-safe path after encountering 
a failure.
Regards,
Aditya