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

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

On 08-10-2025 20:51, 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