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

Eric Dumazet
2025-10-17T18:06:35Z

On Fri, Oct 17, 2025 at 10:41 AM Aditya Garg
<gargaditya@linux.microsoft.com> wrote:
>
> 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.

You must be doing something very wrong.

Difference between TSO and non TSO should not be that high.

ethtool -K eth0 tso on
netperf -H tjbp27
MIGRATED TCP STREAM TEST from ::0 (::) port 0 AF_INET6 to
tjbp27.prod.google.com () port 0 AF_INET6
Recv Send Send
Socket Socket Message Elapsed
Size Size Size Time Throughput
bytes bytes bytes secs. 10^6bits/sec

540000 262144 262144 10.00 92766.69

ethtool -K eth0 tso off
netperf -H tjbp27
MIGRATED TCP STREAM TEST from ::0 (::) port 0 AF_INET6 to
tjbp27.prod.google.com () port 0 AF_INET6
Recv Send Send
Socket Socket Message Elapsed
Size Size Size Time Throughput
bytes bytes bytes secs. 10^6bits/sec

540000 262144 262144 10.00 52218.97

Now if I force linearization, you can definitely see the very high
cost of the copies !

ethtool -K eth1 sg off
tjbp26:/home/edumazet# ./netperf -H tjbp27
MIGRATED TCP STREAM TEST from ::0 (::) port 0 AF_INET6 to
tjbp27.prod.google.com () port 0 AF_INET6
Recv Send Send
Socket Socket Message Elapsed
Size Size Size Time Throughput
bytes bytes bytes secs. 10^6bits/sec

540000 262144 262144 10.00 16951.32

>
> Hence, We propose to linearizing skbs until the first failure occurs.

Hmm... basically hiding a bug then ?

> 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.

Please post your patch (adding the check in ndo_features_check()),
perhaps one of us is able to help.