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Re: [PATCH] net: add per device sg_max_frags for skb

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On 01/06/2016 03:05 PM, Eric Dumazet wrote:
> On Wed, 2016-01-06 at 14:16 +0100, Hans Westgaard Ry wrote:
>> Devices may have limits on the number of fragments in an skb they
>> support. Current codebase uses a constant as maximum for number of
>> fragments (MAX_SKB_FRAGS) one skb can hold and use.
>>
>> When enabling scatter/gather and running traffic with many small
>> messages the codebase uses the maximum number of fragments and thereby
>> violates the max for certain devices.
>>
>> An example of such a violation is when running IPoIB on a HCA
>> supporting 16 SGE on an architecture with 4K pagesize. The
>> MAX_SKB_FRAGS will be 17 (64K/4K+1) and because IPoIB adds yet another
>> segment we end up with send_requests with 18 SGE resulting in
>> kernel-panic.
>>
>> The patch allows the device to limit the maximum number fragments used
>> in one skb.
>>
>> The functionality corresponds to gso_max_size/gso_max_segs for gso.
> Unfortunately this is not the right place to fix this issue.
>
> Think about forwarding workloads, where the SKB is cooked by GRO engine.
>
> Anyway, local TCP stack uses 32KB page fragments, so typical skb has no
> more than 3 frags.
>
> Look at ndo_features_check(), where the problematic device driver can
> add its logic.
>
>
>
I've had a look at ndo_features_check and understand that I could supply 
my own
version of the routine, but I wasn't able to figure out how that would 
solve my problem.
As far as I can see the routine is not called in the part of code 
handling scatter/gather.
Could you help out with more info?


Hans

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[PATCH] net: add per device sg_max_frags for skb Hans Westgaard Ry <hans.westgaard.ry@oracle.com> - 2016-01-06 14:20 +0100
  RE: [PATCH] net: add per device sg_max_frags for skb David Laight <David.Laight@ACULAB.COM> - 2016-01-06 15:10 +0100
    Re: [PATCH] net: add per device sg_max_frags for skb Hans Westgaard Ry <hans.westgaard.ry@oracle.com> - 2016-01-08 11:00 +0100
      RE: [PATCH] net: add per device sg_max_frags for skb David Laight <David.Laight@ACULAB.COM> - 2016-01-08 11:40 +0100
      Re: [PATCH] net: add per device sg_max_frags for skb Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-01-08 12:50 +0100
        Re: [PATCH] net: add per device sg_max_frags for skb Hans Westgaard Ry <hans.westgaard.ry@oracle.com> - 2016-01-13 15:00 +0100
          Re: [PATCH] net: add per device sg_max_frags for skb Eric Dumazet <edumazet@google.com> - 2016-01-13 15:30 +0100
          Re: [PATCH] net: add per device sg_max_frags for skb Eric Dumazet <edumazet@google.com> - 2016-01-13 15:30 +0100
            Re: [PATCH] net: add per device sg_max_frags for skb Hannes Frederic Sowa <hannes@stressinduktion.org> - 2016-01-13 16:10 +0100
            Re: [PATCH] net: add per device sg_max_frags for skb David Miller <davem@davemloft.net> - 2016-01-13 16:40 +0100
              Re: [PATCH] net: add per device sg_max_frags for skb Eric Dumazet <edumazet@google.com> - 2016-01-13 16:50 +0100
          Re: [PATCH] net: add per device sg_max_frags for skb ebiederm@xmission.com (Eric W. Biederman) - 2016-01-13 22:20 +0100
  Re: [PATCH] net: add per device sg_max_frags for skb Eric Dumazet <eric.dumazet@gmail.com> - 2016-01-06 15:10 +0100
    Re: [PATCH] net: add per device sg_max_frags for skb Hans Westgaard Ry <hans.westgaard.ry@oracle.com> - 2016-01-08 11:10 +0100

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