Messages in this thread | | | Subject | Re: [PATCH] net: add per device sg_max_frags for skb | From | Hans Westgaard Ry <> | Date | Fri, 8 Jan 2016 11:01:49 +0100 |
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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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