On 2026-09-19 01:46:34 [+0200], Xin Xie wrote: > On 14/08/2026 03:21, Jakub Kicinski wrote: > > 1) could you explain in more detail why GRO is breaking things? > > GRO shouldn't merge packets with different L2/L2.5 and packets > > with trailers? > The reproducer is plain SAN traffic entering a PRP RedBox's > interlink: matching Ethernet headers, no RCT yet, so GRO can merge it. > PRP then appends one RCT to the aggregate, using its length for > LSDU_size and assigning one sequence number. If the aggregate LSDU > exceeds 4095, the 12-bit field truncates it. Later TCP segmentation > cannot generate per-segment RCTs. An aggregate left unsegmented may > instead be dropped for exceeding the lower device's frame-size limit. > Our wire captures showed segments without RCTs and duplicate delivery > through the two LANs.Disabling interlink GRO raised single-stream TCP > throughput from 27-37 Mbit/s to about 94 Mbit/s on the 100 Mbit/s rig. > > > 2) the code seems inspired by LRO but LRO also has > > NETIF_F_UPPER_DISABLES. This change looks rather incomplete. > > Calling it "best effort" seems like an excuse. > NETIF_F_UPPER_DISABLES prevents a lower's ethtool request from > overriding an upper's LRO-off policy. GRO is outside that mask and > remains changeable; extending the global mask would affect other > stacked devices too. Patch 1 sets the GRO-off default, while patch 3 > segments plain, trailer-free aggregates before tagging, including > when GRO is re-enabled or GSO arrives directly. GRO_HW can also be > fixed on, so feature disabling alone cannot exclude all aggregates; > device-specific hardware-coalesced formats remain outside v6's > segmentation guarantee. I should have explained this distinction > from LRO explicitly.
Would adding a NETIF_F_HSR_SLAVE flag to slave device(s) and then filter invalid flags in netdev_fix_features() fix this? This should help against enabling and add a warning if a flag can not be removed. A netdev_change_features() might do the job then. Sebastian
