Enabling PUNKTFUNK_GSO on a host whose egress MTU is below our UDP segment size
made every GSO send return EMSGSIZE (code 90, "Message too long") — the kernel
validates each GSO segment against the device MTU at send time, which plain
sendmmsg does not. EMSGSIZE wasn't in gso_unsupported() (nor is_transient_io), so
it propagated as a fatal "send failed — stopping stream" and instantly killed
every session the moment GSO was on (observed live: connection fails instantly /
speed-test 0 Mbps).
Add EMSGSIZE to gso_unsupported() so it latches GSO off for the process and
finishes via sendmmsg — the standard "GSO not usable on this path" fallback.
Measured after: the same host+path does 1 Gbps at 0.0% loss over the real LAN via
sendmmsg (and the host send path sustains a 2 Gbps probe with send_dropped=0), so
GSO is a >2 Gbps optimization, not required for 1 Gbps.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
enricobuehler
62613055c1