Re: [PATCH v3 net] udp: gso: do not drop small packets when PMTU reduces

[Willem de Bruijn]

Yan Zhai wrote:
> Commit 4094871db1d6 ("udp: only do GSO if # of segs > 1") avoided GSO
> for small packets. But the kernel currently dismisses GSO requests only
> after checking MTU/PMTU on gso_size. This means any packets, regardless
> of their payload sizes, could be dropped when PMTU becomes smaller than
> requested gso_size. We encountered this issue in production and it
> caused a reliability problem that new QUIC connection cannot be
> established before PMTU cache expired, while non GSO sockets still
> worked fine at the same time.
> 
> Ideally, do not check any GSO related constraints when payload size is
> smaller than requested gso_size, and return EMSGSIZE instead of EINVAL
> on MTU/PMTU check failure to be more specific on the error cause.
> 
> Fixes: 4094871db1d6 ("udp: only do GSO if # of segs > 1")
> Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
> Suggested-by: Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx>

Reviewed-by: Willem de Bruijn <willemb@xxxxxxxxxx>