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

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Message-ID: <679d107b78924_2741f92949a@willemb.c.googlers.com.notmuch>
Date: Fri, 31 Jan 2025 13:03:39 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Yan Zhai <yan@...udflare.com>, 
 netdev@...r.kernel.org
Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 "David S. Miller" <davem@...emloft.net>, 
 David Ahern <dsahern@...nel.org>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Simon Horman <horms@...nel.org>, 
 Shuah Khan <shuah@...nel.org>, 
 Josh Hunt <johunt@...mai.com>, 
 Alexander Duyck <alexander.h.duyck@...ux.intel.com>, 
 netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 linux-kselftest@...r.kernel.org, 
 kernel-team@...udflare.com
Subject: Re: [PATCH v3 net] udp: gso: do not drop small packets when PMTU
 reduces

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@...udflare.com>
> Suggested-by: Willem de Bruijn <willemdebruijn.kernel@...il.com>

Reviewed-by: Willem de Bruijn <willemb@...gle.com>