netdev - [PATCH v5 net-next 0/3] ipv6: avoid atomic fragment on GSO output

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Date: Tue, 24 Oct 2023 07:26:28 -0700
From: Yan Zhai <yan@...udflare.com>
To: netdev@...r.kernel.org
Cc: "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>,
	Aya Levin <ayal@...dia.com>, Tariq Toukan <tariqt@...dia.com>,
	linux-kernel@...r.kernel.org, kernel-team@...udflare.com,
	Florian Westphal <fw@...len.de>,
	Willem de Bruijn <willemdebruijn.kernel@...il.com>,
	Alexander H Duyck <alexander.duyck@...il.com>
Subject: [PATCH v5 net-next 0/3] ipv6: avoid atomic fragment on GSO output

When the ipv6 stack output a GSO packet, if its gso_size is larger than
dst MTU, then all segments would be fragmented. However, it is possible
for a GSO packet to have a trailing segment with smaller actual size
than both gso_size as well as the MTU, which leads to an "atomic
fragment". Atomic fragments are considered harmful in RFC-8021. An
Existing report from APNIC also shows that atomic fragments are more
likely to be dropped even it is equivalent to a no-op [1].

The series contains following changes:
* drop feature RTAX_FEATURE_ALLFRAG, which has been broken. This helps
  simplifying other changes in this set.
* refactor __ip6_finish_output code to separate GSO and non-GSO packet
  processing, mirroring IPv4 side logic.
* avoid generating atomic fragment on GSO packets.

Link: https://www.potaroo.net/presentations/2022-03-01-ipv6-frag.pdf [1]

change log:
V4 -> V5: minor fixup
V3 -> V4: cleaned up all RTAX_FEATURE_ALLFRAG code, rather than just
drop the check at IPv6 output.
V2 -> V3: split the changes to separate commits as Willem de Bruijn suggested
V1 is incorrect and omitted

V4: https://lore.kernel.org/netdev/cover.1698114636.git.yan@cloudflare.com/
V3: https://lore.kernel.org/netdev/cover.1697779681.git.yan@cloudflare.com/
V2: https://lore.kernel.org/netdev/ZS1%2Fqtr0dZJ35VII@debian.debian/

Yan Zhai (3):
  ipv6: drop feature RTAX_FEATURE_ALLFRAG
  ipv6: refactor ip6_finish_output for GSO handling
  ipv6: avoid atomic fragment on GSO packets

 include/net/dst.h                  |  7 -----
 include/net/inet_connection_sock.h |  1 -
 include/net/inet_sock.h            |  1 -
 include/uapi/linux/rtnetlink.h     |  2 +-
 net/ipv4/tcp_output.c              | 20 +------------
 net/ipv6/ip6_output.c              | 45 ++++++++++++++++--------------
 net/ipv6/tcp_ipv6.c                |  1 -
 net/ipv6/xfrm6_output.c            |  2 +-
 net/mptcp/subflow.c                |  1 -
 9 files changed, 27 insertions(+), 53 deletions(-)

-- 
2.30.2