[PATCH net-next v2 0/4] netconsole: Support messages ratelimit-ing

[Breno Leitao <leitao@debian.org>]

netconsole hands every console message to every enabled target, with no
bound on the rate.

At Meta, a few hosts caused some DoS and netconsd was OOM killed in
some regions, fed by 3 billion RCU messages from thousand hosts and by
a swap fault that retries forever, which logged 6 million copies of one
line on a single host in a few hours.

Both floods are being fixed where they are printed, by Paul and me for RCU
Tasks [1][2] and by me for the swap fault fix [3], but that only ever
covers the flood already found, not new issues that might be happening.

Use the ratelimit in the kernel to ratelimit messages from netconsole.

Give each target a token bucket, consulted once per message so that a
message split into several ncfrag packets is sent whole or dropped
whole.

It starts with a zero interval, which struct ratelimit_state treats as
unlimited, so nothing changes until ratelimit_interval_ms and
ratelimit_burst are written.

What the bucket discards leaves nothing on the wire. An extended target
sees the loss as a gap in the sequence number the header carries:

	7.2.0-rc7-01460-g75848acaf136,13,628,252392647,-;netconsole selftest: netcons_AGujw 1
	7.2.0-rc7-01460-g75848acaf136,13,629,252392782,-;netconsole selftest: netcons_AGujw 2
	7.2.0-rc7-01460-g75848acaf136,13,678,254742125,-;netconsole selftest: netcons_AGujw 1

Crash output is exempt. The bucket is skipped while oops_in_progress is
set, so a limit configured for steady-state logging cannot cost a target
part of an oops or a panic.

[1] https://lore.kernel.org/all/anw8Qw8lfeIykski@gmail.com/
[2] https://lore.kernel.org/all/20260810-rcu_task_shrink_lruvec-v1-1-4d9f7d5251cb@debian.org/
[3] https://lore.kernel.org/all/20260813-swap-v2-0-4a625ccabdae@debian.org/

Signed-off-by: Breno Leitao <leitao@debian.org>
---
Changes in v2:
- Drop the patch that sent a "N messages dropped by rate limit" notice
  to the target. (Gustavo Luiz Duarte).
- Link to v1: https://patch.msgid.link/20260818-netcons_ratelimit-v1-0-8c5d2d17789c@debian.org

---
Breno Leitao (4):
      netconsole: add a per-target message rate limit
      netconsole: allow configuring the rate limit interval through configfs
      netconsole: allow configuring the rate limit burst through configfs
      docs: netconsole: document rate limit feature

 Documentation/networking/netconsole.rst | 38 +++++++++++++
 drivers/net/netconsole.c                | 99 +++++++++++++++++++++++++++++++++
 2 files changed, 137 insertions(+)
---
base-commit: c68a982815dcce5464e3bf2a31ac94f5146c04ca
change-id: 20260817-netcons_ratelimit-629b04a73c57

Best regards,
--  
Breno Leitao <leitao@debian.org>