4.15 kernel ip_vs --ops causes performance and hang problem

=== SRU Justification ===
[Impact]
From the commit message:
"Connections in One-packet scheduling mode (-o, --ops) are
removed with refcnt=0 because they are not hashed in conn table."

[Fix]
From the commit message:
"To avoid refcount_dec reporting this as error, change them to be
removed with refcount_dec_if_one as all other connections."

[Test]
The bug reporter has a reproducer and confirmed this commit fixes the
issue.

[Regression Potential]
Low. Fix for a specific use case, and it's in upstream for a while.

=== Original Bug Report ===
On our 16.04LTS (and earlier) systems we used the ipvsadm --ops UDP
support (one-packet scheduling) to get a better distribution amongst
our real servers behind the load-balancer for some small subset of
applications.

This has worked fine through the 4.4.0-xxx kernels. But when we started
a program to upgrade systems to use the 4.15 series of kernels to take
advantage of new facilities, the subset of systems which used the --ops
option ran into problems. Everything else with the 4.15 kernels appeared
to work well.

This issue was reported in #1817247 against 16.04LTS with the HWE 4.15 kernel
but has not received any acknowledgement after having been reported weeks
ago. So we have moved on to confirm that a stock 18.04LTS system with the
latest expected/standard 4.15 kernel also has this issue as well and report
that here. Perhaps this will get more attention.

The issue appears to have been the change in the ip_vs module from using
"atomic_*()" increment/decrement functions in the 4.4 kernel to instead
use "refcount_*()" functions in a later kernel, including the 4.15 one
we switched to. Unfortunately, the simple refcount_dec() function was
inadequate, in putting out a time-consuming message and handling when
the refcount dropped to zero, which is expected in the case of --ops
support that retains no state post packet delivery. I will upload an
attachment with the sample messages that get put out at packet arrival
rate, which destroys performance of course. This test VM reports the
identical errors we see in our production servers, but at least throwing
only a couple of test --ops packets at it doesn't crash/hang the 18.04 system
as it did in the 16.04 VM reported earlier. And in production, with the
far greater packet rates, our systems fail since the attached call backtrace
*** appears on every packet!! ***

This issue was apparently already recognized as an error and has appeared
as a fix in upstream kernels. This is a reference to the 4.17 version
of the fix that we'd like to see incorporated into the next possible
kernel maintenance release:

https://github.com/torvalds/linux/commit/a050d345cef0dc6249263540da1e902bba617e43#diff-75923493f6e3f314b196a8223b0d6342

We have successfully used the livepatch facility to build a livepatch .ko
with the above diffs on our 4.15.0-36 system and successfully demonstrated
the contrast in good/bad behavior with/without the livepatch module
loaded. But we'd rather not have to build a version of livepatch.ko for
each kernel maintenance release, such as the 4.5.0-46 kernel here used to
demonstrate the issue persists in the Ubuntu mainline distro.

The problem is easy to generate, with only a couple of packets
and a simple configuration. Here's a very basic test (addresses
rewritten/obscured) version of an example configuration for 2 servers
that worked on my test VM:

ipvsadm -A -f 100 -s rr --ops
ipvsadm -a -f 100 -r 10.129.131.227:0 -g -w 9999
ipvsadm -a -f 100 -r 10.129.131.228:0 -g -w 9999
iptables -t mangle -A PREROUTING -d 172.16.5.1/32 -j MARK --set-xmark 0x64/0xffffffff
ifconfig lo:0 172.16.5.1/32 up

Routing and addressing to achieve the above, or adaptation for one's
own test environment, is left to the tester. I just added alias 10.129.131.x
addresses on my "outbound" interface and a static route for 172.16.5.1 to my
client system so the test packets arrived on the "inbound" interface.

I set up routing and addresses on my 2 NIC test such that packets arrived
on my test machine's eth1 NIC and were directed by ip_vs out the eth2. To
test, all I did was throw a few UDP packets via traceroute at the address
on the iptables/firewall mark rule so that the eth1 interface of the
test system was the traceroute system's default gateway:

  traceroute -m 2 172.16.5.1

Without the fix my test ip_vs system either hangs or puts out messages
as per the attached. With our livepatch module using the above commit's
contents, all is well. Both of the test ("real" as opposed to "virtual")
servers configured above via ipvsadm, get packets and no errors are
reported in the logs.

Let me know of anything I can do to help accelerate addressing of this
issue or understanding. It seems that the fix incorporation is fairly
straightforward, and is a performance disaster without it for anyone
using the --ops facility to any significant degree.

Thanks!

$ lsb_release -rd
Description: Ubuntu 18.04.2 LTS
Release: 18.04
$ uname -a
Linux direct-18-04 4.15.0-46-generic #49-Ubuntu SMP Wed Feb 6 09:33:07 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: linux-image-4.15.0-46-generic 4.15.0-46.49
ProcVersionSignature: Ubuntu 4.15.0-46.49-generic 4.15.18
Uname: Linux 4.15.0-46-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: marc 1980 F.... pulseaudio
CurrentDesktop: ubuntu:GNOME
Date: Tue Mar 12 13:14:31 2019
InstallationDate: Installed on 2018-08-01 (222 days ago)
InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
Lsusb:
 Bus 001 Device 003: ID 0e0f:0002 VMware, Inc. Virtual USB Hub
 Bus 001 Device 002: ID 0e0f:0003 VMware, Inc. Virtual Mouse
 Bus 001 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
MachineType: VMware, Inc. VMware Virtual Platform
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcFB: 0 svgadrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-46-generic root=UUID=103a235a-d768-45d6-9f14-c7414f72320d ro quiet splash nopti nospectre_v2 nospec_store_bypass_disable
RelatedPackageVersions:
 linux-restricted-modules-4.15.0-46-generic N/A
 linux-backports-modules-4.15.0-46-generic N/A
 linux-firmware 1.173.3
RfKill:

SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 05/20/2014
dmi.bios.vendor: Phoenix Technologies LTD
dmi.bios.version: 6.00
dmi.board.name: 440BX Desktop Reference Platform
dmi.board.vendor: Intel Corporation
dmi.board.version: None
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 1
dmi.chassis.vendor: No Enclosure
dmi.chassis.version: N/A
dmi.modalias: dmi:bvnPhoenixTechnologiesLTD:bvr6.00:bd05/20/2014:svnVMware,Inc.:pnVMwareVirtualPlatform:pvrNone:rvnIntelCorporation:rn440BXDesktopReferencePlatform:rvrNone:cvnNoEnclosure:ct1:cvrN/A:
dmi.product.name: VMware Virtual Platform
dmi.product.version: None
dmi.sys.vendor: VMware, Inc.