Kernel Oops: Rsyncing to bcache device w/o backing cache kernel panic

Platform details:

1) version.log attached
2) lspci-vnvn.log attached
3) lsb_release -rd
Description: Ubuntu 18.04.5 LTS
Release: 18.04
4) apt-cache policy bcache-tools
bcache-tools:
  Installed: 1.0.8-2ubuntu0.18.04.1
  Candidate: 1.0.8-2ubuntu0.18.04.1
  Version table:
 *** 1.0.8-2ubuntu0.18.04.1 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1.0.8-2build1 500
        500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages

5) uname -a:
Linux timber4 4.15.0-117-generic #118-Ubuntu SMP Fri Sep 4 20:02:41 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

Short problem description:

Rsyncing from a 1.4TB home dir to a an new filesystem on a bcache device without an attached cache eventually results in kernel panic.

What I expected to happen: no kernel panic.

Long problem description:

1) Devices / Filesystems:

bfd01: 8*4TB configured as RAID6 using md LVM2 volume group.
bfd02: 8*4TB configured as RAID6 using Dell PERC H810 LVM2 volume group.

The tree contains 1069685 files / dirs.

2) Steps to reproduce:

# Make the device, format and mount it
make-bcache -B /dev/vg-bfd02/delme01_bc
mkfs.xfs -f -L delme01bc /dev/bcache0
mount /dev/bcache0 /dev/bfd02/delme01bc

# Rsync
flags="-a --delete" ; info_cmd="--info=progress2" ; excludes="";
src=/mnt/bfd01/delme01 ; tgt=/mnt/bfd02/delme01bc/ ;
time rsync $flags $info_cmd $excludes $src/ $tgt/
...
# eventually kernel panic

Using incremental rsync it will kernel panic after reading about 21GB.

# If instead I use non-incremental rsync it will kernel panic after reading about 1.2-1.3TB.

flags="--no-inc-recursive -ax -HAXS --delete" ; info_cmd="--info=progress2" ; excludes="";
...
# eventually kernel panic

Additional detail:

1) I was *not* able to reproduce if bfd01/delm01bc was a target and bfd02 was the source e.g.

# Rsync
flags="-a --delete" ; info_cmd="--info=progress2" ; excludes="";
src=/mnt/bfd02/delme01 ; tgt=/mnt/bfd01/delme01bc/ ;
time rsync $flags $info_cmd $excludes $src/ $tgt/
...
# rsync completes successfully

2) I was *not* able to reproduce if I attached and then detached a cache set:

ceho 'd3b93488-714e-4efa-af94-cd80fd2db11f' > /sys/block/bcache0/bcache/attach
echo 1 > /sys/block/bcache0/bcache/detach
...
time rsync $flags $info_cmd $excludes $src/ $tgt/
...
# rsync completes successfully

I have 11 compressed kernel crash dumps. I will upload 3 after filing this. Let me know if you want the other 8.

ProblemType: KernelCrash
DistroRelease: Ubuntu 18.04
Package: linux-image-4.15.0-117-generic 4.15.0-117.118
ProcVersionSignature: Ubuntu 4.15.0-117.118-generic 4.15.18
Uname: Linux 4.15.0-117-generic x86_64
AlsaDevices:
 total 0
 crw-rw---- 1 root audio 116, 1 Sep 13 12:53 seq
 crw-rw---- 1 root audio 116, 33 Sep 13 12:53 timer
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay': 'aplay'
ApportVersion: 2.20.9-0ubuntu7.17
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord': 'arecord'
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
Date: Sat Sep 12 09:51:37 2020
HibernationDevice: RESUME=none
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig': 'iwconfig'
MachineType: Dell Inc. PowerEdge R620
PciMultimedia:

ProcFB: 0 mgadrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-117-generic root=UUID=de68d15b-3948-4666-9bc5-5cbecc83971c ro maybe-ubiquity crashkernel=512M-:512M
RelatedPackageVersions:
 linux-restricted-modules-4.15.0-117-generic N/A
 linux-backports-modules-4.15.0-117-generic N/A
 linux-firmware 1.173.19
RfKill: Error: [Errno 2] No such file or directory: 'rfkill': 'rfkill'
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 12/06/2019
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 2.9.0
dmi.board.name: 0KCKR5
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 23
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvr2.9.0:bd12/06/2019:svnDellInc.:pnPowerEdgeR620:pvr:rvnDellInc.:rn0KCKR5:rvrA00:cvnDellInc.:ct23:cvr:
dmi.product.name: PowerEdge R620
dmi.sys.vendor: Dell Inc.
---
ProblemType: Bug
AlsaDevices:
 total 0
 crw-rw---- 1 root audio 116, 1 Sep 13 12:53 seq
 crw-rw---- 1 root audio 116, 33 Sep 13 12:53 timer
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay': 'aplay'
ApportVersion: 2.20.9-0ubuntu7.17
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord': 'arecord'
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
DistroRelease: Ubuntu 18.04
HibernationDevice: RESUME=none
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig': 'iwconfig'
MachineType: Dell Inc. PowerEdge R620
Package: linux (not installed)
PciMultimedia:

ProcFB: 0 mgadrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-117-generic root=UUID=de68d15b-3948-4666-9bc5-5cbecc83971c ro maybe-ubiquity crashkernel=512M-:512M
ProcVersionSignature: Ubuntu 4.15.0-117.118-generic 4.15.18
RelatedPackageVersions:
 linux-restricted-modules-4.15.0-117-generic N/A
 linux-backports-modules-4.15.0-117-generic N/A
 linux-firmware 1.173.19
RfKill: Error: [Errno 2] No such file or directory: 'rfkill': 'rfkill'
Tags: bionic uec-images
Uname: Linux 4.15.0-117-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups:

_MarkForUpload: True
dmi.bios.date: 12/06/2019
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 2.9.0
dmi.board.name: 0KCKR5
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 23
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvr2.9.0:bd12/06/2019:svnDellInc.:pnPowerEdgeR620:pvr:rvnDellInc.:rn0KCKR5:rvrA00:cvnDellInc.:ct23:cvr:
dmi.product.name: PowerEdge R620
dmi.sys.vendor: Dell Inc.
---
ProblemType: Bug
AlsaDevices:
 total 0
 crw-rw---- 1 root audio 116, 1 Sep 13 12:53 seq
 crw-rw---- 1 root audio 116, 33 Sep 13 12:53 timer
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay': 'aplay'
ApportVersion: 2.20.9-0ubuntu7.17
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord': 'arecord'
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
DistroRelease: Ubuntu 18.04
HibernationDevice: RESUME=none
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig': 'iwconfig'
MachineType: Dell Inc. PowerEdge R620
Package: linux (not installed)
PciMultimedia:

ProcFB: 0 mgadrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-117-generic root=UUID=de68d15b-3948-4666-9bc5-5cbecc83971c ro maybe-ubiquity crashkernel=512M-:512M
ProcVersionSignature: Ubuntu 4.15.0-117.118-generic 4.15.18
RelatedPackageVersions:
 linux-restricted-modules-4.15.0-117-generic N/A
 linux-backports-modules-4.15.0-117-generic N/A
 linux-firmware 1.173.19
RfKill: Error: [Errno 2] No such file or directory: 'rfkill': 'rfkill'
Tags: bionic uec-images
Uname: Linux 4.15.0-117-generic x86_64
UnreportableReason: This report is about a package that is not installed.
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups:

_MarkForUpload: False
dmi.bios.date: 12/06/2019
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 2.9.0
dmi.board.name: 0KCKR5
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 23
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvr2.9.0:bd12/06/2019:svnDellInc.:pnPowerEdgeR620:pvr:rvnDellInc.:rn0KCKR5:rvrA00:cvnDellInc.:ct23:cvr:
dmi.product.name: PowerEdge R620
dmi.sys.vendor: Dell Inc.