Comment 0 for bug 1896350

I'm trying to use nbd on ubuntu 20.04 like so:

qemu-img create -f qcow2 foo.img 500M
sudo modprobe nbd
sudo qemu-nbd --disconnect /dev/nbd15 || true
sudo qemu-nbd --connect=/dev/nbd15 --cache=writeback --format=qcow2 foo.img
sudo mkfs.ext4 -L root -O "^64bit" -E nodiscard /dev/nbd15
sudo qemu-nbd --disconnect /dev/nbd15

It seems to work on some systems, but even on those systems, strange things show up
in syslog, e.g.

Sep 19 15:46:02 thinky kernel: [27042.757768] block nbd15: Possible stuck request 00000000dd2d7f93: control (read@135168,126976B). Runtime 450 seconds
Sep 19 15:46:26 thinky kernel: [27067.333867] INFO: task systemd-udevd:17364 blocked for more than 241 seconds.

and in kern.log, e.g.

...
Sep 19 16:17:27 thinky kernel: [ 71.676026] Buffer I/O error on dev nbd15, logical block 7, async page read
Sep 19 16:17:27 thinky kernel: [ 71.676049] blk_update_request: I/O error, dev nbd15, sector 0 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
Sep 19 16:17:27 thinky kernel: [ 71.676051] Buffer I/O error on dev nbd15, logical block 0, async page read
Sep 19 16:17:27 thinky kernel: [ 71.676061] blk_update_request: I/O error, dev nbd15, sector 1 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
Sep 19 16:17:27 thinky kernel: [ 71.676063] Buffer I/O error on dev nbd15, logical block 1, async page read
Sep 19 16:17:27 thinky kernel: [ 71.676147] ldm_validate_partition_table(): Disk read failed.
Sep 19 16:17:27 thinky kernel: [ 71.676324] Dev nbd15: unable to read RDB block 0
Sep 19 16:17:27 thinky kernel: [ 71.676473] nbd15: unable to read partition table
Sep 19 16:17:27 thinky kernel: [ 71.678340] nbd15: detected capacity change from 0 to 524288000
Sep 19 16:17:27 thinky kernel: [ 71.678536] ldm_validate_partition_table(): Disk read failed.
Sep 19 16:17:27 thinky kernel: [ 71.678719] Dev nbd15: unable to read RDB block 0
Sep 19 16:17:27 thinky kernel: [ 71.678947] nbd15: unable to read partition table
Sep 19 16:17:27 thinky kernel: [ 71.679985] ldm_validate_partition_table(): Disk read failed.
Sep 19 16:17:27 thinky kernel: [ 71.680172] Dev nbd15: unable to read RDB block 0
Sep 19 16:17:27 thinky kernel: [ 71.680366] nbd15: unable to read partition table
Sep 19 16:17:27 thinky kernel: [ 71.680647] block nbd15: Attempted send on invalid socket

and the system seems to hang on reboot.

Also, on another system (an up to date mac, running ubuntu 20.04 in virtualbox),
the mkfs.ext4 fails, complaining the device is zero size.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: linux-image-5.4.0-47-generic 5.4.0-47.51
ProcVersionSignature: Ubuntu 5.4.0-47.51-generic 5.4.55
Uname: Linux 5.4.0-47-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27.6
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC1: dank 2948 F.... pulseaudio
 /dev/snd/controlC2: dank 2948 F.... pulseaudio
 /dev/snd/controlC0: dank 2948 F.... pulseaudio
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Sat Sep 19 16:28:43 2020
HibernationDevice: RESUME=UUID=7aecd5d3-2f5b-4c07-8867-801e63868a0f
InstallationDate: Installed on 2017-04-29 (1239 days ago)
InstallationMedia: Ubuntu 17.04 "Zesty Zapus" - Release amd64 (20170412)
MachineType: LENOVO ThinkServer TS140
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcFB: 0 VESA VGA
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.4.0-47-generic root=UUID=ff9a24e4-277d-4baf-819b-826a33d5bb5e ro quiet splash vt.handoff=7
RelatedPackageVersions:
 linux-restricted-modules-5.4.0-47-generic N/A
 linux-backports-modules-5.4.0-47-generic N/A
 linux-firmware 1.187.2
RfKill:

SourcePackage: linux
UpgradeStatus: Upgraded to focal on 2020-05-01 (141 days ago)
dmi.bios.date: 09/19/2014
dmi.bios.vendor: LENOVO
dmi.bios.version: FBKT99AUS
dmi.board.name: ThinkServer TS140
dmi.board.vendor: LENOVO
dmi.board.version: Not Defined
dmi.chassis.type: 7
dmi.chassis.vendor: LENOVO
dmi.chassis.version: To Be Filled By O.E.M.
dmi.modalias: dmi:bvnLENOVO:bvrFBKT99AUS:bd09/19/2014:svnLENOVO:pnThinkServerTS140:pvr70A4000HUX:rvnLENOVO:rnThinkServerTS140:rvrNotDefined:cvnLENOVO:ct7:cvrToBeFilledByO.E.M.:
dmi.product.family: To be filled by O.E.M.
dmi.product.name: ThinkServer TS140
dmi.product.sku: LENOVO_PN_ThinkServer TS140
dmi.product.version: 70A4000HUX
dmi.sys.vendor: LENOVO