Comment 2 for bug 1908090

From my notes:

=====
OK, I did the following and made some forward progress:

/etc/default/grub: added
 console=ttyS0
update-grub
reboot
virsh console vm843
echo 8 > /proc/sys/kernel/printk
rm -fr /var/crash/*
sync
echo c > /proc/sysrq-trigger

I now see the capture kernel panic messages.
=====

The important item here is the additional serial console which allowed me to view the messages in an xterm and scroll back (on a vnc console, there is no such ability).
eric

________________________________
From: <email address hidden> <email address hidden> on behalf of norman shen <email address hidden>
Sent: Wednesday, June 16, 2021 1:57 AM
To: Eric DeVolder <email address hidden>
Subject: [Bug 1908090] Re: ubuntu 20.04 kdump fails

hello, may I ask how did you capture the log? My kdump also stuck but
unfortunately does not print anything. the system looks dead though. the
vm got a memory size of 4GiB

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https://bugs.launchpad.net/bugs/1908090

Title:
  ubuntu 20.04 kdump fails

Status in kexec-tools package in Ubuntu:
  New

Bug description:
  When linux-crashdump (5.4.0.58.61) is enabled on Ubuntu 20.04 LTS,
  everything appears to be in good working order, according to
  "systemctl status kdump-tools" and "kdump-config status". However,
  upon an actual crash, the system hangs, and no crash files are
  produced. I've investigated and have learned that the capture kernel
  does indeed start, but it is unable to unpack the rootfs/initrd, and
  thus fails and hangs.

  [ 1.070469] Trying to unpack rootfs image as initramfs...
  [ 1.333182] swapper/0 invoked oom-killer: gfp_mask=0x100cc2(GFP_HIGHUSER), order=0, oom_score_adj=0
  [ 1.335074] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.4.0-26-generic #30-Ubuntu
  [ 1.336396] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
  [ 1.336396] Call Trace:
  [ 1.336396] dump_stack+0x6d/0x9a
  [ 1.336396] dump_header+0x4f/0x1eb
  [ 1.336396] out_of_memory.part.0.cold+0x39/0x83
  [ 1.336396] out_of_memory+0x6d/0xd0
  ...
  [ 1.413202] ---[ end Kernel panic - not syncing: System is deadlocked on memory ]---

  On this system with 8G of memory, the crash memory as specified on the kernel command line is "crashkernel=512M-:192M". I changed the 192M to 256M, and now kdump works.

  Not sure how the 192M value is chosen, but it does not work. I think
  this used value used to work for 16.04 and maybe 18.04 (I didn't try),
  but is no longer useful for 20.04.

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