Same issue here, with the same workaround for me:
Edit /etc/default/grub.d/kdump-tools.cfg
Change the crashkernel part to read
crashkernel=512M-:256M
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https://bugs.launchpad.net/bugs/1908090
Title:
ubuntu 20.04 kdump fails
Status in kexec-tools package in Ubuntu:
Confirmed
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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