kexec may fail with kernel panic when crashkernel parameter is passed

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Environment

SUSE Linux Enterprise Server 15 Service Pack 1 (SLES 15 SP1)

Situation

With kexec boot, system could fail to boot further with below error:

Kernel panic - not syncing: ERROR: Failed to allocate 0x0000000000004000 bytes below 0x0000000020000000.

CPU: 0 PID: 0 Comm: swapper Not tainted 4.12.14-135.gbb021ec-default #1 SLE15-SP1 (unreleased)

Call Trace:

[c00000000122fcf0] [c000000000a19368] dump_stack+0xb0/0xf0 (unreliable)

[c00000000122fd30] [c000000000a15efc] panic+0x144/0x31c

[c00000000122fdc0] [c000000000d3c8e4] memblock_alloc_base+0x44/0x58

This issue is seen on large configuration systems when "crashkernel=" parameter is passed to the kernel.

Resolution

Working on a fix to ensure the memory available in first memory block (other than
what is used for KDump) is larger than 128MB. This change can have impact on
older configurations. While a fix that works for all cases is arrived at, 
use an offset greater than 128MB for crashkernel during kexec boot. For example:

  kexec -l --append="`cat /proc/cmdline` crashkernel=X@160M" --initrd=/boot/initrd-`uname -r` /boot/vmlinux-`uname -r` 

where X is the memory to be reserved for KDump.

Cause

System resources need memory to be reserved in the first memory block. While the
size of first memory block is usually 512MB or greater, memory for crashkernel is
reserved at an offset of 128MB by default. This leaves less than 128MB memory to
reserve for system resources that need memory in the first memory block. On large
configuration systems this memory may not be sifficient, especially with kexec
boot where initrd is also placed below 128MB adding to these memory constraints
and eventually leading to allocation failure.

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