Re: [PATCH v2 00/14] Split crash out from kexec and clean up related config items

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Hi Baoquan,

On 19/01/24 8:22 pm, Baoquan He wrote:
Motivation:
=============
Previously, LKP reported a building error. When investigating, it can't
be resolved reasonablly with the present messy kdump config items.

  https://lore.kernel.org/oe-kbuild-all/202312182200.Ka7MzifQ-lkp@xxxxxxxxx/

The kdump (crash dumping) related config items could causes confusions:

Firstly,
---
CRASH_CORE enables codes including
  - crashkernel reservation;
  - elfcorehdr updating;
  - vmcoreinfo exporting;
  - crash hotplug handling;

Now fadump of powerpc, kcore dynamic debugging and kdump all selects
CRASH_CORE, while fadump
  - fadump needs crashkernel parsing, vmcoreinfo exporting, and accessing
    global variable 'elfcorehdr_addr';
  - kcore only needs vmcoreinfo exporting;
  - kdump needs all of the current kernel/crash_core.c.

So only enabling PROC_CORE or FA_DUMP will enable CRASH_CORE, this
mislead people that we enable crash dumping, actual it's not.

Secondly,
---
It's not reasonable to allow KEXEC_CORE select CRASH_CORE.

Because KEXEC_CORE enables codes which allocate control pages, copy
kexec/kdump segments, and prepare for switching. These codes are
shared by both kexec reboot and kdump. We could want kexec reboot,
but disable kdump. In that case, CRASH_CORE should not be selected.

  --------------------
  CONFIG_CRASH_CORE=y
  CONFIG_KEXEC_CORE=y
  CONFIG_KEXEC=y
  CONFIG_KEXEC_FILE=y
     ---------------------

Thirdly,
---
It's not reasonable to allow CRASH_DUMP select KEXEC_CORE.

That could make KEXEC_CORE, CRASH_DUMP are enabled independently from
KEXEC or KEXEC_FILE. However, w/o KEXEC or KEXEC_FILE, the KEXEC_CORE
code built in doesn't make any sense because no kernel loading or
switching will happen to utilize the KEXEC_CORE code.
  ---------------------
  CONFIG_CRASH_CORE=y
  CONFIG_KEXEC_CORE=y
  CONFIG_CRASH_DUMP=y
  ---------------------

In this case, what is worse, on arch sh and arm, KEXEC relies on MMU,
while CRASH_DUMP can still be enabled when !MMU, then compiling error is
seen as the lkp test robot reported in above link.

  ------arch/sh/Kconfig------
  config ARCH_SUPPORTS_KEXEC
          def_bool MMU

  config ARCH_SUPPORTS_CRASH_DUMP
          def_bool BROKEN_ON_SMP
  ---------------------------

Changes:
===========
1, split out crash_reserve.c from crash_core.c;
2, split out vmcore_infoc. from crash_core.c;
3, move crash related codes in kexec_core.c into crash_core.c;
4, remove dependency of FA_DUMP on CRASH_DUMP;
5, clean up kdump related config items;
6, wrap up crash codes in crash related ifdefs on all 9 arch-es
    which support crash dumping;

Achievement:
===========
With above changes, I can rearrange the config item logic as below (the right
item depends on or is selected by the left item):

     PROC_KCORE -----------> VMCORE_INFO

                |----------> VMCORE_INFO
     FA_DUMP----|
                |----------> CRASH_RESERVE

FA_DUMP also needs PROC_VMCORE (CRASH_DUMP by dependency, I guess).
So, the FA_DUMP related changes here will need a relook..


                                                     ---->VMCORE_INFO
                                                    /
                                                    |---->CRASH_RESERVE
     KEXEC      --|                                /|
                  |--> KEXEC_CORE--> CRASH_DUMP-->/-|---->PROC_VMCORE
     KEXEC_FILE --|                               \ |
                                                    \---->CRASH_HOTPLUG


     KEXEC      --|
                  |--> KEXEC_CORE (for kexec reboot only)
     KEXEC_FILE --|

Test
========
On all 8 architectures, including x86_64, arm64, s390x, sh, arm, mips,
riscv, loongarch, I did below three cases of config item setting and
building all passed. Let me take configs on x86_64 as exampmle here:

(1) Both CONFIG_KEXEC and KEXEC_FILE is unset, then all kexec/kdump
items are unset automatically:
# Kexec and crash features
# CONFIG_KEXEC is not set
# CONFIG_KEXEC_FILE is not set
# end of Kexec and crash features

(2) set CONFIG_KEXEC_FILE and 'make olddefconfig':
---------------
# Kexec and crash features
CONFIG_CRASH_RESERVE=y
CONFIG_VMCORE_INFO=y
CONFIG_KEXEC_CORE=y
CONFIG_KEXEC_FILE=y
CONFIG_CRASH_DUMP=y
CONFIG_CRASH_HOTPLUG=y
CONFIG_CRASH_MAX_MEMORY_RANGES=8192
# end of Kexec and crash features
---------------

(3) unset CONFIG_CRASH_DUMP in case 2 and execute 'make olddefconfig':
------------------------
# Kexec and crash features
CONFIG_KEXEC_CORE=y
CONFIG_KEXEC_FILE=y
# end of Kexec and crash features
------------------------

Note:
For ppc, it needs investigation to make clear how to split out crash
code in arch folder.

On powerpc, both kdump and fadump need PROC_VMCORE & CRASH_DUMP.
Hope that clears things. So, patch 3/14 breaks things for FA_DUMP..

Hope Hari and Pingfan can help have a look, see if
it's doable. Now, I make it either have both kexec and crash enabled, or
disable both of them altogether.


Sure. I will take a closer look...

Thanks
Hari




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