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Message-ID: <9101bb07-70f1-476c-bec9-ec67e9899744@linux.ibm.com>
Date: Fri, 2 Feb 2024 10:53:23 +0530
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Subject: Re: [PATCH v2 00/14] Split crash out from kexec and clean up related
 config items
To: Baoquan He <bhe@redhat.com>, linux-kernel@vger.kernel.org
Cc: kexec@lists.infradead.org, x86@kernel.org,
        linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org,
        linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
        linux-mips@vger.kernel.org, linux-riscv@lists.infradead.org,
        loongarch@lists.linux.dev, akpm@linux-foundation.org,
        ebiederm@xmission.com, piliu@redhat.com, viro@zeniv.linux.org.uk
References: <20240119145241.769622-1-bhe@redhat.com>
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From: Hari Bathini <hbathini@linux.ibm.com>
In-Reply-To: <20240119145241.769622-1-bhe@redhat.com>
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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@intel.com/
> 
> 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