[PATCH v22 5/8] x86/crash: add x86 crash hotplug support

[Eric DeVolder]

On 5/9/23 17:52, Thomas Gleixner wrote:
> On Wed, May 03 2023 at 18:41, Eric DeVolder wrote:
>> In the patch 'kexec: exclude elfcorehdr from the segment digest'
> 
> See reply to 8/8
yep

>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index 53bab123a8ee..80538524c494 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -2119,6 +2119,19 @@ config CRASH_DUMP
>>   	  (CONFIG_RELOCATABLE=y).
>>   	  For more details see Documentation/admin-guide/kdump/kdump.rst
>>   
>> +config CRASH_HOTPLUG
>> +	bool "Update the crash elfcorehdr on system configuration changes"
>> +	default y
>> +	depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG)
>> +	help
>> +	  Enable direct update to the crash elfcorehdr (which contains
>> +	  the list of CPUs and memory regions to be dumped upon a crash)
>> +	  in response to hot plug/unplug or online/offline of CPUs or
>> +	  memory. This is a much more advanced approach than userspace
>> +	  attempting that.
>> +
>> +	  If unsure, say Y.
> 
> Why is this config an X86 specific thing?
> 
> Neither CRASH_DUMP nor HOTPLUG_CPU nor MEMORY_HOTPLUG are in any way X86
> specific at all. So why can't you stick that into a place where it can
> be reused by other architectures?
> 
> It's not rocket science to do
> 
> +	depends on WANTS_CRASH_HOTPLUG && CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG)
> 
> or something like that. It's so tiring to have x86 Kconfig be the dump
> ground for the initial implementation, then having the sh*t copied to
> every other architecture and the cleanup is left to the maintainers.
> 
> It's not rocket science to differentiate between a real architecture
> specific option and a generally useful option in the first place, right?

Right. To your point, CRASH_DUMP has been copied in all the archs:
arch/arm/Kconfig:config CRASH_DUMP
arch/arm64/Kconfig:config CRASH_DUMP
arch/ia64/Kconfig:config CRASH_DUMP
arch/mips/Kconfig:config CRASH_DUMP
arch/powerpc/Kconfig:config CRASH_DUMP
arch/riscv/Kconfig:config CRASH_DUMP
arch/s390/Kconfig:config CRASH_DUMP
arch/sh/Kconfig:config CRASH_DUMP
arch/x86/Kconfig:config CRASH_DUMP
arch/loongarch/Kconfig:config CRASH_DUMP

Likewise for KEXEC and KEXEC_FILE.

I've looked into this in the past, and looking again today, I don't see a natural
place to put the option. Perhaps starting a kernel/Kconfig.kexec?

> 
> 
>> +#ifdef CONFIG_CRASH_HOTPLUG
>> +	/*
>> +	 * Ensure the elfcorehdr segment large enough for hotplug changes.
>> +	 * Account for VMCOREINFO and kernel_map and maximum CPUs.
> 
> Neither the first line nor the second one qualifies as parseable sentences.
> 

What about:
Ensure the elfcorehdr segment is large enough for hotplug changes.
The segment size accounts for VMCOREINFO, kernel_map, maximum CPUs
and maximum memory ranges.


>> +/**
>> + * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
>> + * @image: the active struct kimage
> 
> What is an active struct kimage?
> 
How about this:
@image: a pointer to kexec_crash_image

>> + *
>> + * The new elfcorehdr is prepared in a kernel buffer, and then it is
>> + * written on top of the existing/old elfcorehdr.
> 
> -ENOPARSE
> 
How about:
Prepare the new elfcorehdr and replace the existing elfcorehdr.

>> + */
>> +void arch_crash_handle_hotplug_event(struct kimage *image)
>> +{
>> +	void *elfbuf = NULL, *old_elfcorehdr;
>> +	unsigned long nr_mem_ranges;
>> +	unsigned long mem, memsz;
>> +	unsigned long elfsz = 0;
>> +
>> +	/*
>> +	 * Create the new elfcorehdr reflecting the changes to CPU and/or
>> +	 * memory resources.
>> +	 */
>> +	if (prepare_elf_headers(image, &elfbuf, &elfsz, &nr_mem_ranges)) {
>> +		pr_err("unable to prepare elfcore headers");
>> +		goto out;
> 
> So this can fail. Why is there just a pr_err() and no return value which
> tells the caller that this failed?
An error in the crash elfcorehdr infrastructure introduced in this series
is not a reason to rollback state. The cpuhp and memory notifier callbacks
always return an OK.
The primary errors that might occur are failure to obtain the kexec_lock,
and failure to obtain a temporary kernel buffer to stage the new elfcorehdr.
How about:
pr_err("prepare_elf_headers() failed");

> 
>> +	/*
>> +	 * Copy new elfcorehdr over the old elfcorehdr at destination.
>> +	 */
>> +	old_elfcorehdr = kmap_local_page(pfn_to_page(mem >> PAGE_SHIFT));
>> +	if (!old_elfcorehdr) {
>> +		pr_err("updating elfcorehdr failed\n");
> 
> How hard is it to write an error message which is clearly describing the
> problem?
> 
How about:
pr_err("mapping elfcorehdr segment failed");

> Thanks,
> 
>          tglx
Again, thanks for the fresh eyes!
eric