linux-kernel - Re: [PATCH v16 7/7] x86/crash: add x86 crash hotplug support

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Message-ID: <bc4ade74-dcbc-88f6-2d88-2bd4c2be38ba@oracle.com>
Date:   Mon, 9 Jan 2023 13:43:39 -0600
From:   Eric DeVolder <eric.devolder@...cle.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, vgoyal@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org, david@...hat.com,
        sourabhjain@...ux.ibm.com, konrad.wilk@...cle.com,
        boris.ostrovsky@...cle.com
Subject: Re: [PATCH v16 7/7] x86/crash: add x86 crash hotplug support



On 1/9/23 01:36, Baoquan He wrote:
> On 01/05/23 at 10:17am, Eric DeVolder wrote:
> ......
>> @@ -394,10 +409,37 @@ int crash_load_segments(struct kimage *image)
>>   	if (ret)
>>   		return ret;
>>   
>> -	image->elf_headers = kbuf.buffer;
>> -	image->elf_headers_sz = kbuf.bufsz;
>> +	image->elf_headers	= kbuf.buffer;
>> +	image->elf_headers_sz	= kbuf.bufsz;
>> +	kbuf.memsz		= kbuf.bufsz;
>> +
>> +	if (IS_ENABLED(CONFIG_CRASH_HOTPLUG)) {
>> +		/*
>> +		 * Ensure the elfcorehdr segment large enough for hotplug changes.
>> +		 * Start with VMCOREINFO and kernel_map.
>> +		 */
>> +		unsigned long pnum = 2;
>> +
>> +		if (IS_ENABLED(CONFIG_HOTPLUG_CPU))
>> +			pnum += CONFIG_NR_CPUS_DEFAULT;
>> +
>> +		if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
>> +			pnum += CRASH_MAX_MEMORY_RANGES;
> 
> Logic of pnum calculating is a little confusing to me. If I only enable
> one of CONFIG_HOTPLUG_CPU and CONFIG_MEMORY_HOTPLUG, is it OK? Say I
> only enable CONFIG_HOTPLUG_CPU and CONFIG_SMP, on x86_64,
> CONFIG_NR_CPUS_DEFAULT will be 64. pnum will be 64, is it OK. Am I miss
> anything?

Ah, your understanding is correct, and the issue you point out I need to fix.
Specifically is only one of HOTPLUG_CPU or MEMORY_HOTPLUG is set, then I need
to allow a default for the other. Using the example you have above, there would
not be room for a single memory region, so I need to allow for a default number
of memory regions.

Good find!
eric

> 
>> +
>> +		if (pnum < (unsigned long)PN_XNUM) {
>> +			kbuf.memsz = pnum * sizeof(Elf64_Phdr);
>> +			kbuf.memsz += sizeof(Elf64_Ehdr);
>> +
>> +			image->elfcorehdr_index = image->nr_segments;
>> +			image->elfcorehdr_index_valid = true;
>> +
>> +			/* Mark as usable to crash kernel, else crash kernel fails on boot */
>> +			image->elf_headers_sz = kbuf.memsz;
>> +		} else {
>> +			pr_err("number of Phdrs %lu exceeds max\n", pnum);
>> +		}
>> +	}
>>   
>> -	kbuf.memsz = kbuf.bufsz;
>>   	kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
>>   	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
>>   	ret = kexec_add_buffer(&kbuf);
>> @@ -412,3 +454,67 @@ int crash_load_segments(struct kimage *image)
>>   	return ret;
>>   }
>>   #endif /* CONFIG_KEXEC_FILE */
>