Re: [PATCH v15 7/7] x86/crash: add x86 crash hotplug support

[Eric DeVolder <eric.devolder@oracle.com>]

On 1/4/23 03:08, Baoquan He wrote:
> On 12/09/22 at 10:36am, Eric DeVolder wrote:
> ...... snip out
>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>> index 9ceb93c176a6..5186df48ce6c 100644
>> --- a/arch/x86/kernel/crash.c
>> +++ b/arch/x86/kernel/crash.c
>> @@ -42,6 +42,21 @@
>>   #include <asm/crash.h>
>>   #include <asm/cmdline.h>
>>   
>> +/*
>> + * For the kexec_file_load() syscall path, specify the maximum number of
>> + * memory regions that the elfcorehdr buffer/segment can accommodate.
>> + * These regions are obtained via walk_system_ram_res(); eg. the
>> + * 'System RAM' entries in /proc/iomem.
>> + * This value is combined with NR_CPUS_DEFAULT and multiplied by
>> + * sizeof(Elf64_Phdr) to determine the final elfcorehdr memory buffer/
>> + * segment size.
>> + * The value 8192, for example, covers a (sparsely populated) 1TiB system
>> + * consisting of 128MiB memblocks, while resulting in an elfcorehdr
>> + * memory buffer/segment size under 1MiB. This represents a sane choice
>> + * to accommodate both baremetal and virtual machine configurations.
>> + */
>> +#define CRASH_MAX_MEMORY_RANGES 8192
>> +
>>   /* Used while preparing memory map entries for second kernel */
>>   struct crash_memmap_data {
>>   	struct boot_params *params;
>> @@ -394,10 +409,39 @@ 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;
>> +
>> +#ifdef CONFIG_CRASH_HOTPLUG
>> +	if (IS_ENABLED(CONFIG_HOTPLUG_CPU)) {
> 
> Do I miss anything important during reviewing? I can't see why memory
> hotplug is also relying on HOTPLUG_CPU.

No, looks like I made a mistake. The ifdef needs to go, and the IS_ENABLED() immediately following 
needs to be CRASH_HOTPLUG. Having not looked at this in a few weeks, it's obvious now.
Thanks!
eric


> 
>> +		/*
>> +		 * 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;
>> +
>> +		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);
>> +		}
>> +	}
>> +#endif
>>   
>> -	kbuf.memsz = kbuf.bufsz;
>>   	kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
>>   	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
>>   	ret = kexec_add_buffer(&kbuf);
>> @@ -412,3 +456,67 @@ int crash_load_segments(struct kimage *image)
>>   	return ret;
>>   }
>>   #endif /* CONFIG_KEXEC_FILE */
>> +
>> +#ifdef CONFIG_CRASH_HOTPLUG
>> +
>> +#undef pr_fmt
>> +#define pr_fmt(fmt) "crash hp: " fmt
>> +
>> +/**
>> + * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
>> + * @image: the active struct kimage
>> + *
>> + * To accurately reflect hot un/plug changes, the new elfcorehdr
>> + * is prepared in a kernel buffer, and then it is written on top
>> + * of the existing/old elfcorehdr.
>> + */
>> +void arch_crash_handle_hotplug_event(struct kimage *image)
>> +{
>> +	void *elfbuf = NULL, *old_elfcorehdr;
>> +	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)) {
>> +		pr_err("unable to prepare elfcore headers");
>> +		goto out;
>> +	}
>> +
>> +	/*
>> +	 * Obtain address and size of the elfcorehdr segment, and
>> +	 * check it against the new elfcorehdr buffer.
>> +	 */
>> +	mem = image->segment[image->elfcorehdr_index].mem;
>> +	memsz = image->segment[image->elfcorehdr_index].memsz;
>> +	if (elfsz > memsz) {
>> +		pr_err("update elfcorehdr elfsz %lu > memsz %lu",
>> +			elfsz, memsz);
>> +		goto out;
>> +	}
>> +
>> +	/*
>> +	 * 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");
>> +		goto out;
>> +	}
>> +
>> +	/*
>> +	 * Temporarily invalidate the crash image while the
>> +	 * elfcorehdr is updated.
>> +	 */
>> +	xchg(&kexec_crash_image, NULL);
>> +	memcpy_flushcache(old_elfcorehdr, elfbuf, elfsz);
>> +	xchg(&kexec_crash_image, image);
>> +	kunmap_local(old_elfcorehdr);
>> +	pr_debug("updated elfcorehdr\n");
>> +
>> +out:
>> +	vfree(elfbuf);
>> +}
>> +#endif
>> -- 
>> 2.31.1
>>
> 

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