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

[Eric DeVolder]

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 */