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

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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 */


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