Re: [PATCH v10 8/8] x86/crash: Add x86 crash hotplug support
From: Eric DeVolder
Date: Tue Aug 16 2022 - 11:25:26 EST
On 8/12/22 19:34, Baoquan He wrote:
On 07/21/22 at 02:17pm, Eric DeVolder wrote:
...snip....
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e58798f636d4..bb59596c8bea 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2065,6 +2065,17 @@ config CRASH_DUMP
(CONFIG_RELOCATABLE=y).
For more details see Documentation/admin-guide/kdump/kdump.rst
+config CRASH_MAX_MEMORY_RANGES
+ depends on CRASH_DUMP && KEXEC_FILE && (HOTPLUG_CPU || MEMORY_HOTPLUG)
+ int
+ default 32768
Do we need to enforce the value with page align and minimal size? I
Are you asking about the value CRASH_MAX_MEMORY_RANGES? This value represents
the maximum number of memory ranges, and there Elf64_Phdrs, that we need to
allow for elfcorehdr memory. So I'm not sure what the concern for alignment
is. I suppose we could also institute a minimum size for this value, say 1024.
checked crash_load_segments() in arch/x86/kernel/crash.c, it does the
page size aligning in kexec_add_buffer(). And in
load_crashdump_segments() of
kexec-tools/kexec/arch/i386/crashdump-x86.c, it creates elfcorehdr at
below code, the align is 1024, and in generic add_buffer()
implementation, it enforces the memsz page aligned, and changes the
passed align as page alignment.
elfcorehdr = add_buffer(info, tmp, bufsz, memsz, align, min_base,
max_addr, -1);
Maybe we should at least mention this in the help text to notice people.
Unfortunately I do not yet understand the concern being raised.
+ help
+ For the kexec_file_load path, specify the maximum number of
+ memory regions, eg. as represented by the 'System RAM' entries
+ in /proc/iomem, that the elfcorehdr buffer/segment can accommodate.
+ This value is combined with NR_CPUS and multiplied by Elf64_Phdr
+ size to determine the final buffer size.
+
config KEXEC_JUMP
bool "kexec jump"
depends on KEXEC && HIBERNATION
diff --git a/arch/x86/include/asm/crash.h b/arch/x86/include/asm/crash.h
index 8b6bd63530dc..96051d8e4b45 100644
--- a/arch/x86/include/asm/crash.h
+++ b/arch/x86/include/asm/crash.h
@@ -9,4 +9,24 @@ int crash_setup_memmap_entries(struct kimage *image,
struct boot_params *params);
void crash_smp_send_stop(void);
+void *arch_map_crash_pages(unsigned long paddr, unsigned long size);
+#define arch_map_crash_pages arch_map_crash_pages
+
+void arch_unmap_crash_pages(void **ptr);
+#define arch_unmap_crash_pages arch_unmap_crash_pages
+
+void arch_crash_handle_hotplug_event(struct kimage *image,
+ unsigned int hp_action, unsigned int cpu);
+#define arch_crash_handle_hotplug_event arch_crash_handle_hotplug_event
+
+#ifdef CONFIG_HOTPLUG_CPU
+static inline int crash_hotplug_cpu_support(void) { return 1; }
+#define crash_hotplug_cpu_support crash_hotplug_cpu_support
+#endif
+
+#ifdef CONFIG_MEMORY_HOTPLUG
+static inline int crash_hotplug_memory_support(void) { return 1; }
+#define crash_hotplug_memory_support crash_hotplug_memory_support
+#endif
+
#endif /* _ASM_X86_CRASH_H */
diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index 9ceb93c176a6..55dda4fcde6e 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -25,6 +25,7 @@
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/memblock.h>
+#include <linux/highmem.h>
#include <asm/processor.h>
#include <asm/hardirq.h>
@@ -397,7 +398,17 @@ int crash_load_segments(struct kimage *image)
image->elf_headers = kbuf.buffer;
image->elf_headers_sz = kbuf.bufsz;
+#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
+ /* Ensure elfcorehdr segment large enough for hotplug changes */
+ kbuf.memsz = (CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES) * sizeof(Elf64_Phdr);
Do we need to break the line to 80 chars?
Sure, I will do so.
+ /* For marking as usable to crash kernel */
+ image->elf_headers_sz = kbuf.memsz;
Do we need this code comment?
Well, it did take me a while to figure this particular item out in order for all
this code to work right (else the crash kernel would fail at boot time). So I
think it best to keep this comment.
+ /* Record the index of the elfcorehdr segment */
+ image->elfcorehdr_index = image->nr_segments;
And this place?
Not necessarily needed, but I've found it useful.
+ image->elfcorehdr_index_valid = true;
+#else
kbuf.memsz = kbuf.bufsz;
+#endif
kbuf.buf_align = ELF_CORE_HEADER_ALIGN;
kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
ret = kexec_add_buffer(&kbuf);
@@ -412,3 +423,107 @@ int crash_load_segments(struct kimage *image)
return ret;
}
#endif /* CONFIG_KEXEC_FILE */
+
+#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
+void *arch_map_crash_pages(unsigned long paddr, unsigned long size)
+{
+ /*
+ * NOTE: The addresses and sizes passed to this routine have
+ * already been fully aligned on page boundaries. There is no
+ * need for massaging the address or size.
+ */
Can we move the code comment above function interface?
Yes
+ void *ptr = NULL;
+
+ /* NOTE: requires arch_kexec_[un]protect_crashkres() for write access */
Do we need this code comment? On ARCH where proctionion is made, we
surely need to the protect/unprotect.
I will remove this; I've mentioned this in handle_hotplug_event() where these
protect/unprotect functions are called.
+ if (size > 0) {
+ struct page *page = pfn_to_page(paddr >> PAGE_SHIFT);
+
+ ptr = kmap_local_page(page);
+ }
+
+ return ptr;
+}
+
+void arch_unmap_crash_pages(void **ptr)
+{
+ if (ptr) {
+ if (*ptr)
+ kunmap_local(*ptr);
+ *ptr = NULL;
+ }
+}
+
+/**
+ * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
+ * @image: the active struct kimage
+ * @hp_action: the hot un/plug action being handled
+ * @cpu: when KEXEC_CRASH_HP_ADD/REMOVE_CPU, the cpu affected
+ *
+ * To accurately reflect hot un/plug changes, the elfcorehdr (which
+ * is passed to the crash kernel via the elfcorehdr= parameter)
+ * must be updated with the new list of CPUs and memories. The new
+ * elfcorehdr is prepared in a kernel buffer, and then it is
+ * written on top of the existing/old elfcorehdr.
+ *
+ * For hotplug changes to elfcorehdr to work, two conditions are
+ * needed:
+ * First, the segment containing the elfcorehdr must be large enough
+ * to permit a growing number of resources. See the
+ * CONFIG_CRASH_MAX_MEMORY_RANGES description.
+ * Second, purgatory must explicitly exclude the elfcorehdr from the
+ * list of segments it checks (since the elfcorehdr changes and thus
+ * would require an update to purgatory itself to update the digest).
Isn't this generic concept to crash hotplug? Should we move it out to
some generic place?
Yes, so I will relocate this.
+ *
+ */
+void arch_crash_handle_hotplug_event(struct kimage *image,
+ unsigned int hp_action, unsigned int cpu)
The passed in 'cpu' is not used at all, what is it added for? I didn't
see explanation about it.
Well its not used for x86, but as I recall, Sourabh Jain needed it for the PowerPC handler.
+{
+ struct kexec_segment *ksegment;
+ unsigned char *ptr = NULL;
+ unsigned long elfsz = 0;
+ void *elfbuf = NULL;
+ unsigned long mem, memsz;
+
+ /*
+ * Elfcorehdr_index_valid checked in crash_core:handle_hotplug_event()
+ */
+ ksegment = &image->segment[image->elfcorehdr_index];
+ mem = ksegment->mem;
+ memsz = ksegment->memsz;
+
+ /*
+ * Create the new elfcorehdr reflecting the changes to CPU and/or
+ * memory resources.
+ */
+ if (prepare_elf_headers(image, &elfbuf, &elfsz)) {
+ pr_err("crash hp: unable to prepare elfcore headers");
+ goto out;
+ }
+ if (elfsz > memsz) {
+ pr_err("crash hp: update elfcorehdr elfsz %lu > memsz %lu",
+ elfsz, memsz);
+ goto out;
+ }
+
+ /*
+ * At this point, we are all but assured of success.
+ * Copy new elfcorehdr into destination.
+ */
+ ptr = arch_map_crash_pages(mem, memsz);
+ if (ptr) {
+ /*
+ * Temporarily invalidate the crash image while the
+ * elfcorehdr is updated.
+ */
+ xchg(&kexec_crash_image, NULL);
+ memcpy_flushcache((void *)ptr, elfbuf, elfsz);
+ xchg(&kexec_crash_image, image);
+ }
+ arch_unmap_crash_pages((void **)&ptr);
+ pr_debug("crash hp: re-loaded elfcorehdr at 0x%lx\n", mem);
+
+out:
+ if (elfbuf)
+ vfree(elfbuf);
+}
+#endif
--
2.31.1