Re: [PATCH v9 7/7] x86/crash: Add x86 crash hotplug support

Fabio M. De Francesco
2022-07-08T19:38:48Z

On martedì 14 giugno 2022 00:42:40 CEST Eric DeVolder wrote:
> For x86_64, when CPU or memory is hot un/plugged, the crash
> elfcorehdr, which describes the CPUs and memory in the system,
> must also be updated.
>
> When loading the crash kernel via kexec_load or kexec_file_load,
> the elfcorehdr is identified at run time in
> crash_core:handle_hotplug_event().
>
> To update the elfcorehdr for x86_64, a new elfcorehdr must be
> generated from the available CPUs and memory. The new elfcorehdr
> is prepared into a buffer, and then installed over the top of
> the existing elfcorehdr.
>
> In the patch 'kexec: exclude elfcorehdr from the segment digest'
> the need to update purgatory due to the change in elfcorehdr was
> eliminated. As a result, no changes to purgatory or boot_params
> (as the elfcorehdr= kernel command line parameter pointer
> remains unchanged and correct) are needed, just elfcorehdr.
>
> To accommodate a growing number of resources via hotplug, the
> elfcorehdr segment must be sufficiently large enough to accommodate
> changes, see the CRASH_MAX_MEMORY_RANGES configure item.
>
> With this change, crash hotplug for kexec_file_load syscall
> is supported. The kexec_load is also supported, but also
> requires a corresponding change to userspace kexec-tools.
>
> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
> ---
> arch/x86/Kconfig | 11 ++++
> arch/x86/kernel/crash.c | 116 ++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 127 insertions(+)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 762a0b6ab8b6..e9eecff3b97d 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2082,6 +2082,17 @@ 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
> + 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/kernel/crash.c b/arch/x86/kernel/crash.c
> index 9db41cce8d97..b9cdf7a0d868 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>
> @@ -398,7 +399,17 @@ 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);
> + /* For marking as usable to crash kernel */
> + image->elf_headers_sz = kbuf.memsz;
> + /* Record the index of the elfcorehdr segment */
> + image->elfcorehdr_index = image->nr_segments;
> + 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);
> @@ -413,3 +424,108 @@ int crash_load_segments(struct kimage *image)
> return ret;
> }
> #endif /* CONFIG_KEXEC_FILE */
> +
> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
> +int crash_hotplug_support(void) { return 1; }
> +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.
> + */
> + void *ptr = NULL;
> +
> + /* NOTE: requires arch_kexec_[un]protect_crashkres() for write
access */
> + if (size > 0) {
> + struct page *page = pfn_to_page(paddr >> PAGE_SHIFT);
> +
> + ptr = kmap(page);
> + }

kmap_local_page() is preferred over kmap() which should be avoided in new
code (please take a look at highmem.rst which I recently updated).

Can you please check if you can easily avoid using kmap()? Otherwise, this
code will be added to my (long) queue and changed one day or the other :-)

Thanks,

Fabio

> +
> + return ptr;
> +}
> +
> +void arch_unmap_crash_pages(void **ptr)
> +{
> + if (ptr) {
> + if (*ptr)
> + kunmap(*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).
> + *
> + */
> +void arch_crash_handle_hotplug_event(struct kimage *image,
> + unsigned int hp_action, unsigned int cpu)
> +{
> + 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
>
>