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

Fabio M. De Francesco
2022-07-09T18:41:57Z

On venerdì 8 luglio 2022 23:07:20 CEST Eric DeVolder wrote:
>
> On 7/8/22 14:38, Fabio M. De Francesco wrote:
> > 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
> Fabio,
> I converted this to kmap_local_page()!
> Thanks!
> eric
>
> >
Eric,

Thanks for the very quick reaction to my suggestion.

Fabio