Re: [PATCH v8 6/7] x86/crash: Add x86 crash hotplug support for kexec_file_load

Eric DeVolder
2022-05-25T13:52:50Z

On 5/25/22 00:25, Sourabh Jain wrote:
> Hello Eric,
>
> On 06/05/22 00:16, 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.
>>
>> 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. When loading the crash kernel via kexec_file_load,
>> the elfcorehdr is identified at load time in crash_load_segments().
>>
>> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
>> ---
>>   arch/x86/Kconfig        |  11 ++++
>>   arch/x86/kernel/crash.c | 117 ++++++++++++++++++++++++++++++++++++++++
>>   2 files changed, 128 insertions(+)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index 4bed3abf444d..bf1201fe6981 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -2072,6 +2072,17 @@ 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..951ef365f0a7 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 +424,109 @@ int crash_load_segments(struct kimage *image)
>>       return ret;
>>   }
>>   #endif /* CONFIG_KEXEC_FILE */
>> +
>> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
>> +static void *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);
>> +    }
>> +
>> +    return ptr;
>> +}
>> +
>> +static void 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;
>> +
>> +    if (!image->elfcorehdr_index_valid) {
>> +        pr_err("crash hp: unable to locate elfcorehdr segment");
>> +        goto out;
>> +    }
>> +
>> +    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;
>
> Prepare_elf_header uses crash_prepare_elf64_headers function to generate elfcorehdr.
> Since crash_prepare_elf64_headers is defined under CONFIG_KEXEC_FILE option we
> will have build issues if CONFIG_KEXEC is enabled and CONFIG_KEXEC_FILE is disabled.
>
> How about moving crash_prepare_elf64_headers function to kernel/kexec_core.c?

Yes, of course. I originally protected against this scenario with having the
CRASH_HOTPLUG dependent on KEXEC_FILE, but as we eliminated CRASH_HOTPLUG, your
suggestion above is on point.

Thanks!
eric

>
> Thanks,
> Sourabh Jain
>