Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support

[Sourabh Jain <sourabhjain@linux.ibm.com>]

On 08/10/22 01:03, Eric DeVolder wrote:
>
>
> On 9/19/22 02:06, Sourabh Jain wrote:
>>
>> On 10/09/22 02:35, 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>
>>> Acked-by: Baoquan He <bhe@redhat.com>
>>> ---
>>>   arch/x86/Kconfig             |  11 ++++
>>>   arch/x86/include/asm/kexec.h |  20 +++++++
>>>   arch/x86/kernel/crash.c      | 102 
>>> +++++++++++++++++++++++++++++++++++
>>>   3 files changed, 133 insertions(+)
>>>
>>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>>> index f9920f1341c8..cdfc9b2fdf98 100644
>>> --- a/arch/x86/Kconfig
>>> +++ b/arch/x86/Kconfig
>>> @@ -2056,6 +2056,17 @@ 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/include/asm/kexec.h 
>>> b/arch/x86/include/asm/kexec.h
>>> index a3760ca796aa..432073385b2d 100644
>>> --- a/arch/x86/include/asm/kexec.h
>>> +++ b/arch/x86/include/asm/kexec.h
>>> @@ -212,6 +212,26 @@ typedef void crash_vmclear_fn(void);
>>>   extern crash_vmclear_fn __rcu *crash_vmclear_loaded_vmcss;
>>>   extern void kdump_nmi_shootdown_cpus(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);
>>> +#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 /* __ASSEMBLY__ */
>>>   #endif /* _ASM_X86_KEXEC_H */
>>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>>> index 9ceb93c176a6..8fc7d678ac72 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 +398,18 @@ 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);
>>> +    /* Mark as usable to crash kernel, else crash kernel fails on 
>>> boot */
>>> +    image->elf_headers_sz = kbuf.memsz;
>>> +    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);
>>> @@ -412,3 +424,93 @@ int crash_load_segments(struct kimage *image)
>>>       return ret;
>>>   }
>>>   #endif /* CONFIG_KEXEC_FILE */
>>> +
>>> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
>>> +/*
>>> + * 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 *arch_map_crash_pages(unsigned long paddr, unsigned long size)
>>> +{
>>> +    void *ptr = NULL;
>>> +
>>> +    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
>>> + *
>>> + * To accurately reflect hot un/plug changes, the new elfcorehdr
>>> + * is prepared in a kernel buffer, and then it is written on top
>>> + * of the existing/old elfcorehdr.
>>> + */
>>> +void arch_crash_handle_hotplug_event(struct kimage *image,
>>> +    unsigned int hp_action)
>>> +{
>>> +    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;
>>
>> On PowerPC, while preparing the elf core header the memblock 
>> structure is used to prepare program header for memory regions of 
>> elfcorehdr. Since the above arch specific hotplug handler gets 
>> invoked when memory is marked offline (MEM_OFFLINE) which is before 
>> memblock structure gets updated so on PowerPC the above handler may 
>> not work for memory hotplug case.
>>
>> Just wondering which data structure is used to get the list of memory 
>> regions while preparing program header for memory regions of 
>> elfcorehdr on other architectures?
>>
>> Thanks,
>> Sourabh Jain
>
> I think your request to report the memory block address in comments of 
> patch 3/7 "crash: add generic infrastructure" cover this scenario now.
Yes, the asked changes will make easy for PowerPC to recreate elfcorehdr.

Thanks,
Sourabh Jain