Re: [PATCH v5 7/8] x86/crash: Add x86 crash hotplug support for kexec_file_load

[Baoquan He <bhe@redhat.com>]

On 03/03/22 at 11:27am, 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 if no errors occur, it is
> installed over the top of the existing elfcorehdr.
> 
> In the patch 'crash hp: 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_HOTPLUG_ELFCOREHDR_SZ configure item.
> 
> With this change, the kexec_file_load syscall (not kexec_load)
> is supported. When loading the crash kernel via kexec_file_load,
> the elfcorehdr is identified at load time in crash_load_segments().

I tune the log a little, please check:

[PATCH v5 7/8] x86/crash: Add x86 crash hotplug support for kexec_file_load

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 need be
generated from the available CPUs and memory. The new elfcorehdr
is prepared into a buffer, 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_HOTPLUG_ELFCOREHDR_SZ configure item.

With this change, crash hotplug for kexec_file_load syscall 
is supported. When loading kdump 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/kernel/crash.c | 120 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 120 insertions(+)
> 
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index 9db41cce8d97..5da30e2bc780 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;
>  
> +#ifdef CONFIG_CRASH_HOTPLUG
> +	/* Ensure elfcorehdr segment large enough for hotplug changes */
> +	kbuf.memsz = CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ;
> +	/* For marking as usable to crash kernel */
> +	image->elf_headers_sz = kbuf.memsz;
> +	/* Record the index of the elfcorehdr segment */
> +	image->elf_index = image->nr_segments;
> +	image->elf_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 +424,112 @@ int crash_load_segments(struct kimage *image)
>  	return ret;
>  }
>  #endif /* CONFIG_KEXEC_FILE */
> +
> +#ifdef CONFIG_CRASH_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_hotplug_handler() - Handle hotplug elfcorehdr changes
> + * @image: the active struct kimage
> + * @hp_action: the hot un/plug action being handled
> + * @a: first parameter dependent upon hp_action
> + * @b: first parameter dependent upon hp_action
> + *
> + * 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 if no errors,
                                                 ~~~~~~~~~~~~~~~~~~
                                                 can be removed.
> + * 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
> + * CONFIG_CRASH_HOTPLUG_ELFCOREHDR_SZ.
> + * 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_hotplug_handler(struct kimage *image,
> +	unsigned int hp_action, unsigned long a, unsigned long b)
> +{
> +	struct kexec_segment *ksegment;
> +	unsigned char *ptr = NULL;
> +	unsigned long elfsz = 0;
> +	void *elfbuf = NULL;
> +	unsigned long mem, memsz;
> +
> +	/* Must have valid elfcorehdr index */
        Redundant code comment can be removed.

> +	if (!image->elf_index_valid) {
> +		pr_err("crash hp: unable to locate elfcorehdr segment");
> +		goto out;
> +	}
> +
> +	ksegment = &image->segment[image->elf_index];
> +	mem = ksegment->mem;
> +	memsz = ksegment->memsz;
> +
> +	/*
> +	 * Create the new elfcorehdr reflecting the changes to CPU and/or
> +	 * memory resources. The elfcorehdr segment memsz must be
> +	 * sufficiently large to accommodate increases due to hotplug
> +	 * activity. See CRASH_HOTPLUG_ELFCOREHDR_SZ.
> +	 */
        This paragraph could be duplicated with the part in kernel-doc.
        Considering to drop one?
> +	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 = map_crash_pages(mem, memsz);
> +	if (ptr) {
> +		/* Temporarily invalidate the crash image while it is replaced */
> +		xchg(&kexec_crash_image, NULL);
> +		/* Write the new elfcorehdr into memory */
                No need, the code is self explanatory.
> +		memcpy_flushcache((void *)ptr, elfbuf, elfsz);
> +		/* The crash image is now valid once again */

                ditto. Sometime over commenting is not suggested.

> +		xchg(&kexec_crash_image, image);
> +	}
> +	unmap_crash_pages((void **)&ptr);
> +	pr_debug("crash hp: re-loaded elfcorehdr at 0x%lx\n", mem);
> +
> +out:
> +	if (elfbuf)
> +		vfree(elfbuf);
> +}
> +#endif /* CONFIG_CRASH_HOTPLUG */
> -- 
> 2.27.0
>