Re: [PATCH v2 5/5] arm64: crash: Add crash hotplug support

Hi Jinjie,

On Wed, Jul 29, 2026 at 11:12:35AM +0800, Jinjie Ruan wrote:
> +/**
> + * update_crash_elfcorehdr() - Recreate the elfcorehdr and replace it with old
> + *			       elfcorehdr in the kexec segment array.
> + * @image: the active struct kimage
> + */
> +static void update_crash_elfcorehdr(struct kimage *image)
> +{
> +	void *elfbuf = NULL, *old_elfcorehdr;
> +	unsigned long mem, memsz;
> +	unsigned long elfsz = 0;
> +
> +	/*
> +	 * Create the new elfcorehdr reflecting the changes to CPU and/or
> +	 * memory resources.
> +	 */
> +	if (crash_prepare_headers(true, &elfbuf, &elfsz, NULL)) {
> +		pr_err("unable to create new elfcorehdr");
> +		goto out;
> +	}
> +
> +	/*
> +	 * Obtain address and size of the elfcorehdr segment, and
> +	 * check it against the new elfcorehdr buffer.
> +	 */
> +	mem = image->segment[image->elfcorehdr_index].mem;
> +	memsz = image->segment[image->elfcorehdr_index].memsz;
> +	if (elfsz > memsz) {
> +		pr_err("update elfcorehdr elfsz %lu > memsz %lu",
> +			elfsz, memsz);
> +		goto out;
> +	}
> +
> +	/*
> +	 * Copy new elfcorehdr over the old elfcorehdr at destination.
> +	 */
> +	old_elfcorehdr = phys_to_virt(mem);
> +
> +	/*
> +	 * Temporarily invalidate the crash image while the
> +	 * elfcorehdr is updated.
> +	 */
> +	xchg(&kexec_crash_image, NULL);
> +	memcpy(old_elfcorehdr, elfbuf, elfsz);
> +	dcache_clean_inval_poc((unsigned long)old_elfcorehdr,
> +			       (unsigned long)(old_elfcorehdr + elfsz));
> +	xchg(&kexec_crash_image, image);
> +	pr_debug("updated elfcorehdr\n");
> +
> +out:
> +	vfree(elfbuf);
> +}
> +
> +/**
> + * arch_crash_handle_hotplug_event() - Handle hotplug elfcorehdr changes
> + * @image: a pointer to kexec_crash_image
> + * @arg: struct memory_notify handler for memory hotplug case and
> + *       NULL for CPU hotplug case.
> + *
> + * Update the kdump image based on the type of hotplug event:
> + * - CPU add and remove: No action is needed.
> + * - Memory add/remove: Update the elfcorehdr to reflect the current memory layout.
> + *
> + * Prepare the new elfcorehdr and replace the existing elfcorehdr.
> + */
> +void arch_crash_handle_hotplug_event(struct kimage *image, void *arg)
> +{
> +	if (image->hp_action == KEXEC_CRASH_HP_ADD_CPU ||
> +	    image->hp_action == KEXEC_CRASH_HP_REMOVE_CPU)
> +		return;
> +
> +	update_crash_elfcorehdr(image);
> +}

Looking at powerpc, they pass the arg down to update_crash_elfcorehdr()
to handle the memory hot-unplug case. It looks like the notifier is
called before the memblock_remove(), so the update above will still
count the memory being removed.

There are a few Sashiko comments as well, though some might be about
existing issues (it would be nice to have them fixed ;)).

-- 
Catalin