Re: [PATCH v5 4/8] crash: generic crash hotplug support infrastructure

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

Hello Eric,

On 03/03/22 21:57, Eric DeVolder wrote:
> This patch introduces a generic crash hot plug/unplug infrastructure
> for CPU and memory changes. Upon CPU and memory changes, a generic
> crash_hotplug_handler() obtains the appropriate lock, does some
> important house keeping and then dispatches the hot plug/unplug event
> to the architecture specific arch_crash_hotplug_handler(), and when
> that handler returns, the lock is released.
>
> This patch modifies crash_core.c to implement a subsys_initcall()
> function that installs handlers for hot plug/unplug events. If CPU
> hotplug is enabled, then cpuhp_setup_state() is invoked to register a
> handler for CPU changes. Similarly, if memory hotplug is enabled, then
> register_memory_notifier() is invoked to install a handler for memory
> changes. These handlers in turn invoke the common generic handler
> crash_hotplug_handler().
>
> On the CPU side, cpuhp_setup_state_nocalls() is invoked with parameter
> CPUHP_AP_ONLINE_DYN. While this works, when a CPU is being unplugged,
> the CPU still shows up in foreach_present_cpu() during the regeneration
> of the new CPU list, thus the need to explicitly check and exclude the
> soon-to-be offlined CPU in crash_prepare_elf64_headers().
>
> On the memory side, each un/plugged memory block passes through the
> handler. For example, if a 1GiB DIMM is hotplugged, that generate 8
> memory events, one for each 128MiB memblock.
>
> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
> ---
>   include/linux/kexec.h |  16 +++++++
>   kernel/crash_core.c   | 108 ++++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 124 insertions(+)
>
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index d7b59248441b..b11d75a6b2bc 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -300,6 +300,13 @@ struct kimage {
>   
>   	/* Information for loading purgatory */
>   	struct purgatory_info purgatory_info;
> +
> +#ifdef CONFIG_CRASH_HOTPLUG
> +	bool hotplug_event;
> +	int offlinecpu;
> +	bool elf_index_valid;
> +	int elf_index;

How about keeping an array to track all kexec segment index need to be 
updated in
crash hotplug handler.

struct hp_segment {
    name;
    index;
    is_valid;
  }

It will be helpful if architecture need to updated multiple kexec 
segments  for a hotplug event.

For example, on PowerPC, we might need to update FDT and elfcorehdr on 
memory hot plug/unplug.

Thanks,
Sourabh Jain


> +#endif
>   #endif
>   
>   #ifdef CONFIG_IMA_KEXEC
> @@ -316,6 +323,15 @@ struct kimage {
>   	unsigned long elf_load_addr;
>   };
>   
> +#ifdef CONFIG_CRASH_HOTPLUG
> +void arch_crash_hotplug_handler(struct kimage *image,
> +	unsigned int hp_action, unsigned long a, unsigned long b);
> +#define KEXEC_CRASH_HP_REMOVE_CPU   0
> +#define KEXEC_CRASH_HP_ADD_CPU      1
> +#define KEXEC_CRASH_HP_REMOVE_MEMORY 2
> +#define KEXEC_CRASH_HP_ADD_MEMORY   3
> +#endif /* CONFIG_CRASH_HOTPLUG */
> +
>   /* kexec interface functions */
>   extern void machine_kexec(struct kimage *image);
>   extern int machine_kexec_prepare(struct kimage *image);
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 256cf6db573c..76959d440f71 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -9,12 +9,17 @@
>   #include <linux/init.h>
>   #include <linux/utsname.h>
>   #include <linux/vmalloc.h>
> +#include <linux/highmem.h>
> +#include <linux/memory.h>
> +#include <linux/cpuhotplug.h>
>   
>   #include <asm/page.h>
>   #include <asm/sections.h>
>   
>   #include <crypto/sha1.h>
>   
> +#include "kexec_internal.h"
> +
>   /* vmcoreinfo stuff */
>   unsigned char *vmcoreinfo_data;
>   size_t vmcoreinfo_size;
> @@ -491,3 +496,106 @@ static int __init crash_save_vmcoreinfo_init(void)
>   }
>   
>   subsys_initcall(crash_save_vmcoreinfo_init);
> +
> +#ifdef CONFIG_CRASH_HOTPLUG
> +void __weak arch_crash_hotplug_handler(struct kimage *image,
> +	unsigned int hp_action, unsigned long a, unsigned long b)
> +{
> +	pr_warn("crash hp: %s not implemented", __func__);
> +}
> +
> +static void crash_hotplug_handler(unsigned int hp_action,
> +	unsigned long a, unsigned long b)
> +{
> +	/* Obtain lock while changing crash information */
> +	if (!mutex_trylock(&kexec_mutex))
> +		return;
> +
> +	/* Check kdump is loaded */
> +	if (kexec_crash_image) {
> +		pr_debug("crash hp: hp_action %u, a %lu, b %lu", hp_action,
> +			a, b);
> +
> +		/* Needed in order for the segments to be updated */
> +		arch_kexec_unprotect_crashkres();
> +
> +		/* Flag to differentiate between normal load and hotplug */
> +		kexec_crash_image->hotplug_event = true;
> +
> +		/* Now invoke arch-specific update handler */
> +		arch_crash_hotplug_handler(kexec_crash_image, hp_action, a, b);
> +
> +		/* No longer handling a hotplug event */
> +		kexec_crash_image->hotplug_event = false;
> +
> +		/* Change back to read-only */
> +		arch_kexec_protect_crashkres();
> +	}
> +
> +	/* Release lock now that update complete */
> +	mutex_unlock(&kexec_mutex);
> +}
> +
> +#if defined(CONFIG_MEMORY_HOTPLUG)
> +static int crash_memhp_notifier(struct notifier_block *nb,
> +	unsigned long val, void *v)
> +{
> +	struct memory_notify *mhp = v;
> +	unsigned long start, end;
> +
> +	start = mhp->start_pfn << PAGE_SHIFT;
> +	end = ((mhp->start_pfn + mhp->nr_pages) << PAGE_SHIFT) - 1;
> +
> +	switch (val) {
> +	case MEM_ONLINE:
> +		crash_hotplug_handler(KEXEC_CRASH_HP_ADD_MEMORY,
> +			start, end-start);
> +		break;
> +
> +	case MEM_OFFLINE:
> +		crash_hotplug_handler(KEXEC_CRASH_HP_REMOVE_MEMORY,
> +			start, end-start);
> +		break;
> +	}
> +	return NOTIFY_OK;
> +}
> +
> +static struct notifier_block crash_memhp_nb = {
> +	.notifier_call = crash_memhp_notifier,
> +	.priority = 0
> +};
> +#endif
> +
> +#if defined(CONFIG_HOTPLUG_CPU)
> +static int crash_cpuhp_online(unsigned int cpu)
> +{
> +	crash_hotplug_handler(KEXEC_CRASH_HP_ADD_CPU, cpu, 0);
> +	return 0;
> +}
> +
> +static int crash_cpuhp_offline(unsigned int cpu)
> +{
> +	crash_hotplug_handler(KEXEC_CRASH_HP_REMOVE_CPU, cpu, 0);
> +	return 0;
> +}
> +#endif
> +
> +static int __init crash_hotplug_init(void)
> +{
> +	int result = 0;
> +
> +#if defined(CONFIG_MEMORY_HOTPLUG)
> +	register_memory_notifier(&crash_memhp_nb);
> +#endif
> +
> +#if defined(CONFIG_HOTPLUG_CPU)
> +	result = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
> +				"crash/cpuhp",
> +				crash_cpuhp_online, crash_cpuhp_offline);
> +#endif
> +
> +	return result;
> +}
> +
> +subsys_initcall(crash_hotplug_init);
> +#endif /* CONFIG_CRASH_HOTPLUG */