Re: [PATCH v10 4/8] crash: add generic infrastructure for crash hotplug support

[Baoquan He <bhe@redhat.com>]

On 07/21/22 at 02:17pm, Eric DeVolder wrote:
> CPU and memory change notifications are received in order to
> regenerate the elfcorehdr.
> 
> To support cpu hotplug, a callback is registered to capture the
> CPUHP_AP_ONLINE_DYN online and offline events via
> cpuhp_setup_state_nocalls().
> 
> To support memory hotplug, a notifier is registered to capture the
> MEM_ONLINE and MEM_OFFLINE events via register_memory_notifier().
> 
> The cpu callback and memory notifiers call handle_hotplug_event()
> which performs needed tasks and then dispatches the event to the
> architecture specific arch_crash_handle_hotplug_event(). During the
> process, the kexec_mutex is held.
> 
> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
> ---
>  include/linux/crash_core.h |  24 ++++++++
>  include/linux/kexec.h      |   7 +++
>  kernel/crash_core.c        | 118 +++++++++++++++++++++++++++++++++++++
>  3 files changed, 149 insertions(+)
> 
> diff --git a/include/linux/crash_core.h b/include/linux/crash_core.h
> index cb0f1916fbf5..c9705b6872e7 100644
> --- a/include/linux/crash_core.h
> +++ b/include/linux/crash_core.h
> @@ -86,4 +86,28 @@ int parse_crashkernel_high(char *cmdline, unsigned long long system_ram,
>  int parse_crashkernel_low(char *cmdline, unsigned long long system_ram,
>  		unsigned long long *crash_size, unsigned long long *crash_base);
>  
> +#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
> +#define KEXEC_CRASH_HP_INVALID_CPU		-1U
> +
> +struct kimage;
> +#ifndef arch_map_crash_pages
> +static inline void *arch_map_crash_pages(unsigned long paddr,
> +		unsigned long size)
> +{
> +	return NULL;
> +}
> +#endif
> +#ifndef arch_unmap_crash_pages
> +static inline void arch_unmap_crash_pages(void **ptr) { }
> +#endif
> +#ifndef arch_crash_handle_hotplug_event
> +static inline void arch_crash_handle_hotplug_event(struct kimage *image,
> +		unsigned int hp_action, unsigned int cpu)
> +{
> +}
> +#endif
> +
>  #endif /* LINUX_CRASH_CORE_H */
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index 5f4969cf3f4e..7694aa77b92b 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -340,6 +340,13 @@ struct kimage {
>  	struct purgatory_info purgatory_info;
>  #endif
>  
> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
> +	bool hotplug_event;
> +	unsigned int offlinecpu;
> +	bool elfcorehdr_index_valid;
> +	int elfcorehdr_index;
> +#endif
> +
>  #ifdef CONFIG_IMA_KEXEC
>  	/* Virtual address of IMA measurement buffer for kexec syscall */
>  	void *ima_buffer;
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 212d4dad0ec7..154ef532d45a 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -10,12 +10,16 @@
>  #include <linux/utsname.h>
>  #include <linux/vmalloc.h>
>  #include <linux/kexec.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;
> @@ -587,3 +591,117 @@ static int __init crash_save_vmcoreinfo_init(void)
>  }
>  
>  subsys_initcall(crash_save_vmcoreinfo_init);
> +
> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
> +static void handle_hotplug_event(unsigned int hp_action, unsigned int cpu)
> +{
> +	/* Obtain lock while changing crash information */
> +	mutex_lock(&kexec_mutex);
> +
> +	/* Check kdump is loaded */
> +	if (kexec_crash_image) {
> +		struct kimage *image = kexec_crash_image;
> +
> +		pr_debug("crash hp: hp_action %u, cpu %u", hp_action, cpu);

We don't need to check if it's cpu hotplug action here? Then for mem
hotplug actions, we also get:
                "crash hp: hp_action 2, cpu 0"
That cpu 0 will be confusing. And you forgot the line break.

Can we do like this:
		pr_debug("crash hp: hp_action %u ", hp_action);
		if (hp_action == KEXEC_CRASH_HP_REMOVE_CPU ||
		    hp_action == KEXEC_CRASH_HP_REMOVE_CPU)
			pr_debug("cpu %u \n", cpu);

Other than this, the patch looks good to me.

> +
> +		/*
> +		 * When the struct kimage is alloced, it is wiped to zero, so
> +		 * the elfcorehdr_index_valid defaults to false. Find the
> +		 * segment containing the elfcorehdr, if not already found.
> +		 * This works for both the kexec_load and kexec_file_load paths.
> +		 */
> +		if (!image->elfcorehdr_index_valid) {
> +			unsigned char *ptr;
> +			unsigned long mem, memsz;
> +			unsigned int n;
> +
> +			for (n = 0; n < image->nr_segments; n++) {
> +				mem = image->segment[n].mem;
> +				memsz = image->segment[n].memsz;
> +				ptr = arch_map_crash_pages(mem, memsz);
> +				if (ptr) {
> +					/* The segment containing elfcorehdr */
> +					if (memcmp(ptr, ELFMAG, SELFMAG) == 0) {
> +						image->elfcorehdr_index = (int)n;
> +						image->elfcorehdr_index_valid = true;
> +					}
> +				}
> +				arch_unmap_crash_pages((void **)&ptr);
> +			}
> +		}
> +
> +		if (!image->elfcorehdr_index_valid) {
> +			pr_err("crash hp: unable to locate elfcorehdr segment");
> +			goto out;
> +		}
> +
> +		/* Needed in order for the segments to be updated */
> +		arch_kexec_unprotect_crashkres();
> +
> +		/* Flag to differentiate between normal load and hotplug */
> +		image->hotplug_event = true;
> +
> +		/* Now invoke arch-specific update handler */
> +		arch_crash_handle_hotplug_event(image, hp_action, cpu);
> +
> +		/* No longer handling a hotplug event */
> +		image->hotplug_event = false;
> +
> +		/* Change back to read-only */
> +		arch_kexec_protect_crashkres();
> +	}
> +
> +out:
> +	/* Release lock now that update complete */
> +	mutex_unlock(&kexec_mutex);
> +}
> +
> +static int crash_memhp_notifier(struct notifier_block *nb, unsigned long val, void *v)
> +{
> +	switch (val) {
> +	case MEM_ONLINE:
> +		handle_hotplug_event(KEXEC_CRASH_HP_ADD_MEMORY, 0);
> +		break;
> +
> +	case MEM_OFFLINE:
> +		handle_hotplug_event(KEXEC_CRASH_HP_REMOVE_MEMORY, 0);
> +		break;
> +	}
> +	return NOTIFY_OK;
> +}
> +
> +static struct notifier_block crash_memhp_nb = {
> +	.notifier_call = crash_memhp_notifier,
> +	.priority = 0
> +};
> +
> +static int crash_cpuhp_online(unsigned int cpu)
> +{
> +	handle_hotplug_event(KEXEC_CRASH_HP_ADD_CPU, cpu);
> +	return 0;
> +}
> +
> +static int crash_cpuhp_offline(unsigned int cpu)
> +{
> +	handle_hotplug_event(KEXEC_CRASH_HP_REMOVE_CPU, cpu);
> +	return 0;
> +}
> +
> +static int __init crash_hotplug_init(void)
> +{
> +	int result = 0;
> +
> +	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
> +		register_memory_notifier(&crash_memhp_nb);
> +
> +	if (IS_ENABLED(CONFIG_HOTPLUG_CPU))
> +		result = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
> +						   "crash/cpuhp",
> +						   crash_cpuhp_online,
> +						   crash_cpuhp_offline);
> +
> +	return result;
> +}
> +
> +subsys_initcall(crash_hotplug_init);
> +#endif
> -- 
> 2.31.1
>