Re: [PATCH v4 00/10] crash: Kernel handling of CPU and memory hot un/plug

[Baoquan He <bhe@redhat.com>]

Hi Eric,

On 02/09/22 at 02:56pm, Eric DeVolder wrote:
> When the kdump service is loaded, if a CPU or memory is hot
> un/plugged, the crash elfcorehdr (for x86), which describes the CPUs
> and memory in the system, must also be updated, else the resulting
> vmcore is inaccurate (eg. missing either CPU context or memory
> regions).

This version looks much better. I am gonna to review this week and
give it a shot on a machine. Can you share your test steps? I have
knowledge about mem hotplug, but haven't tried cpu hotplug yet.

Thanks
Baoquan

> 
> The current solution utilizes udev to initiate an unload-then-reload
> of the kdump image (e. kernel, initrd, boot_params, puratory and
> elfcorehdr) by the userspace kexec utility. In previous posts I have
> outlined the significant performance problems related to offloading
> this activity to userspace.
> 
> This patchset introduces a generic crash hot un/plug handler that
> registers with the CPU and memory notifiers. Upon CPU or memory
> changes, this generic handler is invoked and performs important
> housekeeping, for example obtaining the appropriate lock, and then
> invokes an architecture specific handler to do the appropriate
> updates.
> 
> In the case of x86_64, the arch specific handler generates a new
> elfcorehdr, and overwrites the old one in memory. No involvement
> with userspace needed.
> 
> To realize the benefits/test this patchset, one must make a couple
> of minor changes to userspace:
> 
>  - Disable the udev rule for updating kdump on hot un/plug changes
>    Eg. on RHEL: rm -f /usr/lib/udev/rules.d/98-kexec.rules
>    or other technique to neuter the rule.
> 
>  - Change to the kexec_file_load for loading the kdump kernel:
>    Eg. on RHEL: in /usr/bin/kdumpctl, change to:
>     standard_kexec_args="-p -d -s"
>    which adds the -s to select kexec_file_load syscall.
> 
> This patchset supports kexec_load with a modified kexec userspace
> utility, and a working changeset to the kexec userspace utility
> is provided here (and to use, the above change to standard_kexec_args
> would be, for example, to append --hotplug-size=262144 instead of -s).
> 
>  diff --git a/kexec/arch/i386/crashdump-x86.c b/kexec/arch/i386/crashdump-x86.c
>  index 9826f6d..06adb7e 100644
>  --- a/kexec/arch/i386/crashdump-x86.c
>  +++ b/kexec/arch/i386/crashdump-x86.c
>  @@ -48,6 +48,7 @@
>   #include <x86/x86-linux.h>
>   
>   extern struct arch_options_t arch_options;
>  +extern unsigned long long hotplug_size;
>   
>   static int get_kernel_page_offset(struct kexec_info *UNUSED(info),
>   				  struct crash_elf_info *elf_info)
>  @@ -975,6 +976,13 @@ int load_crashdump_segments(struct kexec_info *info, char* mod_cmdline,
>   	} else {
>   		memsz = bufsz;
>   	}
>  +
>  +    /* If hotplug support enabled, use that size */
>  +    if (hotplug_size) {
>  +        memsz = hotplug_size;
>  +    }
>  +
>  +    info->elfcorehdr =
>   	elfcorehdr = add_buffer(info, tmp, bufsz, memsz, align, min_base,
>   							max_addr, -1);
>   	dbgprintf("Created elf header segment at 0x%lx\n", elfcorehdr);
>  diff --git a/kexec/kexec.c b/kexec/kexec.c
>  index f63b36b..9569d9a 100644
>  --- a/kexec/kexec.c
>  +++ b/kexec/kexec.c
>  @@ -58,6 +58,7 @@
>   
>   unsigned long long mem_min = 0;
>   unsigned long long mem_max = ULONG_MAX;
>  +unsigned long long hotplug_size = 0;
>   static unsigned long kexec_flags = 0;
>   /* Flags for kexec file (fd) based syscall */
>   static unsigned long kexec_file_flags = 0;
>  @@ -672,6 +673,12 @@ static void update_purgatory(struct kexec_info *info)
>   		if (info->segment[i].mem == (void *)info->rhdr.rel_addr) {
>   			continue;
>   		}
>  +        /* Don't include elfcorehdr in the checksum, if hotplug
>  +         * support enabled.
>  +         */
>  +        if (hotplug_size && (info->segment[i].mem == (void *)info->elfcorehdr)) {
>  +			continue;
>  +		}
>   		sha256_update(&ctx, info->segment[i].buf,
>   			      info->segment[i].bufsz);
>   		nullsz = info->segment[i].memsz - info->segment[i].bufsz;
>  @@ -1504,6 +1511,17 @@ int main(int argc, char *argv[])
>   		case OPT_PRINT_CKR_SIZE:
>   			print_crashkernel_region_size();
>   			return 0;
>  +		case OPT_HOTPLUG_SIZE:
>  +            /* Reserved the specified size for hotplug growth */
>  +			hotplug_size = strtoul(optarg, &endptr, 0);
>  +			if (*endptr) {
>  +				fprintf(stderr,
>  +					"Bad option value in --hotplug-size=%s\n",
>  +					optarg);
>  +				usage();
>  +				return 1;
>  +			}
>  +			break;
>   		default:
>   			break;
>   		}
>  diff --git a/kexec/kexec.h b/kexec/kexec.h
>  index 595dd68..b30dda4 100644
>  --- a/kexec/kexec.h
>  +++ b/kexec/kexec.h
>  @@ -169,6 +169,7 @@ struct kexec_info {
>   	int command_line_len;
>   
>   	int skip_checks;
>  +    unsigned long elfcorehdr;
>    };
>   
>   struct arch_map_entry {
>  @@ -231,7 +232,8 @@ extern int file_types;
>   #define OPT_PRINT_CKR_SIZE	262
>   #define OPT_LOAD_LIVE_UPDATE	263
>   #define OPT_EXEC_LIVE_UPDATE	264
>  -#define OPT_MAX			265
>  +#define OPT_HOTPLUG_SIZE	265
>  +#define OPT_MAX			266
>   #define KEXEC_OPTIONS \
>   	{ "help",		0, 0, OPT_HELP }, \
>   	{ "version",		0, 0, OPT_VERSION }, \
>  @@ -258,6 +260,7 @@ extern int file_types;
>   	{ "debug",		0, 0, OPT_DEBUG }, \
>   	{ "status",		0, 0, OPT_STATUS }, \
>   	{ "print-ckr-size",     0, 0, OPT_PRINT_CKR_SIZE }, \
>  +	{ "hotplug-size",     2, 0, OPT_HOTPLUG_SIZE }, \
>   
>   #define KEXEC_OPT_STR "h?vdfixyluet:pscaS"
>  
> 
> Regards,
> eric
> ---
> v4: 9feb2022
>  - Refactored patches per Baoquan suggestsions.
>  - A few corrections, per Baoquan.
> 
> v3: 10jan2022
>  https://lkml.org/lkml/2022/1/10/1212
>  - Rebasing per Baoquan He request.
>  - Changed memory notifier per David Hildenbrand.
>  - Providing example kexec userspace change in cover letter.
> 
> RFC v2: 7dec2021
>  https://lkml.org/lkml/2021/12/7/1088
>  - Acting upon Baoquan He suggestion of removing elfcorehdr from
>    the purgatory list of segments, removed purgatory code from
>    patchset, and it is signficiantly simpler now.
> 
> RFC v1: 18nov2021
>  https://lkml.org/lkml/2021/11/18/845
>  - working patchset demonstrating kernel handling of hotplug
>    updates to x86 elfcorehdr for kexec_file_load
> 
> RFC: 14dec2020
>  https://lkml.org/lkml/2020/12/14/532
>  - proposed concept of allowing kernel to handle hotplug update
>    of elfcorehdr
> ---
> 
> Eric DeVolder (10):
>   crash: fix minor typo/bug in debug message
>   crash hp: Introduce CRASH_HOTPLUG configuration options
>   crash hp: definitions and prototype changes
>   crash hp: prototype change for crash_prepare_elf64_headers
>   crash hp: introduce helper functions un/map_crash_pages
>   crash hp: generic crash hotplug support infrastructure
>   crash hp: exclude elfcorehdr from the segment digest
>   crash hp: exclude hot remove cpu from elfcorehdr notes
>   crash hp: Add x86 crash hotplug support for kexec_file_load
>   crash hp: Add x86 crash hotplug support for kexec_load
> 
>  arch/arm64/kernel/machine_kexec_file.c |   6 +-
>  arch/powerpc/kexec/file_load_64.c      |   2 +-
>  arch/x86/Kconfig                       |  26 +++++
>  arch/x86/kernel/crash.c                | 123 ++++++++++++++++++++-
>  include/linux/kexec.h                  |  23 +++-
>  kernel/crash_core.c                    | 146 +++++++++++++++++++++++++
>  kernel/kexec_file.c                    |  15 ++-
>  7 files changed, 331 insertions(+), 10 deletions(-)
> 
> -- 
> 2.27.0
>