Re: [RFC v2 0/6] crash: Kernel handling of CPU and memory hot un/plug

[Eric DeVolder]

On 1/10/22 02:04, Baoquan He wrote:
> Hi Eric,
>
> On 01/05/22 at 08:25am, Eric DeVolder wrote:
> > Nudge...
> >
> > Fwiw, below is a working changeset to kexec userspace utility that allows the kexec_load
> > path to work similarly to the kexec_file_load path of this RFC. With both the following
> > userspace kexec patch and this RFC, both kexec_load and kexec_file_load work with changes
> > due to hotplug *without* unloading-then-reloading the kdump/capture kernel.
>
> Thanks for taking a try on that, and sorry for late response because of
> somethings at hand.
>
> I will review this v2 round. When applying them, I encountered some
> conflict, could you please rebase these on the ltest 5.16 and send me
> a tar ball privately, or a github branch is also welcome. Thanks in
> advance.

Baoquan, thank you for your time and interest on this patch.
I posted as v3 as I incorporated changes from David Hildenbrand and some other minor tweaks.
Regards,
eric

> Thanks
> Baoquan
>
> > 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"
> >
> >
> >
> > On 12/7/21 13:51, 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).
> > >
> > > 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, on which I am current working to provide separately.
> > >
> > > Regards,
> > > eric
> > > ---
> > > RFC v2: 7dec2021
> > >    - 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 (6):
> > >     crash: fix minor typo/bug in debug message
> > >     crash hp: Introduce CRASH_HOTPLUG configuration options
> > >     crash hp: definitions and prototype changes
> > >     crash hp: generic crash hotplug support infrastructure
> > >     crash hp: kexec_file changes for crash hotplug support
> > >     crash hp: Add x86 crash hotplug support
> > >
> > >    arch/x86/Kconfig        |  26 ++++++++
> > >    arch/x86/kernel/crash.c | 140 +++++++++++++++++++++++++++++++++++++++-
> > >    include/linux/kexec.h   |  21 +++++-
> > >    kernel/crash_core.c     | 118 +++++++++++++++++++++++++++++++++
> > >    kernel/kexec_file.c     |  15 ++++-
> > >    5 files changed, 314 insertions(+), 6 deletions(-)