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SubjectRe: [PATCH v3 0/6] crash: Kernel handling of CPU and memory hot un/plug
From
Baoquan,
Thanks for the feedback! I think I've incorporated all suggestions in this message and others in v3.
As a result, I just posted v4.
Thank you!
eric

On 1/27/22 00:51, Baoquan He wrote:
> Hi Eric,
>
> On 01/10/22 at 02:57pm, 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.
>
> About patch splitting, it is always a headache to me when I post
> patches. Both too coarse or too small granularity are not good. But we
> should obey two rules:
> 1) commits had better be bisectable, so that later we find out which
> commit cause issue or regression easier. Here, your patch 5/6 and 6/6 are
> obviously breaking the rule. Because crash_prepare_elf64_headers()'s
> protype has been changed, its invokation is not modified accodingly.
>
> 2) Each patch only cover one action, bad better not stack too much in
> one patch. Surely, we should avoid to scatter one thing into several
> patch, it's not easy to reivew. E.g in this patchset, kexec_load can be
> taken out, for this we have discussed. Skipping the elfcorehdr digest
> can be taken out, its code change is simple, while it's worth to tell
> the reason in a patch.
>
> So my suggestion to rearrange the patchset as below, please take it as a
> reference.
> 1)crash: fix minor typo/bug in debug message
> 2)crash hp: Introduce CRASH_HOTPLUG configuration options
> 3)crash hp: definitions and prototype changes
> Note: this patch need cover those invokation of crash_prepare_elf64_headers.
> Otherwise it's not bisectable.
> 4)crash hp: introduce helper function map_crash_pages/unmap_crash_pages
> 5)crash hp: generic crash hotplug support infrastructure
> 6)crash hp: exclude elfcorehdr from the segment checksum calculation
> 7)crash hp: exclude hot removed cpu from cpu notes of elfcorehdr
> this includes the assigment part in crash_hotplug_handler() and
> usage part in crash_prepare_elf64_headers().
> 8)crash hp/x86: add the crash hp support for x86
>
> Above patch splitting and patch subject may not be appropriate, please
> adust or change with your understanding.
>
>>
>> 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
>> ---
>> v3: 10jan2022
>> - 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 (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 | 117 +++++++++++++++++++++++++++++++++
>> kernel/kexec_file.c | 15 ++++-
>> 5 files changed, 313 insertions(+), 6 deletions(-)
>>
>> --
>> 2.27.0
>>
>

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