Re: [PATCH v9 0/7] crash: Kernel handling of CPU and memory hot un/plug

Eric DeVolder
2022-07-07T13:06:10Z

On 7/5/22 20:16, Baoquan He wrote:
> On 07/05/22 at 10:17am, Eric DeVolder wrote:
>> Nudge...
>> Thanks!
>
> Seems DavidH has concern and rasied it after you posted v9. Is that
> solved?

In looking at David's comments on v8:
- [PATCH v8 2/7] crash: prototype change for crash_prepare_elf64_headers
I adjusted per feedback and responded as such. He provided an Acked-by.

- [PATCH v8 3/7] crash: add generic infrastructure for crash hotplug support
He says do not use WARN* on expected error paths. I instead used WARN_ONCE(),
but I didn't consider this an error path. Nonetheless, I'll remove the
message altogether in v11.

- [PATCH v8 0/7] crash: Kernel handling of CPU and memory hot un/plug
David muses about a better name, but that was in response to Sourabh's
and I exchange on the sysfs attributes to use to report the presence
of this feature. In the end, I used something altogether different, as
presented in v10 (and the corresponding udev solution/change that goes
hand in hand with it).

Let me know if I've missed anything else? Sourabh points out a build problem
for architectures without this feature support, so a v11 is warranted. But I'll
wait to post that until I hear that I've addressed the previous questions.

Thanks,
eric

>
>>
>> On 6/13/22 17:42, 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:
>>>
>>> - Prevent udev from updating kdump crash kernel on hot un/plug changes.
>>> Add the following as the first lines to the udev rule file
>>> /usr/lib/udev/rules.d/98-kexec.rules:
>>>
>>> # The kernel handles updates to crash elfcorehdr for cpu and memory changes
>>> SUBSYSTEM=="cpu", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end"
>>> SUBSYSTEM=="memory", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end"
>>>
>>> These lines will cause cpu and memory hot un/plug events to be
>>> skipped within this rule file, if the kernel has these changes
>>> enabled.
>>>
>>> - 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 instead of -s).
>>>
>>> diff --git a/kexec/arch/i386/crashdump-x86.c b/kexec/arch/i386/crashdump-x86.c
>>> index 9826f6d..4ed395a 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 int do_hotplug;
>>> static int get_kernel_page_offset(struct kexec_info *UNUSED(info),
>>> struct crash_elf_info *elf_info)
>>> @@ -975,6 +976,14 @@ int load_crashdump_segments(struct kexec_info *info, char* mod_cmdline,
>>> } else {
>>> memsz = bufsz;
>>> }
>>> +
>>> + /* If hotplug support enabled, use larger size to accomodate changes */
>>> + if (do_hotplug) {
>>> + long int nr_cpus = get_nr_cpus();
>>> + memsz = (nr_cpus + CRASH_MAX_MEMORY_RANGES) * sizeof(Elf64_Phdr);
>>> + }
>>> +
>>> + 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/crashdump-elf.c b/kexec/crashdump-elf.c
>>> index b8bb686..5e29f7a 100644
>>> --- a/kexec/crashdump-elf.c
>>> +++ b/kexec/crashdump-elf.c
>>> @@ -43,11 +43,7 @@ int FUNC(struct kexec_info *info,
>>> int (*get_note_info)(int cpu, uint64_t *addr, uint64_t *len);
>>> long int count_cpu;
>>> - if (xen_present())
>>> - nr_cpus = xen_get_nr_phys_cpus();
>>> - else
>>> - nr_cpus = sysconf(_SC_NPROCESSORS_CONF);
>>> -
>>> + nr_cpus = get_nr_cpus();
>>> if (nr_cpus < 0) {
>>> return -1;
>>> }
>>> diff --git a/kexec/crashdump.h b/kexec/crashdump.h
>>> index 18bd691..28d3278 100644
>>> --- a/kexec/crashdump.h
>>> +++ b/kexec/crashdump.h
>>> @@ -57,7 +57,6 @@ unsigned long phys_to_virt(struct crash_elf_info *elf_info,
>>> unsigned long long paddr);
>>> unsigned long xen_architecture(struct crash_elf_info *elf_info);
>>> -int xen_get_nr_phys_cpus(void);
>>> int xen_get_note(int cpu, uint64_t *addr, uint64_t *len);
>>> int xen_get_crashkernel_region(uint64_t *start, uint64_t *end);
>>> diff --git a/kexec/kexec-xen.h b/kexec/kexec-xen.h
>>> index 70fb576..f54a2dd 100644
>>> --- a/kexec/kexec-xen.h
>>> +++ b/kexec/kexec-xen.h
>>> @@ -83,5 +83,6 @@ extern int __xc_interface_close(xc_interface *xch);
>>> #endif
>>> int xen_get_kexec_range(int range, uint64_t *start, uint64_t *end);
>>> +int xen_get_nr_phys_cpus(void);
>>> #endif /* KEXEC_XEN_H */
>>> diff --git a/kexec/kexec.c b/kexec/kexec.c
>>> index 829a6ea..3668b73 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;
>>> +int do_hotplug = 0;
>>> static unsigned long kexec_flags = 0;
>>> /* Flags for kexec file (fd) based syscall */
>>> static unsigned long kexec_file_flags = 0;
>>> @@ -489,6 +490,17 @@ static int add_backup_segments(struct kexec_info *info,
>>> return 0;
>>> }
>>> +long int get_nr_cpus(void)
>>> +{
>>> + long int nr_cpus;
>>> +
>>> + if (xen_present())
>>> + nr_cpus = xen_get_nr_phys_cpus();
>>> + else
>>> + nr_cpus = sysconf(_SC_NPROCESSORS_CONF);
>>> + return nr_cpus;
>>> +}
>>> +
>>> static char *slurp_fd(int fd, const char *filename, off_t size, off_t *nread)
>>> {
>>> char *buf;
>>> @@ -672,6 +684,14 @@ 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 (do_hotplug && (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;
>>> @@ -1565,6 +1585,9 @@ int main(int argc, char *argv[])
>>> case OPT_PRINT_CKR_SIZE:
>>> print_crashkernel_region_size();
>>> return 0;
>>> + case OPT_HOTPLUG:
>>> + do_hotplug = 1;
>>> + break;
>>> default:
>>> break;
>>> }
>>> diff --git a/kexec/kexec.h b/kexec/kexec.h
>>> index 0f97a97..b0428cc 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 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", 0, 0, OPT_HOTPLUG }, \
>>> #define KEXEC_OPT_STR "h?vdfixyluet:pscaS"
>>> @@ -290,6 +293,8 @@ extern unsigned long add_buffer_phys_virt(struct kexec_info *info,
>>> int buf_end, int phys);
>>> extern void arch_reuse_initrd(void);
>>> +extern long int get_nr_cpus(void);
>>> +
>>> extern int ifdown(void);
>>> extern char purgatory[];
>>>
>>> Regards,
>>> eric
>>> ---
>>>
>>> v9: 13jun2022
>>> - Rebased to 5.18.0
>>> - Per Sourabh, moved crash_prepare_elf64_headers() into common
>>> crash_core.c to avoid compile issues with kexec_load only path.
>>> - Per David Hildebrand, replaced mutex_trylock() with mutex_lock().
>>> - Changed the __weak arch_crash_handle_hotplug_event() to utilize
>>> WARN_ONCE() instead of WARN(). Fix some formatting issues.
>>> - Per Sourabh, introduced sysfs attribute crash_hotplug for memory
>>> and CPUs; for use by userspace (udev) to determine if the kernel
>>> performs crash hot un/plug support.
>>> - Per Sourabh, moved the code detecting the elfcorehdr segment from
>>> arch/x86 into crash_core:handle_hotplug_event() so both kexec_load
>>> and kexec_file_load can benefit.
>>> - Updated userspace kexec-tools kexec utility to reflect change to
>>> using CRASH_MAX_MEMORY_RANGES and get_nr_cpus().
>>>
>>> v8: 5may2022
>>> https://lkml.org/lkml/2022/5/5/1133
>>> - Per Borislav Petkov, eliminated CONFIG_CRASH_HOTPLUG in favor
>>> of CONFIG_HOTPLUG_CPU || CONFIG_MEMORY_HOTPLUG, ie a new define
>>> is not needed. Also use of IS_ENABLED() rather than #ifdef's.
>>> Renamed crash_hotplug_handler() to handle_hotplug_event().
>>> And other corrections.
>>> - Per Baoquan, minimized the parameters to the arch_crash_
>>> handle_hotplug_event() to hp_action and cpu.
>>> - Introduce KEXEC_CRASH_HP_INVALID_CPU definition, per Baoquan.
>>> - Per Sourabh Jain, renamed and repurposed CRASH_HOTPLUG_ELFCOREHDR_SZ
>>> to CONFIG_CRASH_MAX_MEMORY_RANGES, mirroring kexec-tools change
>>> by David Hildebrand. Folded this patch into the x86
>>> kexec_file_load support patch.
>>>
>>> v7: 13apr2022
>>> https://lkml.org/lkml/2022/4/13/850
>>> - Resolved parameter usage to crash_hotplug_handler(), per Baoquan.
>>>
>>> v6: 1apr2022
>>> https://lkml.org/lkml/2022/4/1/1203
>>> - Reword commit messages and some comment cleanup per Baoquan.
>>> - Changed elf_index to elfcorehdr_index for clarity.
>>> - Minor code changes per Baoquan.
>>>
>>> v5: 3mar2022
>>> https://lkml.org/lkml/2022/3/3/674
>>> - Reworded description of CRASH_HOTPLUG_ELFCOREHDR_SZ, per
>>> David Hildenbrand.
>>> - Refactored slightly a few patches per Baoquan recommendation.
>>>
>>> v4: 9feb2022
>>> https://lkml.org/lkml/2022/2/9/1406
>>> - 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 (7):
>>> crash: move crash_prepare_elf64_headers
>>> crash: prototype change for crash_prepare_elf64_headers
>>> crash: add generic infrastructure for crash hotplug support
>>> kexec: exclude elfcorehdr from the segment digest
>>> kexec: exclude hot remove cpu from elfcorehdr notes
>>> crash: memory and cpu hotplug sysfs attributes
>>> x86/crash: Add x86 crash hotplug support
>>>
>>> .../admin-guide/mm/memory-hotplug.rst | 8 +
>>> Documentation/core-api/cpu_hotplug.rst | 18 ++
>>> arch/arm64/kernel/machine_kexec_file.c | 6 +-
>>> arch/powerpc/kexec/file_load_64.c | 2 +-
>>> arch/x86/Kconfig | 11 +
>>> arch/x86/kernel/crash.c | 119 ++++++++-
>>> drivers/base/cpu.c | 13 +
>>> drivers/base/memory.c | 13 +
>>> include/linux/crash_core.h | 12 +
>>> include/linux/kexec.h | 14 +-
>>> kernel/crash_core.c | 241 ++++++++++++++++++
>>> kernel/kexec_file.c | 105 +-------
>>> 12 files changed, 456 insertions(+), 106 deletions(-)
>>>
>>
>