Re: [PATCH v27 2/8] crash: add generic infrastructure for crash hotplug support

Sourabh Jain
2023-08-12T10:48:25Z

Hello Eric,

On 11/08/23 22:36, Eric DeVolder wrote:
> To support crash hotplug, a mechanism is needed to update the crash
> elfcorehdr upon CPU or memory changes (eg. hot un/plug or off/
> onlining). The crash elfcorehdr describes the CPUs and memory to
> be written into the vmcore.
>
> To track CPU changes, callbacks are registered with the cpuhp
> mechanism via cpuhp_setup_state_nocalls(CPUHP_BP_PREPARE_DYN). The
> crash hotplug elfcorehdr update has no explicit ordering requirement
> (relative to other cpuhp states), so meets the criteria for
> utilizing CPUHP_BP_PREPARE_DYN. CPUHP_BP_PREPARE_DYN is a dynamic
> state and avoids the need to introduce a new state for crash
> hotplug. Also, CPUHP_BP_PREPARE_DYN is the last state in the PREPARE
> group, just prior to the STARTING group, which is very close to the
> CPU starting up in a plug/online situation, or stopping in a unplug/
> offline situation. This minimizes the window of time during an
> actual plug/online or unplug/offline situation in which the
> elfcorehdr would be inaccurate. Note that for a CPU being unplugged
> or offlined, the CPU will still be present in the list of CPUs
> generated by crash_prepare_elf64_headers(). However, there is no
> need to explicitly omit the CPU, see justification in
> 'crash: change crash_prepare_elf64_headers() to for_each_possible_cpu()'.
>
> To track memory changes, a notifier is registered to capture the
> memblock MEM_ONLINE and MEM_OFFLINE events via register_memory_notifier().
>
> The CPU callbacks and memory notifiers invoke crash_handle_hotplug_event()
> which performs needed tasks and then dispatches the event to the
> architecture specific arch_crash_handle_hotplug_event() to update the
> elfcorehdr with the current state of CPUs and memory. During the
> process, the kexec_lock is held.
>
> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
> Reviewed-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> Acked-by: Hari Bathini <hbathini@linux.ibm.com>
> Acked-by: Baoquan He <bhe@redhat.com>
> ---
> include/linux/crash_core.h | 9 +++
> include/linux/kexec.h | 11 +++
> kernel/Kconfig.kexec | 31 ++++++++
> kernel/crash_core.c | 142 +++++++++++++++++++++++++++++++++++++
> kernel/kexec_core.c | 6 ++
> 5 files changed, 199 insertions(+)
>
> diff --git a/include/linux/crash_core.h b/include/linux/crash_core.h
> index de62a722431e..e14345cc7a22 100644
> --- a/include/linux/crash_core.h
> +++ b/include/linux/crash_core.h
> @@ -84,4 +84,13 @@ 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_NONE 0
> +#define KEXEC_CRASH_HP_ADD_CPU 1
> +#define KEXEC_CRASH_HP_REMOVE_CPU 2
> +#define KEXEC_CRASH_HP_ADD_MEMORY 3
> +#define KEXEC_CRASH_HP_REMOVE_MEMORY 4
> +#define KEXEC_CRASH_HP_INVALID_CPU -1U
> +
> +struct kimage;
> +
> #endif /* LINUX_CRASH_CORE_H */
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index 811a90e09698..b9903dd48e24 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -33,6 +33,7 @@ extern note_buf_t __percpu *crash_notes;
> #include <linux/compat.h>
> #include <linux/ioport.h>
> #include <linux/module.h>
> +#include <linux/highmem.h>
> #include <asm/kexec.h>
>
> /* Verify architecture specific macros are defined */
> @@ -360,6 +361,12 @@ struct kimage {
> struct purgatory_info purgatory_info;
> #endif
>
> +#ifdef CONFIG_CRASH_HOTPLUG
> + int hp_action;
> + int elfcorehdr_index;
> + bool elfcorehdr_updated;
> +#endif
> +
> #ifdef CONFIG_IMA_KEXEC
> /* Virtual address of IMA measurement buffer for kexec syscall */
> void *ima_buffer;
> @@ -490,6 +497,10 @@ static inline int arch_kexec_post_alloc_pages(void *vaddr, unsigned int pages, g
> static inline void arch_kexec_pre_free_pages(void *vaddr, unsigned int pages) { }
> #endif
>
> +#ifndef arch_crash_handle_hotplug_event
> +static inline void arch_crash_handle_hotplug_event(struct kimage *image) { }
> +#endif
> +

Isn't the above function should be declare under CONFIG_CRASH_HOTPLUG?

Thanks,
Sourabh

> #else /* !CONFIG_KEXEC_CORE */
> struct pt_regs;
> struct task_struct;
> diff --git a/kernel/Kconfig.kexec b/kernel/Kconfig.kexec
> index ff72e45cfaef..d0a9a5392035 100644
> --- a/kernel/Kconfig.kexec
> +++ b/kernel/Kconfig.kexec
> @@ -113,4 +113,35 @@ config CRASH_DUMP
> For s390, this option also enables zfcpdump.
> See also <file:Documentation/s390/zfcpdump.rst>
>
> +config CRASH_HOTPLUG
> + bool "Update the crash elfcorehdr on system configuration changes"
> + default y
> + depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG)
> + depends on ARCH_SUPPORTS_CRASH_HOTPLUG
> + help
> + Enable direct update to the crash elfcorehdr (which contains
> + the list of CPUs and memory regions to be dumped upon a crash)
> + in response to hot plug/unplug or online/offline of CPUs or
> + memory. This is a much more advanced approach than userspace
> + attempting that.
> +
> + If unsure, say Y.
> +
> +config CRASH_MAX_MEMORY_RANGES
> + int "Specify the maximum number of memory regions for the elfcorehdr"
> + default 8192
> + depends on CRASH_HOTPLUG
> + help
> + For the kexec_file_load() syscall path, specify the maximum number of
> + memory regions that the elfcorehdr buffer/segment can accommodate.
> + These regions are obtained via walk_system_ram_res(); eg. the
> + 'System RAM' entries in /proc/iomem.
> + This value is combined with NR_CPUS_DEFAULT and multiplied by
> + sizeof(Elf64_Phdr) to determine the final elfcorehdr memory buffer/
> + segment size.
> + The value 8192, for example, covers a (sparsely populated) 1TiB system
> + consisting of 128MiB memblocks, while resulting in an elfcorehdr
> + memory buffer/segment size under 1MiB. This represents a sane choice
> + to accommodate both baremetal and virtual machine configurations.
> +
> endmenu
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index b7c30b748a16..53d211c690a1 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -11,6 +11,8 @@
> #include <linux/vmalloc.h>
> #include <linux/sizes.h>
> #include <linux/kexec.h>
> +#include <linux/memory.h>
> +#include <linux/cpuhotplug.h>
>
> #include <asm/page.h>
> #include <asm/sections.h>
> @@ -18,6 +20,7 @@
> #include <crypto/sha1.h>
>
> #include "kallsyms_internal.h"
> +#include "kexec_internal.h"
>
> /* vmcoreinfo stuff */
> unsigned char *vmcoreinfo_data;
> @@ -697,3 +700,142 @@ static int __init crash_save_vmcoreinfo_init(void)
> }
>
> subsys_initcall(crash_save_vmcoreinfo_init);
> +
> +#ifdef CONFIG_CRASH_HOTPLUG
> +#undef pr_fmt
> +#define pr_fmt(fmt) "crash hp: " fmt
> +/*
> + * To accurately reflect hot un/plug changes of cpu and memory resources
> + * (including onling and offlining of those resources), the elfcorehdr
> + * (which is passed to the crash kernel via the elfcorehdr= parameter)
> + * must be updated with the new list of CPUs and memories.
> + *
> + * In order to make changes to elfcorehdr, two conditions are needed:
> + * First, the segment containing the elfcorehdr must be large enough
> + * to permit a growing number of resources; the elfcorehdr memory size
> + * is based on NR_CPUS_DEFAULT and CRASH_MAX_MEMORY_RANGES.
> + * Second, purgatory must explicitly exclude the elfcorehdr from the
> + * list of segments it checks (since the elfcorehdr changes and thus
> + * would require an update to purgatory itself to update the digest).
> + */
> +static void crash_handle_hotplug_event(unsigned int hp_action, unsigned int cpu)
> +{
> + struct kimage *image;
> +
> + /* Obtain lock while changing crash information */
> + if (!kexec_trylock()) {
> + pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n");
> + return;
> + }
> +
> + /* Check kdump is not loaded */
> + if (!kexec_crash_image)
> + goto out;
> +
> + image = kexec_crash_image;
> +
> + if (hp_action == KEXEC_CRASH_HP_ADD_CPU ||
> + hp_action == KEXEC_CRASH_HP_REMOVE_CPU)
> + pr_debug("hp_action %u, cpu %u\n", hp_action, cpu);
> + else
> + pr_debug("hp_action %u\n", hp_action);
> +
> + /*
> + * The elfcorehdr_index is set to -1 when the struct kimage
> + * is allocated. Find the segment containing the elfcorehdr,
> + * if not already found.
> + */
> + if (image->elfcorehdr_index < 0) {
> + unsigned long mem;
> + unsigned char *ptr;
> + unsigned int n;
> +
> + for (n = 0; n < image->nr_segments; n++) {
> + mem = image->segment[n].mem;
> + ptr = kmap_local_page(pfn_to_page(mem >> PAGE_SHIFT));
> + if (ptr) {
> + /* The segment containing elfcorehdr */
> + if (memcmp(ptr, ELFMAG, SELFMAG) == 0)
> + image->elfcorehdr_index = (int)n;
> + kunmap_local(ptr);
> + }
> + }
> + }
> +
> + if (image->elfcorehdr_index < 0) {
> + pr_err("unable to locate elfcorehdr segment");
> + goto out;
> + }
> +
> + /* Needed in order for the segments to be updated */
> + arch_kexec_unprotect_crashkres();
> +
> + /* Differentiate between normal load and hotplug update */
> + image->hp_action = hp_action;
> +
> + /* Now invoke arch-specific update handler */
> + arch_crash_handle_hotplug_event(image);
> +
> + /* No longer handling a hotplug event */
> + image->hp_action = KEXEC_CRASH_HP_NONE;
> + image->elfcorehdr_updated = true;
> +
> + /* Change back to read-only */
> + arch_kexec_protect_crashkres();
> +
> + /* Errors in the callback is not a reason to rollback state */
> +out:
> + /* Release lock now that update complete */
> + kexec_unlock();
> +}
> +
> +static int crash_memhp_notifier(struct notifier_block *nb, unsigned long val, void *v)
> +{
> + switch (val) {
> + case MEM_ONLINE:
> + crash_handle_hotplug_event(KEXEC_CRASH_HP_ADD_MEMORY,
> + KEXEC_CRASH_HP_INVALID_CPU);
> + break;
> +
> + case MEM_OFFLINE:
> + crash_handle_hotplug_event(KEXEC_CRASH_HP_REMOVE_MEMORY,
> + KEXEC_CRASH_HP_INVALID_CPU);
> + 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)
> +{
> + crash_handle_hotplug_event(KEXEC_CRASH_HP_ADD_CPU, cpu);
> + return 0;
> +}
> +
> +static int crash_cpuhp_offline(unsigned int cpu)
> +{
> + crash_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_BP_PREPARE_DYN,
> + "crash/cpuhp", crash_cpuhp_online, crash_cpuhp_offline);
> + }
> +
> + return result;
> +}
> +
> +subsys_initcall(crash_hotplug_init);
> +#endif
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index e2f2574d8b74..5d323255862a 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -277,6 +277,12 @@ struct kimage *do_kimage_alloc_init(void)
> /* Initialize the list of unusable pages */
> INIT_LIST_HEAD(&image->unusable_pages);
>
> +#ifdef CONFIG_CRASH_HOTPLUG
> + image->hp_action = KEXEC_CRASH_HP_NONE;
> + image->elfcorehdr_index = -1;
> + image->elfcorehdr_updated = false;
> +#endif
> +
> return image;
> }
>