[PATCH v26 2/8] crash: add generic infrastructure for crash hotplug support

Eric DeVolder
2023-08-04T21:06:05Z

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
+
#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;
}

--
2.31.1