[RFC v1 4/8] crash hp: generic crash hotplug support infrastructure

[Eric DeVolder]

This patch introduces a generic crash hot plug/unplug handler for CPU
and memory changes. This crash_update_elfcorehdr() handler is invoked
upon CPU and memory changes.  This generic handler obtains the
appropriate lock, does some important house keeping and then
dispatches the hot plug/unplug event to the architecture specific
handler arch_update_crash_elfcorehdr(), and when that architecture
specific handler returns, the lock is released.

This patch modifies crash_core.c to implement a subsys_initcall()
function that installs handlers for hot plug/unplug events. If CPU
hotplug is enabled, then cpuhp_setup_state() is invoked to register a
handler for CPU changes. Similarly, if memory hotplug is enabled, then
register_memory_notifier() is invoked to install a handler for memory
changes. These handlers in turn invoke the common handler
crash_update_elfcorehdr().

On the CPU side, cpuhp_setup_state_nocalls() is invoked with parameter
CPUHP_AP_ONLINE_DYN. While this works, when a CPU is being unplugged,
the CPU still shows up in foreach_present_cpu() during the regeneration
of the elfcorehdr, thus the need to explicitly check and exclude the
soon-to-be offlined CPU in crash_prepare_elf64_headers().

On the memory side, each un/plugged memory block passes through the
handler. For example, if a 1GiB DIMM is hotplugged, that generate 8
memory events, one for each 128MiB memblock.

Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
---
 kernel/crash_core.c | 118 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 118 insertions(+)

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index eb53f5ec62c9..d567839b476c 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -8,12 +8,16 @@
 #include <linux/crash_core.h>
 #include <linux/utsname.h>
 #include <linux/vmalloc.h>
+#include <linux/memory.h>
+#include <linux/cpuhotplug.h>
 
 #include <asm/page.h>
 #include <asm/sections.h>
 
 #include <crypto/sha1.h>
 
+#include "kexec_internal.h"
+
 /* vmcoreinfo stuff */
 unsigned char *vmcoreinfo_data;
 size_t vmcoreinfo_size;
@@ -480,3 +484,117 @@ static int __init crash_save_vmcoreinfo_init(void)
 }
 
 subsys_initcall(crash_save_vmcoreinfo_init);
+
+#ifdef CONFIG_CRASH_HOTPLUG
+void __weak arch_update_crash_elfcorehdr(struct kimage *image,
+	unsigned int hp_action, unsigned long a, unsigned long b)
+{
+	pr_warn("crash hp: %s not implemented", __func__);
+}
+
+static void crash_update_elfcorehdr(unsigned int hp_action,
+	unsigned long a, unsigned long b)
+{
+	/* Obtain lock while changing crash information */
+	if (!mutex_trylock(&kexec_mutex))
+		return;
+
+	/* Check kdump is loaded */
+	if (kexec_crash_image) {
+		pr_debug("crash hp: hp_action %u, a %lu, b %lu", hp_action,
+			a, b);
+
+		/* Needed in order for the segments to be updated */
+		arch_kexec_unprotect_crashkres();
+
+		/* Flag to differentiate between normal load and hotplug */
+		kexec_crash_image->hotplug_event = 1;
+
+		/*
+		 * Due to use of CPUHP_AP_ONLINE_DYN, upon unplug and during
+		 * this callback, the CPU is still in the present list. Must
+		 * explicitly look to exclude it when building new elfcorehdr.
+		 */
+		kexec_crash_image->offlinecpu =
+			(hp_action == KEXEC_CRASH_HP_REMOVE_CPU) ?
+				(unsigned int)a : ~0U;
+
+		/* Now update the elfcorehdr and friends */
+		arch_update_crash_elfcorehdr(kexec_crash_image, hp_action,
+			a, b);
+
+		/* No longer in hotplug */
+		kexec_crash_image->hotplug_event = 0;
+
+		/* Change back to read-only */
+		arch_kexec_protect_crashkres();
+	}
+
+	/* Release lock now that update complete */
+	mutex_unlock(&kexec_mutex);
+}
+
+#if defined(CONFIG_MEMORY_HOTPLUG)
+static int crash_memhp_notifier(struct notifier_block *nb,
+	unsigned long val, void *v)
+{
+	struct memory_notify *mhp = v;
+	unsigned long start, end;
+
+	start = mhp->start_pfn << PAGE_SHIFT;
+	end = ((mhp->start_pfn + mhp->nr_pages) << PAGE_SHIFT) - 1;
+
+	switch (val) {
+	case MEM_GOING_ONLINE:
+		crash_update_elfcorehdr(KEXEC_CRASH_HP_ADD_MEMORY,
+			start, end-start);
+		break;
+
+	case MEM_OFFLINE:
+	case MEM_CANCEL_ONLINE:
+		crash_update_elfcorehdr(KEXEC_CRASH_HP_REMOVE_MEMORY,
+			start, end-start);
+		break;
+	}
+	return NOTIFY_OK;
+}
+
+static struct notifier_block crash_memhp_nb = {
+	.notifier_call = crash_memhp_notifier,
+	.priority = 0
+};
+#endif
+
+#if defined(CONFIG_HOTPLUG_CPU)
+static int crash_cpuhp_online(unsigned int cpu)
+{
+	crash_update_elfcorehdr(KEXEC_CRASH_HP_ADD_CPU, cpu, 0);
+	return 0;
+}
+
+static int crash_cpuhp_offline(unsigned int cpu)
+{
+	crash_update_elfcorehdr(KEXEC_CRASH_HP_REMOVE_CPU, cpu, 0);
+	return 0;
+}
+#endif
+
+static int __init crash_hotplug_init(void)
+{
+	int result = 0;
+
+#if defined(CONFIG_MEMORY_HOTPLUG)
+	register_memory_notifier(&crash_memhp_nb);
+#endif
+
+#if defined(CONFIG_HOTPLUG_CPU)
+	result = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
+				"crash/cpuhp",
+				crash_cpuhp_online, crash_cpuhp_offline);
+#endif
+
+	return result;
+}
+
+subsys_initcall(crash_hotplug_init);
+#endif /* CONFIG_CRASH_HOTPLUG */
-- 
2.27.0