Re: [PATCH v7 4/8] crash: add generic infrastructure for crash hotplug support

From: Sourabh Jain
Date: Thu Apr 28 2022 - 01:19:38 EST

Hi Baoquan,

On 26/04/22 10:52, Baoquan He wrote:
On 04/26/22 at 09:36am, Sourabh Jain wrote:
On 15/04/22 03:59, Eric DeVolder wrote:
Hi Baoquan,
Inline comments below.

On 4/13/22 21:45, Baoquan He wrote:
On 04/13/22 at 12:42pm, Eric DeVolder wrote:
Upon CPU and memory changes, a generic crash_hotplug_handler()
dispatches the hot plug/unplug event to the architecture specific
arch_crash_hotplug_handler(). During the process, the kexec_mutex
is held.

To support cpu hotplug, a callback is registered to capture the
CPUHP_AP_ONLINE_DYN online and ofline events via

To support memory hotplug, a notifier is registered to capture the
MEM_ONLINE and MEM_OFFLINE events via register_memory_notifier().

The cpu callback and memory notifier then call crash_hotplug_handler()
to handle the hot plug/unplug event.

Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
  include/linux/kexec.h |  16 +++++++
  kernel/crash_core.c   | 101
  2 files changed, 117 insertions(+)

diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index f93f2591fc1e..02daff1f47dd 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -306,6 +306,13 @@ struct kimage {
        /* Information for loading purgatory */
      struct purgatory_info purgatory_info;
+    bool hotplug_event;
+    unsigned int offlinecpu;
+    bool elfcorehdr_index_valid;
+    int elfcorehdr_index;
@@ -322,6 +329,15 @@ struct kimage {
      unsigned long elf_load_addr;
+void arch_crash_hotplug_handler(struct kimage *image,
+    unsigned int hp_action, unsigned int cpu);
+#define KEXEC_CRASH_HP_ADD_CPU      1
  /* kexec interface functions */
  extern void machine_kexec(struct kimage *image);
  extern int machine_kexec_prepare(struct kimage *image);
diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 256cf6db573c..ecf746243ab2 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -9,12 +9,17 @@
  #include <linux/init.h>
  #include <linux/utsname.h>
  #include <linux/vmalloc.h>
+#include <linux/highmem.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;
@@ -491,3 +496,99 @@ static int __init crash_save_vmcoreinfo_init(void)
+void __weak arch_crash_hotplug_handler(struct kimage *image,
+    unsigned int hp_action, unsigned int cpu)
+    pr_warn("crash hp: %s not implemented", __func__);
+static void crash_hotplug_handler(unsigned int hp_action,
+    unsigned int cpu)
+    /* 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, cpu %u", hp_action, cpu);
+        /* 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 = true;
+        /* Now invoke arch-specific update handler */
+        arch_crash_hotplug_handler(kexec_crash_image, hp_action, cpu);
+        /* No longer handling a hotplug event */
+        kexec_crash_image->hotplug_event = false;
+        /* Change back to read-only */
+        arch_kexec_protect_crashkres();
+    }
+    /* Release lock now that update complete */
+    mutex_unlock(&kexec_mutex);
+static int crash_memhp_notifier(struct notifier_block *nb,
+    unsigned long val, void *v)
+    struct memory_notify *mhp = v;
+    switch (val) {
+    case MEM_ONLINE:
+        crash_hotplug_handler(KEXEC_CRASH_HP_ADD_MEMORY, -1U);
We don't differentiate the memory add/remove, cpu add, except of cpu
remove. Means the hp_action only differentiate cpu remove from the other
action. Maybe only making two types?


Sourabh Jain's work with PPC uses REMOVE_CPU, REMOVE_MEMORY, and
Do you still want to consolidate these?
On PowerPC different actions are needed for CPU add and memory add/remove.
For CPU add case only FDT is updated whereas for the memory hotplug we will
updating FDT and elfcorehdr.
I don't understand. For elfcorehdr updating, we only need regenerate it.
Do you update them different for memory add/remove?

We have different actions for cpu remove, CPU add and memory add/remove case.

CPU remove: no action
CPU add: update flattened device tree (FDT)
memory add/remove: update FDT and regenerate/update elfcorehdr

Since memory add/remove action is same we can have common hp_action for them.

What I saw is the added action for memory hotplug is only for message
printing. Is this really needed? And memory hotplug is even not
supported. Please correct me if I missed anything.

I agree that currently memory hp_action is only used for printing warning message but
eventually we will be handling memory hotplug case as well.

+ /* crash update on memory hotplug is not support yet */
+ if (hp_action == KEXEC_CRASH_HP_REMOVE_MEMORY || hp_action == KEXEC_CRASH_HP_ADD_MEMORY) {
+ pr_info_once("crash hp: crash update is not supported with memory hotplug\n");
+ return;
+ }

Sourabh Jain