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

[Baoquan He <bhe@redhat.com>]

On 04/14/22 at 05:29pm, Eric DeVolder wrote:
> Hi Baoquan,
> Inline comments below.
> Thanks!
> eric
> 
> 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
> > > cpuhp_setup_state_nocalls().
> > > 
> > > 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@oracle.com>
> > > ---
> > >   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;
> > > +
> > > +#ifdef CONFIG_CRASH_HOTPLUG
> > > +	bool hotplug_event;
> > > +	unsigned int offlinecpu;
> > > +	bool elfcorehdr_index_valid;
> > > +	int elfcorehdr_index;
> > > +#endif
> > >   #endif
> > >   #ifdef CONFIG_IMA_KEXEC
> > > @@ -322,6 +329,15 @@ struct kimage {
> > >   	unsigned long elf_load_addr;
> > >   };
> > > +#ifdef CONFIG_CRASH_HOTPLUG
> > > +void arch_crash_hotplug_handler(struct kimage *image,
> > > +	unsigned int hp_action, unsigned int cpu);
> > > +#define KEXEC_CRASH_HP_REMOVE_CPU   0
> > > +#define KEXEC_CRASH_HP_ADD_CPU      1
> > > +#define KEXEC_CRASH_HP_REMOVE_MEMORY 2
> > > +#define KEXEC_CRASH_HP_ADD_MEMORY   3
> > > +#endif /* CONFIG_CRASH_HOTPLUG */
> > > +
> > >   /* 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)
> > >   }
> > >   subsys_initcall(crash_save_vmcoreinfo_init);
> > > +
> > > +#ifdef CONFIG_CRASH_HOTPLUG
> > > +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);
> > > +}
> > > +
> > > +#if defined(CONFIG_MEMORY_HOTPLUG)
> > > +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?
> > 
> > #define KEXEC_CRASH_HP_REMOVE_CPU   0
> > #define KEXEC_CRASH_HP_UPDATE_OTHER      1
> > 
> Sourabh Jain's work with PPC uses REMOVE_CPU, REMOVE_MEMORY, and ADD_MEMORY.
> Do you still want to consolidate these?

I checked Sourabh's ppc patches, am wondering why memory hotplug is not
handled. Is it going to be that CONFIG_MEMORY_HOTPLUG is enabled on ppc,
while Sourabh is not willing to handle it for now?

Only passing KEXEC_CRASH_HP_ADD_MEMORY/KEXEC_CRASH_HP_REMOVE_MEMORY to
indicate that they are not supported sounds weird to me, honestly.