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Date:   Thu, 24 Mar 2022 09:37:09 -0500
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Subject: Re: [PATCH v5 4/8] crash: generic crash hotplug support
 infrastructure
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To:     Baoquan He <bhe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        kexec@lists.infradead.org, ebiederm@xmission.com,
        dyoung@redhat.com, vgoyal@redhat.com, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        hpa@zytor.com, nramas@linux.microsoft.com, thomas.lendacky@amd.com,
        robh@kernel.org, efault@gmx.de, rppt@kernel.org, david@redhat.com,
        konrad.wilk@oracle.com, boris.ostrovsky@oracle.com
References: <20220303162725.49640-1-eric.devolder@oracle.com>
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From:   Eric DeVolder <eric.devolder@oracle.com>
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On 3/24/22 09:33, Baoquan He wrote:
> On 03/24/22 at 08:53am, Eric DeVolder wrote:
>> Baoquan,
>> Thanks, I've offered a minor correction below.
>> eric
>>
>> On 3/24/22 08:49, Baoquan He wrote:
>>> On 03/24/22 at 09:38pm, Baoquan He wrote:
>>>> On 03/03/22 at 11:27am, Eric DeVolder wrote:
>>>>> This patch introduces a generic crash hot plug/unplug infrastructure
>>>>> for CPU and memory changes. Upon CPU and memory changes, a generic
>>>>> crash_hotplug_handler() obtains the appropriate lock, does some
>>>>> important house keeping and then dispatches the hot plug/unplug event
>>>>> to the architecture specific arch_crash_hotplug_handler(), and when
>>>>> that 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 generic handler
>>>>> crash_hotplug_handler().
>>>>>
>>>>> 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 new CPU list, 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.
>>>>
>>>> I rewrite the log as below with my understanding. Hope it's simpler to
>>>> help people get what's going on here. Please consider to take if it's
>>>> OK to you or adjust based on this. The code looks good to me.
>>>>
>>> Made some tuning:
>>>
>>> crash: add generic infrastructure for crash hotplug support
>>>
>>> Upon CPU and memory changes, a generic crash_hotplug_handler() is added
>>> to dispatch the hot plug/unplug event to the architecture specific
>>> arch_crash_hotplug_handler(). During the process, kexec_mutex need be
>>> held.
>>>
>>> To support cpu hotplug, one callback pair are registered to capture
>>> KEXEC_CRASH_HP_ADD_CPU and KEXEC_CRASH_HP_REMOVE_CPU events via
>>> cpuhp_setup_state_nocalls().
>> s/KEXEC_CRASH_HP_ADD}REMOVE_CPU/CPUHP_AP_ONLINE_DYN/ as the KEXEC_CRASH are the
>> names I've introduced with this patch?
> 
> Right.
> 
> While checking it, I notice hp_action which you don't use actually.
> Can you reconsider that part of design, the hp_action, the a, b
> parameter passed to handler?

Sure I can remove. I initially put in there as this was generic infrastructure and not sure if it 
would benefit others.
eric

> 
>>
>>>
>>> To support memory hotplug, a notifier crash_memhp_nb is registered to
>>> memory_chain to watch MEM_ONLINE and MEM_OFFLINE events.
>>>
>>> These callbacks and notifier will call crash_hotplug_handler() to handle
>>> captured event when invoked.
>>>
>>>>
>>>>>
>>>>> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
>>>>> ---
>>>>>    include/linux/kexec.h |  16 +++++++
>>>>>    kernel/crash_core.c   | 108 ++++++++++++++++++++++++++++++++++++++++++
>>>>>    2 files changed, 124 insertions(+)
>>>>>
>>>>> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
>>>>> index d7b59248441b..b11d75a6b2bc 100644
>>>>> --- a/include/linux/kexec.h
>>>>> +++ b/include/linux/kexec.h
>>>>> @@ -300,6 +300,13 @@ struct kimage {
>>>>>    	/* Information for loading purgatory */
>>>>>    	struct purgatory_info purgatory_info;
>>>>> +
>>>>> +#ifdef CONFIG_CRASH_HOTPLUG
>>>>> +	bool hotplug_event;
>>>>> +	int offlinecpu;
>>>>> +	bool elf_index_valid;
>>>>> +	int elf_index;
>>>>> +#endif
>>>>>    #endif
>>>>>    #ifdef CONFIG_IMA_KEXEC
>>>>> @@ -316,6 +323,15 @@ struct kimage {
>>>>>    	unsigned long elf_load_addr;
>>>>>    };
>>>>> +#ifdef CONFIG_CRASH_HOTPLUG
>>>>> +void arch_crash_hotplug_handler(struct kimage *image,
>>>>> +	unsigned int hp_action, unsigned long a, unsigned long b);
>>>>> +#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..76959d440f71 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 +496,106 @@ 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 long a, unsigned long b)
>>>>> +{
>>>>> +	pr_warn("crash hp: %s not implemented", __func__);
>>>>> +}
>>>>> +
>>>>> +static void crash_hotplug_handler(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 = true;
>>>>> +
>>>>> +		/* Now invoke arch-specific update handler */
>>>>> +		arch_crash_hotplug_handler(kexec_crash_image, hp_action, a, b);
>>>>> +
>>>>> +		/* 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;
>>>>> +	unsigned long start, end;
>>>>> +
>>>>> +	start = mhp->start_pfn << PAGE_SHIFT;
>>>>> +	end = ((mhp->start_pfn + mhp->nr_pages) << PAGE_SHIFT) - 1;
>>>>> +
>>>>> +	switch (val) {
>>>>> +	case MEM_ONLINE:
>>>>> +		crash_hotplug_handler(KEXEC_CRASH_HP_ADD_MEMORY,
>>>>> +			start, end-start);
>>>>> +		break;
>>>>> +
>>>>> +	case MEM_OFFLINE:
>>>>> +		crash_hotplug_handler(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_hotplug_handler(KEXEC_CRASH_HP_ADD_CPU, cpu, 0);
>>>>> +	return 0;
>>>>> +}
>>>>> +
>>>>> +static int crash_cpuhp_offline(unsigned int cpu)
>>>>> +{
>>>>> +	crash_hotplug_handler(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
>>>>>
>>>>
>>>
>>
>