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Date:   Tue, 15 Mar 2022 09:12:17 -0500
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Subject: Re: [PATCH v5 4/8] crash: generic crash hotplug support
 infrastructure
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To:     Sourabh Jain <sourabhjain@linux.ibm.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        kexec@lists.infradead.org, ebiederm@xmission.com,
        dyoung@redhat.com, bhe@redhat.com, vgoyal@redhat.com
Cc:     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>
 <20220303162725.49640-5-eric.devolder@oracle.com>
 <1b7405e4-e6a3-56ff-5068-d598ba2f2b5e@linux.ibm.com>
From:   Eric DeVolder <eric.devolder@oracle.com>
In-Reply-To: <1b7405e4-e6a3-56ff-5068-d598ba2f2b5e@linux.ibm.com>
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On 3/15/22 07:08, Sourabh Jain wrote:
> Hello Eric,
> 
> On 03/03/22 21:57, 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.
>>
>> 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;
> 
> How about keeping an array to track all kexec segment index need to be updated in
> crash hotplug handler.
> 
> struct hp_segment {
>     name;
>     index;
>     is_valid;
>   }
> 
> It will be helpful if architecture need to updated multiple kexec segments  for a hotplug event.
> 
> For example, on PowerPC, we might need to update FDT and elfcorehdr on memory hot plug/unplug.
> 
> Thanks,
> Sourabh Jain

Sourabh,
I'm OK with that. Another idea might be if there are just two, and one of them is elfcorehdr, then 
perhaps in addition to elf_index and elf_index_valid, maybe we add an arch_index and 
arch_index_valid? In the case of PPC, the FDT would be stored in arch_index?

Either way.
Thanks!
eric

> 
> 
>> +#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 */