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Date:   Fri, 7 Oct 2022 14:14:39 -0500
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Subject: Re: [PATCH v12 3/7] crash: add generic infrastructure for crash
 hotplug support
Content-Language: en-US
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: <20220909210509.6286-1-eric.devolder@oracle.com>
 <20220909210509.6286-4-eric.devolder@oracle.com>
 <8cc31b22-a061-d07c-77dc-c555b8b35af3@linux.ibm.com>
From:   Eric DeVolder <eric.devolder@oracle.com>
In-Reply-To: <8cc31b22-a061-d07c-77dc-c555b8b35af3@linux.ibm.com>
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On 10/3/22 12:51, Sourabh Jain wrote:
> Hello Eric,
> 
> On 10/09/22 02:35, Eric DeVolder wrote:
>> CPU and memory change notifications are received in order to
>> regenerate the elfcorehdr.
>>
>> To support cpu hotplug, a callback is registered to capture the
>> CPUHP_AP_ONLINE_DYN online and offline 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 notifiers call handle_hotplug_event()
>> which performs needed tasks and then dispatches the event to the
>> architecture specific arch_crash_handle_hotplug_event(). During the
>> process, the kexec_mutex is held.
>>
>> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
>> Acked-by: Baoquan He <bhe@redhat.com>
>> ---
>>   include/linux/crash_core.h |   8 +++
>>   include/linux/kexec.h      |  26 +++++++
>>   kernel/crash_core.c        | 134 +++++++++++++++++++++++++++++++++++++
>>   3 files changed, 168 insertions(+)
>>
>> diff --git a/include/linux/crash_core.h b/include/linux/crash_core.h
>> index de62a722431e..a270f8660538 100644
>> --- a/include/linux/crash_core.h
>> +++ b/include/linux/crash_core.h
>> @@ -84,4 +84,12 @@ int parse_crashkernel_high(char *cmdline, unsigned long long system_ram,
>>   int parse_crashkernel_low(char *cmdline, unsigned long long system_ram,
>>           unsigned long long *crash_size, unsigned long long *crash_base);
>> +#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
>> +#define KEXEC_CRASH_HP_INVALID_CPU        -1U
>> +
>> +struct kimage;
>> +
>>   #endif /* LINUX_CRASH_CORE_H */
>> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
>> index 4eefa631e0ae..9597b41136ec 100644
>> --- a/include/linux/kexec.h
>> +++ b/include/linux/kexec.h
>> @@ -374,6 +374,13 @@ struct kimage {
>>       struct purgatory_info purgatory_info;
>>   #endif
>> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
>> +    bool hotplug_event;
>> +    unsigned int offlinecpu;
>> +    bool elfcorehdr_index_valid;
>> +    int elfcorehdr_index;
>> +#endif
>> +
>>   #ifdef CONFIG_IMA_KEXEC
>>       /* Virtual address of IMA measurement buffer for kexec syscall */
>>       void *ima_buffer;
>> @@ -503,6 +510,25 @@ static inline int arch_kexec_post_alloc_pages(void *vaddr, unsigned int pages, g
>>   static inline void arch_kexec_pre_free_pages(void *vaddr, unsigned int pages) { }
>>   #endif
>> +#ifndef arch_map_crash_pages
>> +static inline void *arch_map_crash_pages(unsigned long paddr,
>> +        unsigned long size)
>> +{
>> +    return NULL;
>> +}
>> +#endif
>> +
>> +#ifndef arch_unmap_crash_pages
>> +static inline void arch_unmap_crash_pages(void **ptr) { }
>> +#endif
>> +
>> +#ifndef arch_crash_handle_hotplug_event
>> +static inline void arch_crash_handle_hotplug_event(struct kimage *image,
>> +        unsigned int hp_action)
>> +{
>> +}
>> +#endif
>> +
>>   #else /* !CONFIG_KEXEC_CORE */
>>   struct pt_regs;
>>   struct task_struct;
>> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
>> index 8c648fd5897a..4b15d91f0b21 100644
>> --- a/kernel/crash_core.c
>> +++ b/kernel/crash_core.c
>> @@ -11,6 +11,8 @@
>>   #include <linux/vmalloc.h>
>>   #include <linux/sizes.h>
>>   #include <linux/kexec.h>
>> +#include <linux/memory.h>
>> +#include <linux/cpuhotplug.h>
>>   #include <asm/page.h>
>>   #include <asm/sections.h>
>> @@ -18,6 +20,7 @@
>>   #include <crypto/sha1.h>
>>   #include "kallsyms_internal.h"
>> +#include "kexec_internal.h"
>>   /* vmcoreinfo stuff */
>>   unsigned char *vmcoreinfo_data;
>> @@ -612,3 +615,134 @@ static int __init crash_save_vmcoreinfo_init(void)
>>   }
>>   subsys_initcall(crash_save_vmcoreinfo_init);
>> +
>> +#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
>> +/*
>> + * To accurately reflect hot un/plug changes, the elfcorehdr (which
>> + * is passed to the crash kernel via the elfcorehdr= parameter)
>> + * must be updated with the new list of CPUs and memories.
>> + *
>> + * In order to make changes to elfcorehdr, two conditions are needed:
>> + * First, the segment containing the elfcorehdr must be large enough
>> + * to permit a growing number of resources. The elfcorehdr memory is
>> + * typically based on CONFIG_NR_CPUS and CONFIG_CRASH_MAX_MEMORY_RANGES.
>> + * Second, purgatory must explicitly exclude the elfcorehdr from the
>> + * list of segments it checks (since the elfcorehdr changes and thus
>> + * would require an update to purgatory itself to update the digest).
>> + */
>> +static void handle_hotplug_event(unsigned int hp_action, unsigned int cpu)
>> +{
>> +    /* Obtain lock while changing crash information */
>> +    mutex_lock(&kexec_mutex);
>> +
>> +    /* Check kdump is loaded */
>> +    if (kexec_crash_image) {
>> +        struct kimage *image = kexec_crash_image;
>> +
>> +        if (hp_action == KEXEC_CRASH_HP_ADD_CPU ||
>> +            hp_action == KEXEC_CRASH_HP_REMOVE_CPU)
>> +            pr_debug("crash hp: hp_action %u, cpu %u\n", hp_action, cpu);
>> +        else
>> +            pr_debug("crash hp: hp_action %u\n", hp_action);
>> +
>> +        /*
>> +         * When the struct kimage is allocated, it is wiped to zero, so
>> +         * the elfcorehdr_index_valid defaults to false. Find the
>> +         * segment containing the elfcorehdr, if not already found.
>> +         * This works for both the kexec_load and kexec_file_load paths.
>> +         */
>> +        if (!image->elfcorehdr_index_valid) {
>> +            unsigned char *ptr;
>> +            unsigned long mem, memsz;
>> +            unsigned int n;
>> +
>> +            for (n = 0; n < image->nr_segments; n++) {
>> +                mem = image->segment[n].mem;
>> +                memsz = image->segment[n].memsz;
>> +                ptr = arch_map_crash_pages(mem, memsz);
>> +                if (ptr) {
>> +                    /* The segment containing elfcorehdr */
>> +                    if (memcmp(ptr, ELFMAG, SELFMAG) == 0) {
>> +                        image->elfcorehdr_index = (int)n;
>> +                        image->elfcorehdr_index_valid = true;
>> +                    }
>> +                }
>> +                arch_unmap_crash_pages((void **)&ptr);
>> +            }
>> +        }
>> +
>> +        if (!image->elfcorehdr_index_valid) {
>> +            pr_err("crash hp: unable to locate elfcorehdr segment");
>> +            goto out;
>> +        }
>> +
>> +        /* Needed in order for the segments to be updated */
>> +        arch_kexec_unprotect_crashkres();
>> +
>> +        /* Flag to differentiate between normal load and hotplug */
>> +        image->hotplug_event = true;
>> +
>> +        /* Now invoke arch-specific update handler */
>> +        arch_crash_handle_hotplug_event(image, hp_action);
>> +
>> +        /* No longer handling a hotplug event */
>> +        image->hotplug_event = false;
>> +
>> +        /* Change back to read-only */
>> +        arch_kexec_protect_crashkres();
>> +    }
>> +
>> +out:
>> +    /* Release lock now that update complete */
>> +    mutex_unlock(&kexec_mutex);
>> +}
>> +
>> +static int crash_memhp_notifier(struct notifier_block *nb, unsigned long val, void *v)
>> +{
>> +    switch (val) {
>> +    case MEM_ONLINE:
>> +        handle_hotplug_event(KEXEC_CRASH_HP_ADD_MEMORY, 0);
>> +        break;
>> +
>> +    case MEM_OFFLINE:
>> +        handle_hotplug_event(KEXEC_CRASH_HP_REMOVE_MEMORY, 0);
>> +        break;
>> +    }
>> +    return NOTIFY_OK;
> 
> Can we pass v (memory_notify) argument to arch_crash_handle_hotplug_event function
> via handle_hotplug_event?
> 
> Because the way memory hotplug is handled on PowerPC, it is hard to update the elfcorehdr
> without memory_notify args.
> 
> On PowePC memblock data structure is used to prepare elfcorehdr for kdump. Since the notifier
> used for memory hotplug crash handler get initiated before the memblock data structure update
> happens (as depicted below), the newly prepared elfcorehdr still holds the old memory regions.
> So if the system crash with obsolete elfcorehdr, makedumpfile failed to collect vmcore.
> 
> Sequence of actions done on PowerPC to server the memory hotplug:
> 
>   Initiate memory hot remove
>            |
>            v
>   offline pages
>            |
>            v
>   initiate memory notify call chain
>   for MEM_OFFLINE event.
>   (same is used for crash update)
>            |
>            v
>   prepare new elfcorehdr for kdump using
>   memblock data structure
>            |
>            v
>   update memblock data structure
> 
> How passing memory_notify to arch crash hotplug handler will help?
> 
> memory_notify holds the start PFN and page count, with that we can get
> the base address and size of hot unplugged memory and can use the same
> to avoid hot unplugged memeory region to get added in the elfcorehdr..
> 
> Thanks,
> Sourabh Jain
> 

Sourabh, let's see what Baoquan thinks.

Baoquan, are you OK with this request? I once had these parameters to the
crash hotplug handler and since they were unused at the time, you asked
that I remove them, which I did.

To accommodate this, how about this:

static void handle_hotplug_event(unsigned int hp_action, unsigned int cpu,
      unsigned long mem_start, unsigned long mem_size)

For CPU events, I would just pass zeros for mem_start/size. For memory events,
I would pass KEXEC_CRASH_HP_INVALID_CPU.

Thanks,
eric