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Date:   Wed, 9 Feb 2022 14:01:14 -0600
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Subject: Re: [PATCH v3 0/6] crash: Kernel handling of CPU and memory hot
 un/plug
Content-Language: en-US
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,
        konrad.wilk@oracle.com, boris.ostrovsky@oracle.com
References: <20220110195727.1682-1-eric.devolder@oracle.com>
 <20220127065101.GA13508@MiWiFi-R3L-srv>
From:   Eric DeVolder <eric.devolder@oracle.com>
In-Reply-To: <20220127065101.GA13508@MiWiFi-R3L-srv>
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Baoquan,
Thanks for the feedback! I think I've incorporated all suggestions in this message and others in v3. 
As a result, I just posted v4.
Thank you!
eric

On 1/27/22 00:51, Baoquan He wrote:
> Hi Eric,
> 
> On 01/10/22 at 02:57pm, Eric DeVolder wrote:
>> When the kdump service is loaded, if a CPU or memory is hot
>> un/plugged, the crash elfcorehdr (for x86), which describes the CPUs
>> and memory in the system, must also be updated, else the resulting
>> vmcore is inaccurate (eg. missing either CPU context or memory
>> regions).
>>
>> The current solution utilizes udev to initiate an unload-then-reload
>> of the kdump image (e. kernel, initrd, boot_params, puratory and
>> elfcorehdr) by the userspace kexec utility. In previous posts I have
>> outlined the significant performance problems related to offloading
>> this activity to userspace.
>>
>> This patchset introduces a generic crash hot un/plug handler that
>> registers with the CPU and memory notifiers. Upon CPU or memory
>> changes, this generic handler is invoked and performs important
>> housekeeping, for example obtaining the appropriate lock, and then
>> invokes an architecture specific handler to do the appropriate
>> updates.
>>
>> In the case of x86_64, the arch specific handler generates a new
>> elfcorehdr, and overwrites the old one in memory. No involvement
>> with userspace needed.
> 
> About patch splitting, it is always a headache to me when I post
> patches. Both too coarse or too small granularity are not good. But we
> should obey two rules:
> 1) commits had better be bisectable, so that later we find out which
>     commit cause issue or regression easier. Here, your patch 5/6 and 6/6 are
>     obviously breaking the rule. Because crash_prepare_elf64_headers()'s
>     protype has been changed, its invokation is not modified accodingly.
> 
> 2) Each patch only cover one action, bad better not stack too much in
> one patch. Surely, we should avoid to scatter one thing into several
> patch, it's not easy to reivew. E.g in this patchset, kexec_load can be
> taken out, for this we have discussed. Skipping the elfcorehdr digest
> can be taken out, its code change is simple, while it's worth to tell
> the reason in a patch.
> 
> So my suggestion to rearrange the patchset as below, please take it as a
> reference.
> 1)crash: fix minor typo/bug in debug message
> 2)crash hp: Introduce CRASH_HOTPLUG configuration options
> 3)crash hp: definitions and prototype changes
>    Note: this patch need cover those invokation of crash_prepare_elf64_headers.
>    Otherwise it's not bisectable.
> 4)crash hp: introduce helper function map_crash_pages/unmap_crash_pages
> 5)crash hp: generic crash hotplug support infrastructure
> 6)crash hp: exclude elfcorehdr from the segment checksum calculation
> 7)crash hp: exclude hot removed cpu from cpu notes of elfcorehdr
>    this includes the assigment part in crash_hotplug_handler() and
>    usage part in crash_prepare_elf64_headers().
> 8)crash hp/x86: add the crash hp support for x86
> 
> Above patch splitting and patch subject may not be appropriate, please
> adust or change with your understanding.
> 
>>
>> To realize the benefits/test this patchset, one must make a couple
>> of minor changes to userspace:
>>
>>   - Disable the udev rule for updating kdump on hot un/plug changes
>>     Eg. on RHEL: rm -f /usr/lib/udev/rules.d/98-kexec.rules
>>     or other technique to neuter the rule.
>>
>>   - Change to the kexec_file_load for loading the kdump kernel:
>>     Eg. on RHEL: in /usr/bin/kdumpctl, change to:
>>      standard_kexec_args="-p -d -s"
>>     which adds the -s to select kexec_file_load syscall.
>>
>> This patchset supports kexec_load with a modified kexec userspace
>> utility, and a working changeset to the kexec userspace utility
>> is provided here (and to use, the above change to standard_kexec_args
>> would be, for example, to append --hotplug-size=262144 instead of -s).
>>
>>   diff --git a/kexec/arch/i386/crashdump-x86.c b/kexec/arch/i386/crashdump-x86.c
>>   index 9826f6d..06adb7e 100644
>>   --- a/kexec/arch/i386/crashdump-x86.c
>>   +++ b/kexec/arch/i386/crashdump-x86.c
>>   @@ -48,6 +48,7 @@
>>    #include <x86/x86-linux.h>
>>    
>>    extern struct arch_options_t arch_options;
>>   +extern unsigned long long hotplug_size;
>>    
>>    static int get_kernel_page_offset(struct kexec_info *UNUSED(info),
>>    				  struct crash_elf_info *elf_info)
>>   @@ -975,6 +976,13 @@ int load_crashdump_segments(struct kexec_info *info, char* mod_cmdline,
>>    	} else {
>>    		memsz = bufsz;
>>    	}
>>   +
>>   +    /* If hotplug support enabled, use that size */
>>   +    if (hotplug_size) {
>>   +        memsz = hotplug_size;
>>   +    }
>>   +
>>   +    info->elfcorehdr =
>>    	elfcorehdr = add_buffer(info, tmp, bufsz, memsz, align, min_base,
>>    							max_addr, -1);
>>    	dbgprintf("Created elf header segment at 0x%lx\n", elfcorehdr);
>>   diff --git a/kexec/kexec.c b/kexec/kexec.c
>>   index f63b36b..9569d9a 100644
>>   --- a/kexec/kexec.c
>>   +++ b/kexec/kexec.c
>>   @@ -58,6 +58,7 @@
>>    
>>    unsigned long long mem_min = 0;
>>    unsigned long long mem_max = ULONG_MAX;
>>   +unsigned long long hotplug_size = 0;
>>    static unsigned long kexec_flags = 0;
>>    /* Flags for kexec file (fd) based syscall */
>>    static unsigned long kexec_file_flags = 0;
>>   @@ -672,6 +673,12 @@ static void update_purgatory(struct kexec_info *info)
>>    		if (info->segment[i].mem == (void *)info->rhdr.rel_addr) {
>>    			continue;
>>    		}
>>   +        /* Don't include elfcorehdr in the checksum, if hotplug
>>   +         * support enabled.
>>   +         */
>>   +        if (hotplug_size && (info->segment[i].mem == (void *)info->elfcorehdr)) {
>>   +			continue;
>>   +		}
>>    		sha256_update(&ctx, info->segment[i].buf,
>>    			      info->segment[i].bufsz);
>>    		nullsz = info->segment[i].memsz - info->segment[i].bufsz;
>>   @@ -1504,6 +1511,17 @@ int main(int argc, char *argv[])
>>    		case OPT_PRINT_CKR_SIZE:
>>    			print_crashkernel_region_size();
>>    			return 0;
>>   +		case OPT_HOTPLUG_SIZE:
>>   +            /* Reserved the specified size for hotplug growth */
>>   +			hotplug_size = strtoul(optarg, &endptr, 0);
>>   +			if (*endptr) {
>>   +				fprintf(stderr,
>>   +					"Bad option value in --hotplug-size=%s\n",
>>   +					optarg);
>>   +				usage();
>>   +				return 1;
>>   +			}
>>   +			break;
>>    		default:
>>    			break;
>>    		}
>>   diff --git a/kexec/kexec.h b/kexec/kexec.h
>>   index 595dd68..b30dda4 100644
>>   --- a/kexec/kexec.h
>>   +++ b/kexec/kexec.h
>>   @@ -169,6 +169,7 @@ struct kexec_info {
>>    	int command_line_len;
>>    
>>    	int skip_checks;
>>   +    unsigned long elfcorehdr;
>>     };
>>    
>>    struct arch_map_entry {
>>   @@ -231,7 +232,8 @@ extern int file_types;
>>    #define OPT_PRINT_CKR_SIZE	262
>>    #define OPT_LOAD_LIVE_UPDATE	263
>>    #define OPT_EXEC_LIVE_UPDATE	264
>>   -#define OPT_MAX			265
>>   +#define OPT_HOTPLUG_SIZE	265
>>   +#define OPT_MAX			266
>>    #define KEXEC_OPTIONS \
>>    	{ "help",		0, 0, OPT_HELP }, \
>>    	{ "version",		0, 0, OPT_VERSION }, \
>>   @@ -258,6 +260,7 @@ extern int file_types;
>>    	{ "debug",		0, 0, OPT_DEBUG }, \
>>    	{ "status",		0, 0, OPT_STATUS }, \
>>    	{ "print-ckr-size",     0, 0, OPT_PRINT_CKR_SIZE }, \
>>   +	{ "hotplug-size",     2, 0, OPT_HOTPLUG_SIZE }, \
>>    
>>    #define KEXEC_OPT_STR "h?vdfixyluet:pscaS"
>>   
>>
>> Regards,
>> eric
>> ---
>> v3: 10jan2022
>>   - Rebasing per Baoquan He request.
>>   - Changed memory notifier per David Hildenbrand.
>>   - Providing example kexec userspace change in cover letter.
>>
>> RFC v2: 7dec2021
>>   https://lkml.org/lkml/2021/12/7/1088
>>   - Acting upon Baoquan He suggestion of removing elfcorehdr from
>>     the purgatory list of segments, removed purgatory code from
>>     patchset, and it is signficiantly simpler now.
>>
>> RFC v1: 18nov2021
>>   https://lkml.org/lkml/2021/11/18/845
>>   - working patchset demonstrating kernel handling of hotplug
>>     updates to x86 elfcorehdr for kexec_file_load
>>
>> RFC: 14dec2020
>>   https://lkml.org/lkml/2020/12/14/532
>>   - proposed concept of allowing kernel to handle hotplug update
>>     of elfcorehdr
>> ---
>>
>> Eric DeVolder (6):
>>    crash: fix minor typo/bug in debug message
>>    crash hp: Introduce CRASH_HOTPLUG configuration options
>>    crash hp: definitions and prototype changes
>>    crash hp: generic crash hotplug support infrastructure
>>    crash hp: kexec_file changes for crash hotplug support
>>    crash hp: Add x86 crash hotplug support
>>
>>   arch/x86/Kconfig        |  26 ++++++++
>>   arch/x86/kernel/crash.c | 140 +++++++++++++++++++++++++++++++++++++++-
>>   include/linux/kexec.h   |  21 +++++-
>>   kernel/crash_core.c     | 117 +++++++++++++++++++++++++++++++++
>>   kernel/kexec_file.c     |  15 ++++-
>>   5 files changed, 313 insertions(+), 6 deletions(-)
>>
>> -- 
>> 2.27.0
>>
>