kdump failed because of hotplug memory adding in kdump kernel

Yasuaki Ishimatsu isimatu.yasuaki at jp.fujitsu.com
Fri Jan 10 03:06:56 EST 2014

(2014/01/10 16:11), Baoquan wrote:
> On 01/09/14 at 02:56pm, Toshi Kani wrote:
>> On Thu, 2014-01-09 at 16:27 -0500, Vivek Goyal wrote:
>>> On Thu, Jan 09, 2014 at 11:34:30AM -0700, Toshi Kani wrote:
>>>> On Thu, 2014-01-09 at 13:23 -0500, Vivek Goyal wrote:
>>>>> On Thu, Jan 09, 2014 at 10:24:25AM -0700, Toshi Kani wrote:
>>>>> [..]
>>>>>>> I think creating a new command line option is simpler as compared to
>>>>>>> creating a new flag in bootparam which in turn disables memory hotplug.
>>>>>>> More users can use that option. For example, if for some reason hotplug
>>>>>>> code is crashing, one can just disable it on command line as work around
>>>>>>> and move on.
>>>>>> I do not have a strong opinion about having such option.  However, I
>>>>>> think it is more user friendly to keep the exactmap option works alone
>>>>>> on any platforms.
>>>>> I think we should create internally a variable which will disable memory
>>>>> hotplug. And set that variable based on memmap=exactmap, mem=X and also
>>>>> provide a way to disable memory hotplug directly using command line
>>>>> option.
>>>>> Current kexec-tools can use memmap=exactmap and be happy. I am writing
>>>>> a new kexec syscall and will not be using memmap=exactmap and would need
>>>>> to use that command line option to disable memory hotplug behavior.
>>>> Sounds good to me.
>>> Nobody responded to my other question, so I would ask it again.
>>> Assume we have disabled hotplug memory in second kernel. First kernel
>>> saw hotplug memory and assume crash kernel reserved region came from
>>> there. We will pass this memory in bootparams to second kernel and it
>>> will show up in E820 map. It should still be accessible in second kernel,
>>> is that right?
>> Yes.
>>> Or there is some dependency on ACPI doing some magic before this memory
>>> range is available in second kernel?
>> No.  The 1st kernel reserves the crash kernel region, which cannot be
>> hot-deleted.  So, this region continues to be accessible by the 2nd
>> kernel without any operation.

If my understanding is correct:

> Now what I understand is if a several memsection is reserved for
> crashkernel, then in 2nd kernel, they are just like normal memory.


> In ns
> object tree, they are not treated as hotplug memory.

They are treated as hotplug memory. But the memory cannot hot removed
because the memory has kernel memory.

> Otherwise, any hotplug memory which is not reserved for 2nd kernel can
> be parsed and need be added as hotplug memory, and add them into movable
> zone.

The memory is allocated as normal zone and it is offline.

> Am I right?

> The other question, e820 reserve is done earlier than acpi
> initialization, because acpi_early_init() invocation is very late in
> start_kernel(). Does that means at the very beginning all memorys are in
> e820, later when acpi_early_init is called, hotplug memory is detected,
> they will be moved to different place or need be marked with a specific
> flag?


Yasuaki Ishimatsu

>> I am more curious to know how makedumpfile decides what memory ranges to
>> dump.  The 1st kernel may have performed memory hot-add / delete
>> operations before a crash, so it needs to know the valid physical
>> address range at the time of crash, and may not rely on the E820 map
>> from BIOS (which is stale).  Am I right to assume that makedumpfile gets
>> it from the page tables of the 1st kernel?
> makedumpfile just do the dump, what memory ranges to dump is decided in
> 1st kernel by kexec-tools. In 1st kernel, if kexec-tools executed, it
> will find all System Ram memorys which exclude the reserved regions for
> kdump kernel, then build a logical elf file, each load segment is one of
> these System Ram memory regions, its addr and length is written into the
> program header.
> Then makedumpfile just read this elf file, and read all of them and
> dump.
> If after kexec-tools execution and before crash, a hotplug memory is
> removed, udev will check this and trigger a kdump restart, kexec-tools
> is executed again, System Ram region information are stored. The logical
> file header will be passed to 2nd kernel.
>> Thanks,
>> -Toshi
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