Re: [PATCH 2/2] x86, e820 disable ACPI Memory Hotplug if memory mappingis specified by user [v2]

[Prarit Bhargava]

On 01/13/2014 03:31 PM, KOSAKI Motohiro wrote:
> On Sun, Jan 12, 2014 at 6:46 PM, Prarit Bhargava <prarit@xxxxxxxxxx> wrote:
>>
>>
>> On 01/11/2014 11:35 AM, 7eggert@xxxxxx wrote:
>>>
>>>
>>> On Fri, 10 Jan 2014, Prarit Bhargava wrote:
>>>
>>>> kdump uses memmap=exactmap and mem=X values to configure the memory
>>>> mapping for the kdump kernel.  If memory is hotadded during the boot of
>>>> the kdump kernel it is possible that the page tables for the new memory
>>>> cause the kdump kernel to run out of memory.
>>>>
>>>> Since the user has specified a specific mapping ACPI Memory Hotplug should be
>>>> disabled in this case.
>>>
>>> I'll ask just in case: Is it possible to want memory hotplug in spite of
>>> using memmap=exactmap or mem=X?
>>
>> Good question -- I can't think of a case.  When a user specifies "memmap" or
>> "mem" IMO they are asking for a very specific memory configuration.  Having
>> extra memory added above what the user has specified seems to defeat the purpose
>> of "memmap" and "mem".
> 
> May be yes, may be no.
> 
> They are often used for a wrokaround to avoid broken firmware issue.
> If we have no way
> to explicitly enable hotplug. We will lose a workaround.
> 
> Perhaps, there is no matter. Today, memory hotplug is only used on
> high-end machine
> and their firmware is carefully developped and don't have a serious
> issue almostly. Though.

Oof -- sorry Kosaki :(  I didn't see this until just now (and your subsequent
ACK on the updated patch).

I just remembered that we did have a processor vendor's whitebox that would not
boot unless we specified a specific memmap and we did specify memmap=exactmap to
boot the system correctly and the system had hotplug memory.

So it means that I should not key off of "memmap=exactmap".

I will self-NAK the updated patch and submit a new one.

P.


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