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Date:   Wed, 26 Oct 2022 16:54:30 +0200
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To:     Baoquan He <bhe@redhat.com>, Borislav Petkov <bp@alien8.de>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support
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On 26.10.22 16:48, Baoquan He wrote:
> On 10/25/22 at 12:31pm, Borislav Petkov wrote:
>> On Thu, Oct 13, 2022 at 10:57:28AM +0800, Baoquan He wrote:
>>> The concern to range number mainly is on Virt guest systems.
>>
>> And why would virt emulate 1K hotpluggable DIMM slots and not emulate a
>> real machine?

IIRC, ACPI only allows for 256 slots. PPC dlpar might provide more.

> 
> Well, currently, mem hotpug is an important feature on virt system to
> dynamically increase/shrink memory on the system. If only emulating real
> machine, it won't be different than bare metal system.
> 
> IIRC, the ballon driver or virtio-mem feature can add memory board, e.g
> 1G, block size is 128M, 8 blocks added. When shrinking this 1G memory
> later, it will take best effort way to hot remove memory. Means if any
> memory block is occupied, it will be kept there. Finally we could only
> remove every second blocks, 4 blocks altogether. Then the left
> un-removed blocks will produce 4 separate memory regions. Like this, a
> virt guest could have many memory regions in kernel after memory
> being added/removed.
> 
> If I am wrong, Please correct me, David.

Yes, virtio-mem (but also PPC dlpar) can result in many individual 
memory blocks with holes in between after hotunplug. Hotplug OTOH, 
usually tries to "plug" these holes and reduce the total number of 
memory blocks. It might be rare that our range will be heavily 
fragmented after unplug, but it's certainly possible.

[...]

> 
> Yes, now assume we have a HPE SGI system and it has memory hotplug
> capacity. The system itself has already got memory regions more than
> 1024. Then when we hot add extra memory board, we want to include the
> newly added memory regions into elfcorehdr so that it will be dumped out
> in kdump kernel.
> 
> That's why I earlier suggested 2048 for number of memory regions.

The more the better, unless "it hurts". Assuming a single memory block 
is 128 MiB, that would be 256 GiB.

Usually, on big systems, the memory block size is 2 GiB. So 4 TiB.

-- 
Thanks,

David / dhildenb