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Date:   Mon, 30 May 2022 09:41:22 +0200
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To:     zhenwei pi <pizhenwei@bytedance.com>, Peter Xu <peterx@redhat.com>,
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH 0/3] recover hardware corrupted page by virtio balloon
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On 27.05.22 08:32, zhenwei pi wrote:
> On 5/27/22 02:37, Peter Xu wrote:
>> On Wed, May 25, 2022 at 01:16:34PM -0700, Jue Wang wrote:
>>> The hypervisor _must_ emulate poisons identified in guest physical
>>> address space (could be transported from the source VM), this is to
>>> prevent silent data corruption in the guest. With a paravirtual
>>> approach like this patch series, the hypervisor can clear some of the
>>> poisoned HVAs knowing for certain that the guest OS has isolated the
>>> poisoned page. I wonder how much value it provides to the guest if the
>>> guest and workload are _not_ in a pressing need for the extra KB/MB
>>> worth of memory.
>>
>> I'm curious the same on how unpoisoning could help here.  The reasoning
>> behind would be great material to be mentioned in the next cover letter.
>>
>> Shouldn't we consider migrating serious workloads off the host already
>> where there's a sign of more severe hardware issues, instead?
>>
>> Thanks,
>>
> 
> I'm maintaining 1000,000+ virtual machines, from my experience:
> UE is quite unusual and occurs randomly, and I did not hit UE storm case 
> in the past years. The memory also has no obvious performance drop after 
> hitting UE.
> 
> I hit several CE storm case, the performance memory drops a lot. But I 
> can't find obvious relationship between UE and CE.
> 
> So from the point of my view, to fix the corrupted page for VM seems 
> good enough. And yes, unpoisoning several pages does not help 
> significantly, but it is still a chance to make the virtualization better.
> 

I'm curious why we should care about resurrecting a handful of poisoned
pages in a VM. The cover letter doesn't touch on that.

IOW, I'm missing the motivation why we should add additional
code+complexity to unpoison pages at all.

If we're talking about individual 4k pages, it's certainly sub-optimal,
but does it matter in practice? I could understand if we're losing
megabytes of memory. But then, I assume the workload might be seriously
harmed either way already?


I assume when talking about "the performance memory drops a lot", you
imply that this patch set can mitigate that performance drop?

But why do you see a performance drop? Because we might lose some
possible THP candidates (in the host or the guest) and you want to plug
does holes? I assume you'll see a performance drop simply because
poisoning memory is expensive, including migrating pages around on CE.

If you have some numbers to share, especially before/after this change,
that would be great.

-- 
Thanks,

David / dhildenb