Date:   Thu, 2 Jun 2022 11:40:18 +0200
From:   David Hildenbrand <david@...hat.com>
To:     zhenwei pi <pizhenwei@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>
Cc:     Peter Xu <peterx@...hat.com>, Jue Wang <juew@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, jasowang@...hat.com,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, mst@...hat.com,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 0/3] recover hardware corrupted page by virtio balloon

On 02.06.22 11:28, zhenwei pi wrote:
> On 6/1/22 15:59, David Hildenbrand wrote:
>> On 01.06.22 04:17, zhenwei pi wrote:
>>> On 5/31/22 12:08, Jue Wang wrote:
>>>> On Mon, May 30, 2022 at 8:49 AM Peter Xu <peterx@...hat.com> wrote:
>>>>>
>>>>> On Mon, May 30, 2022 at 07:33:35PM +0800, zhenwei pi wrote:
>>>>>> A VM uses RAM of 2M huge page. Once a MCE(@HVAy in [HVAx,HVAz)) occurs, the
>>>>>> 2M([HVAx,HVAz)) of hypervisor becomes unaccessible, but the guest poisons 4K
>>>>>> (@GPAy in [GPAx, GPAz)) only, it may hit another 511 MCE ([GPAx, GPAz)
>>>>>> except GPAy). This is the worse case, so I want to add
>>>>>>    '__le32 corrupted_pages' in struct virtio_balloon_config, it is used in the
>>>>>> next step: reporting 512 * 4K 'corrupted_pages' to the guest, the guest has
>>>>>> a chance to isolate the other 511 pages ahead of time. And the guest
>>>>>> actually loses 2M, fixing 512*4K seems to help significantly.
>>>>>
>>>>> It sounds hackish to teach a virtio device to assume one page will always
>>>>> be poisoned in huge page granule.  That's only a limitation to host kernel
>>>>> not virtio itself.
>>>>>
>>>>> E.g. there're upstream effort ongoing with enabling doublemap on hugetlbfs
>>>>> pages so hugetlb pages can be mapped in 4k with it.  It provides potential
>>>>> possibility to do page poisoning with huge pages in 4k too.  When that'll
>>>>> be ready the assumption can go away, and that does sound like a better
>>>>> approach towards this problem.
>>>>
>>>> +1.
>>>>
>>>> A hypervisor should always strive to minimize the guest memory loss.
>>>>
>>>> The HugeTLB double mapping enlightened memory poisoning behavior (only
>>>> poison 4K out of a 2MB huge page and 4K in guest) is a much better
>>>> solution here. To be completely transparent, it's not _strictly_
>>>> required to poison the page (whatever the granularity it is) on the
>>>> host side, as long as the following are true:
>>>>
>>>> 1. A hypervisor can emulate the _minimized_ (e.g., 4K) the poison to the guest.
>>>> 2. The host page with the UC error is "isolated" (could be PG_HWPOISON
>>>> or in some other way) and prevented from being reused by other
>>>> processes.
>>>>
>>>> For #2, PG_HWPOISON and HugeTLB double mapping enlightened memory
>>>> poisoning is a good solution.
>>>>
>>>>>
>>>>>>
>>>>>>>
>>>>>>> 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.
>>>>>>>
>>>>>>
>>>>>> The CE storm leads 2 problems I have even seen:
>>>>>> 1, the memory bandwidth slows down to 10%~20%, and the cycles per
>>>>>> instruction of CPU increases a lot.
>>>>>> 2, the THR (/proc/interrupts) interrupts frequently, the CPU has to use a
>>>>>> lot time to handle IRQ.
>>>>>
>>>>> Totally no good knowledge on CMCI, but if 2) is true then I'm wondering
>>>>> whether it's necessary to handle the interrupts that frequently.  When I
>>>>> was reading the Intel CMCI vector handler I stumbled over this comment:
>>>>>
>>>>> /*
>>>>>    * The interrupt handler. This is called on every event.
>>>>>    * Just call the poller directly to log any events.
>>>>>    * This could in theory increase the threshold under high load,
>>>>>    * but doesn't for now.
>>>>>    */
>>>>> static void intel_threshold_interrupt(void)
>>>>>
>>>>> I think that matches with what I was thinking..  I mean for 2) not sure
>>>>> whether it can be seen as a CMCI problem and potentially can be optimized
>>>>> by adjust the cmci threshold dynamically.
>>>>
>>>> The CE storm caused performance drop is caused by the extra cycles
>>>> spent by the ECC steps in memory controller, not in CMCI handling.
>>>> This is observed in the Google fleet as well. A good solution is to
>>>> monitor the CE rate closely in user space via /dev/mcelog and migrate
>>>> all VMs to another host once the CE rate exceeds some threshold.
>>>>
>>>> CMCI is a _background_ interrupt that is not handled in the process
>>>> execution context and its handler is setup to switch to poll (1 / 5
>>>> min) mode if there are more than ~ a dozen CEs reported via CMCI per
>>>> second.
>>>>>
>>>>> --
>>>>> Peter Xu
>>>>>
>>>
>>> Hi, Andrew, David, Naoya
>>>
>>> According to the suggestions, I'd give up the improvement of memory
>>> failure on huge page in this series.
>>>
>>> Is it worth recovering corrupted pages for the guest kernel? I'd follow
>>> your decision.
>>
>> Well, as I said, I am not sure if we really need/want this for a handful
>> of 4k poisoned pages in a VM. As I suspected, doing so might primarily
>> be interesting for some sort of de-fragmentation (allow again a higher
>> order page to be placed at the affected PFNs), not because of the slight
>> reduction of available memory. A simple VM reboot would get the job
>> similarly done.
>>
> 
> Sure, Let's drop this idea. Thanks to all for the suggestions.

Thanks for the interesting idea + discussions.

Just a note that if you believe that we want/need something like that,
and there is a reasonable use case, please tell us we're wrong and push
back :)

-- 
Thanks,

David / dhildenb

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