Date:   Thu, 2 Jun 2022 17:28:00 +0800
From:   zhenwei pi <pizhenwei@...edance.com>
To:     David Hildenbrand <david@...hat.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: Re: [PATCH 0/3] recover hardware corrupted page by virtio balloon

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.

Hi, Naoya
It seems that memory failure notifier is not required currently, so I'll 
not push the next version of:
[PATCH 1/3] memory-failure: Introduce memory failure notifier
[PATCH 2/3] mm/memory-failure.c: support reset PTE during unpoison

Thanks you for review work!

> As the poisoning refcount code is already a bit shaky as I learned
> recently in the context of memory offlining, I do wonder if we really
> want to expose the unpoisoning code outside of debugfs (hwpoison) usage.
> 
> Interestingly, unpoison_memory() documents: "This is only done on the
> software-level, so it only works for linux injected failures, not real
> hardware failures" -- ehm?
> 

I guess unpoison_memory() is designed/tested by hwpoison-inject only, I 
have no idea to fix memory failure on a hardware platform. I suppose 
it's the first time that unpoison_memory() is required by hardware-level 
(balloon VQ).

-- 
zhenwei pi

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