Re: Re: [PATCH 0/3] recover hardware corrupted page by virtio balloon

From: zhenwei pi
Date: Thu Jun 02 2022 - 05:32:11 EST

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@xxxxxxxxxx> 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