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Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page pois
From: |
David Hildenbrand |
Subject: |
Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature |
Date: |
Thu, 16 Apr 2020 20:33:56 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 |
> The other thing to keep in mind is that the poison value only really
> comes into play with hinting/reporting. In the case of the standard
> balloon the pages are considered allocated from the guest's
Currently just as free page hinting IMHO. They are temporarily
considered allocated.
> perspective until the balloon is deflated. Then any poison/init will
> occur over again anyway so I don't think the standard balloon should
> really care.
I think we should make this consistent. And as we discuss below, this
allows for a nice optimization in the guest even for ordinary
inflation/deflation (no need to zero out/poison again when giving the
pages back to the buddy).
>
> For hinting it somewhat depends. Currently the implementation is
> inflating a balloon so having poisoning or init_on_free means it is
> written to immediately after it is freed so it defeats the purpose of
> the hinting. However that is a Linux implementation issue, not
Yeah, and as we discuss below, we can optimize that later in Linux. It's
sub-optimal, I agree.
> necessarily an issue with the QEMU implementation. As such may be I
> should fix that in the Linux driver since that has been ignored in
> QEMU up until now anyway. The more interesting bit is what should the
> behavior be from the hypervisor when a page is marked as being hinted.
> I think right now the behavior is to just not migrate the page. I
> wonder though if we shouldn't instead just consider the page a zero
> page, and then maybe modify the zero page behavior for the case where
> we know page poisoning is enabled.
I consider that maybe future work. Let's keep it simple for now (iow,
try to get page poisoning handling right first). The optimize the guest
handling on balloon deflation / end of free page hinting.
[...]
>> I can totally understand if Alex would want to stop working on
>> VIRTIO_BALLOON_F_PAGE_POISON at this point and only fix the guest to not
>> enable free page reporting in case we don't have
>> VIRTIO_BALLOON_F_PAGE_POISON (unless that's already done), lol. :)
>
> I already have a patch for that.
>
> The bigger issue is how to deal with the PAGE_POISON being enabled
> with FREE_PAGE_HINTING. The legacy code at this point is just broken
> and is plowing through with FREE_PAGE_HINTING while it is enabled.
> That is safe for now because it is using a balloon, the side effect is
> that it is going to defer migration. If it switches to a page
> reporting type setup at some point in the future we would need to make
> sure something is written to the other end to identify the poison/zero
> pages.
I think we don't have to worry about that for now. Might be sub-optimal,
but then, I don't think actual page poisoning isn't all that frequently
used in production setups.
--
Thanks,
David / dhildenb
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, (continued)
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, Alexander Duyck, 2020/04/15
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, David Hildenbrand, 2020/04/15
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, Alexander Duyck, 2020/04/15
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, David Hildenbrand, 2020/04/15
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, Alexander Duyck, 2020/04/15
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, David Hildenbrand, 2020/04/16
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, David Hildenbrand, 2020/04/16
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, Michael S. Tsirkin, 2020/04/16
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, David Hildenbrand, 2020/04/16
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, Alexander Duyck, 2020/04/16
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature,
David Hildenbrand <=
[PATCH v19 QEMU 2/4] linux-headers: update to contain virito-balloon free page reporting, Alexander Duyck, 2020/04/09
[PATCH v19 QEMU 3/4] virtio-balloon: Provide an interface for free page reporting, Alexander Duyck, 2020/04/09
[PATCH v19 QEMU 4/4] memory: Do not allow direct write access to rom_device regions, Alexander Duyck, 2020/04/09