linux-kernel - Re: [PATCH v16.1 6/9] virtio-balloon: Add support for providing free page reports to host

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Message-ID: <20200211094357-mutt-send-email-mst@kernel.org>
Date:   Tue, 11 Feb 2020 09:48:44 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, willy@...radead.org,
        mhocko@...nel.org, linux-mm@...ck.org, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, vbabka@...e.cz,
        yang.zhang.wz@...il.com, nitesh@...hat.com, konrad.wilk@...cle.com,
        pagupta@...hat.com, riel@...riel.com, lcapitulino@...hat.com,
        dave.hansen@...el.com, wei.w.wang@...el.com, aarcange@...hat.com,
        pbonzini@...hat.com, dan.j.williams@...el.com,
        alexander.h.duyck@...ux.intel.com, osalvador@...e.de
Subject: Re: [PATCH v16.1 6/9] virtio-balloon: Add support for providing free
 page reports to host

On Tue, Feb 11, 2020 at 03:31:18PM +0100, David Hildenbrand wrote:
> On 11.02.20 15:07, Michael S. Tsirkin wrote:
> > On Tue, Feb 11, 2020 at 01:19:31PM +0100, David Hildenbrand wrote:
> >>>>
> >>>> Did you see the discussion regarding unifying handling of
> >>>> inflate/deflate/free_page_hinting_free_page_reporting, requested by
> >>>> Michael? I think free page reporting is special and shall be left alone.
> >>>
> >>> Not sure what do you mean by "left alone here". Could you clarify?
> >>
> >> Don't try to unify handling like I proposed below, because it's
> >> semantics are special.
> >>
> >>>
> >>>> VIRTIO_BALLOON_F_REPORTING is nothing but a more advanced inflate, right
> >>>> (sg, inflate based on size - not "virtio pages")?
> >>>
> >>>
> >>> Not exactly - it's also initiated by guest as opposed to host, and
> >>> not guided by the ballon size request set by the host.
> >>
> >> True, but AFAIKS you could use existing INFLATE/DEFLATE in a similar
> >> way. There is no way for the hypervisor to nack a request. The balloon
> >> size is not glued to inflate/deflate requests. The guests manually
> >> updates it.
> > 
> > Hmm how isn't it? num_pages is the only way to inflate/deflate.
> 
> Usually, guests are nice and respond to num_pages changes in an
> appropriate way, except:
> - Triggering deflate: Unload the driver. Suspend/hibernate. OOM.
>   (+ Reboot, although that's special)
> - Triggering inflate + deflate: Simple balloon compaction / page
>   migration.


These are all real situations but balloon always has been best effort.


> But that's not what I meant.
> 
> "actual" is updated by the guest, not by the host. So the "actual
> balloon size" is set by the guest. It's not glued to inflation/deflation
> requests. "num_pages" is the host request.

Well the expectation is that as long as guest has ample
available memory, when num_pages changes then
guest starts sending inflate/deflate requests,
until actual matches num_pages.

If it does not match, and we wait and it still doesn't,
then something unusual happened. People do depend on that
behaviour.

> AFAIKs, the guest could inflate/deflate (esp. temporarily) and
> communicate via "actual" the actual balloon size as he sees it.

OK so you want hinted but unused pages counted, and reported
in "actual"? That's a vmexit before each page use ...



> > Spec also says:
> > The device is driven either by the receipt of a configuration change notification, or by changing guest memory
> > needs, such as performing memory compaction or responding to out of memory conditions.
> > 
> > so ignoring compaction/oom (later is under-specified, not a good example
> > to follow) yes inflate/deflate are tied to host specified configuration
> Yes, "num_pages" is the host request. But I'd say the statement (esp.
> "the device is driven by") in the spec is rather weak. It does not
> explicitly state when inflation/deflation is allowed IMHO.

Right since it's all best effort anyway.


> -- 
> Thanks,
> 
> David / dhildenb