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Re: [PATCH resend v2 5/5] softmmu/memory_mapping: optimize for RamDiscar

From: Peter Xu
Subject: Re: [PATCH resend v2 5/5] softmmu/memory_mapping: optimize for RamDiscardManager sections
Date: Fri, 23 Jul 2021 18:33:21 -0400

On Fri, Jul 23, 2021 at 08:56:54PM +0200, David Hildenbrand wrote:
> > 
> > As I've asked this question previously elsewhere, it's more or less also
> > related to the design decision of having virtio-mem being able to sparsely
> > plugged in such a small granularity rather than making the plug/unplug still
> > continuous within GPA range (so we move page when unplug).
> Yes, in an ideal world that would be optimal solution. Unfortunately, we're
> not living in an ideal world :)
> virtio-mem in Linux guests will as default try unplugging highest-to-lowest
> address, and I have on my TODO list an item to shrink the usable region (->
> later, shrinking the actual RAMBlock) once possible.
> So virtio-mem is prepared for that, but it will only apply in some cases.
> > 
> > There's definitely reasons there and I believe you're the expert on that (as
> > you mentioned once: some guest GUPed pages cannot migrate so cannot get 
> > those
> > ranges offlined otherwise), but so far I still not sure whether that's a 
> > kernel
> > issue to solve on GUP, although I agree it's a complicated one anyway!
> To do something like that reliably, you have to manage hotplugged memory in
> a special way, for example, in a movable zone.
> We have a at least 4 cases:
> a) The guest OS supports the movable zone and uses it for all hotplugged
>    memory
> b) The guest OS supports the movable zone and uses it for some
>    hotplugged memory
> c) The guest OS supports the movable zone and uses it for no hotplugged
>    memory
> d) The guest OS does not support the concept of movable zones
> a) is the dream but only applies in some cases if Linux is properly
> configured (e.g., never hotplug more than 3 times boot memory)
> b) will be possible under Linux soon (e.g., when hotplugging more than 3
> times boot memory)
> c) is the default under Linux for most Linux distributions
> d) Is Windows
> In addition, we can still have random unplug errors when using the movable
> zone, for example, if someone references a page just a little too long.
> Maybe that helps.

Yes, thanks.

> > 
> > Maybe it's a trade-off you made at last, I don't have enough knowledge to 
> > tell.
> That's the precise description of what virtio-mem is. It's a trade-off
> between which OSs we want to support, what the guest OS can actually do, how
> we can manage memory in the hypervisor efficiently, ...
> > 
> > The patch itself looks okay to me, there's just a slight worry on not sure 
> > how
> > long would the list be at last; if it's chopped in 1M/2M small chunks.
> I don't think that's really an issue: take a look at
> qemu_get_guest_memory_mapping(), which will create as many entries as
> necessary to express the guest physical mapping of the guest virtual (!)
> address space with such chunks. That can be a lot :)

I'm indeed a bit surprised by the "paging" parameter.. I gave it a try, the
list grows into tens of thousands.

One last question: will virtio-mem still do best-effort to move the pages, so
as to grant as less holes as possible?


Peter Xu

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