Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble hinting"

[Michael S. Tsirkin]

On Wed, Jul 24, 2019 at 03:27:37PM -0700, Alexander Duyck wrote:
> On Wed, 2019-07-24 at 18:08 -0400, Michael S. Tsirkin wrote:
> > On Wed, Jul 24, 2019 at 03:03:56PM -0700, Alexander Duyck wrote:
> > > On Wed, 2019-07-24 at 17:38 -0400, Michael S. Tsirkin wrote:
> > > > On Wed, Jul 24, 2019 at 10:12:10AM -0700, Alexander Duyck wrote:
> > > > > From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> > > > > 
> > > > > Add support for what I am referring to as "bubble hinting". Basically the
> > > > > idea is to function very similar to how the balloon works in that we
> > > > > basically end up madvising the page as not being used. However we don't
> > > > > really need to bother with any deflate type logic since the page will be
> > > > > faulted back into the guest when it is read or written to.
> > > > > 
> > > > > This is meant to be a simplification of the existing balloon interface
> > > > > to use for providing hints to what memory needs to be freed. I am assuming
> > > > > this is safe to do as the deflate logic does not actually appear to do very
> > > > > much other than tracking what subpages have been released and which ones
> > > > > haven't.
> > > > > 
> > > > > Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> > > > 
> > > > BTW I wonder about migration here.  When we migrate we lose all hints
> > > > right?  Well destination could be smarter, detect that page is full of
> > > > 0s and just map a zero page. Then we don't need a hint as such - but I
> > > > don't think it's done like that ATM.
> > > 
> > > I was wondering about that a bit myself. If you migrate with a balloon
> > > active what currently happens with the pages in the balloon? Do you
> > > actually migrate them, or do you ignore them and just assume a zero page?
> > 
> > Ignore and assume zero page.
> > 
> > > I'm just reusing the ram_block_discard_range logic that was being used for
> > > the balloon inflation so I would assume the behavior would be the same.
> > > 
> > > > I also wonder about interaction with deflate.  ATM deflate will add
> > > > pages to the free list, then balloon will come right back and report
> > > > them as free.
> > > 
> > > I don't know how likely it is that somebody who is getting the free page
> > > reporting is likely to want to also use the balloon to take up memory.
> > 
> > Why not?
> 
> The two functions are essentially doing the same thing. The only real
> difference is enforcement. If the balloon takes the pages the guest cannot
> get them back. I suppose there might be some advantage if you are wanting
> for force shrink a guest but that would be about it.

Yea, that's a common use of the balloon ATM. Helps partition
the host so guests don't conflict. OTOH deflate on oom thing
probably will never be used with hinting.

> > > However hinting on a page that came out of deflate might make sense when
> > > you consider that the balloon operates on 4K pages and the hints are on 2M
> > > pages. You are likely going to lose track of it all anyway as you have to
> > > work to merge the 4K pages up to the higher order page.
> > 
> > Right - we need to fix inflate/deflate anyway.
> > When we do, we can do whatever :)
> 
> One thing we could probably look at for the future would be to more
> closely merge the balloon and this reporting logic. Ideally the balloon
> would grab pages that were already hinted in order to enforce a certain
> size limit on the guest, and then when it gave the pages back they would
> retain their hinted status if possible.
> 
> The only problem is that right now both of those require that
> hinting/reporting be active for the zone being accessed since we otherwise
> don't have pointers to the pages at the head of the "hinted" list.

I guess I was talking about reworking host/guest ABI, you were
talking about the internals. So both need to change :)