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

[Michael S. Tsirkin]

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?

> 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 :)

-- 
MST