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

[Alexander Duyck <alexander.h.duyck@linux.intel.com>]

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@linux.intel.com>
> > > > 
> > > > 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@linux.intel.com>
> > > 
> > > 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.

> > 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.