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 :)
