Re: [Qemu-devel] [PATCH v11 4/6] mm: function to offer a page block on the free list

[Michael S. Tsirkin]

On Tue, Jun 20, 2017 at 08:54:29PM +0200, David Hildenbrand wrote:
> On 20.06.2017 20:17, Michael S. Tsirkin wrote:
> > On Tue, Jun 20, 2017 at 06:49:33PM +0200, David Hildenbrand wrote:
> >> On 20.06.2017 18:44, Rik van Riel wrote:
> >>> On Mon, 2017-06-12 at 07:10 -0700, Dave Hansen wrote:
> >>>
> >>>> The hypervisor is going to throw away the contents of these pages,
> >>>> right?  As soon as the spinlock is released, someone can allocate a
> >>>> page, and put good data in it.  What keeps the hypervisor from
> >>>> throwing
> >>>> away good data?
> >>>
> >>> That looks like it may be the wrong API, then?
> >>>
> >>> We already have hooks called arch_free_page and
> >>> arch_alloc_page in the VM, which are called when
> >>> pages are freed, and allocated, respectively.
> >>>
> >>> Nitesh Lal (on the CC list) is working on a way
> >>> to efficiently batch recently freed pages for
> >>> free page hinting to the hypervisor.
> >>>
> >>> If that is done efficiently enough (eg. with
> >>> MADV_FREE on the hypervisor side for lazy freeing,
> >>> and lazy later re-use of the pages), do we still
> >>> need the harder to use batch interface from this
> >>> patch?
> >>>
> >> David's opinion incoming:
> >>
> >> No, I think proper free page hinting would be the optimum solution, if
> >> done right. This would avoid the batch interface and even turn
> >> virtio-balloon in some sense useless.
> > 
> > I agree generally. But we have to balance that against the fact that
> > this was discussed since at least 2011 and no one built this solution
> > yet.
> 
> I totally agree, and I still think it will be hard to get a decent
> performance for free page hinting (let's call it challenging). But I
> heard of some interesting ideas. Surprise me.
> 
> Still, I would favor such an interface over a mm interface where people
> start asking the same question over and over again ("how can this even
> work"). Not only because it wasn't explained sufficiently enough, but
> also because this interface is so special for one use case and one
> scenario (concurrent dirty tracking in the host during migration).
> 
> IMHO even simply writing all-zeros to all free pages before starting
> migration (or even when freeing a page) would be a cleaner interface
> than this (because it atomically works with the entity the host cares
> about for migration). But yes, performance is horrible that's why I am
> not even suggesting it. Just saying that this mm interface is very very
> special and if we could find something better, I'd favor it.

As long as there's a single user, changing to a better interface
once it's found won't be hard at all :)

> -- 
> 
> Thanks,
> 
> David