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

[Rik van Riel]

On Tue, 2017-06-20 at 18:49 +0200, David Hildenbrand wrote:
> On 20.06.2017 18:44, Rik van Riel wrote:

> > 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 with that.  Let me go into some more detail of
what Nitesh is implementing:

1) In arch_free_page, the being-freed page is added
   to a per-cpu set of freed pages.
2) Once that set is full, arch_free_pages goes into a
   slow path, which:
   2a) Iterates over the set of freed pages, and
   2b) Checks whether they are still free, and
   2c) Adds the still free pages to a list that is
       to be passed to the hypervisor, to be MADV_FREEd.
   2d) Makes that hypercall.

Meanwhile all arch_alloc_pages has to do is make sure it
does not allocate a page while it is currently being
MADV_FREEd on the hypervisor side.

The code Wei is working on looks like it could be 
suitable for steps (2c) and (2d) above. Nitesh already
has code for steps 1 through 2b.

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