Re: [Qemu-devel] [PATCH kernel 1/2] mm: add the related functions to build the free page bitmap

[Michael S. Tsirkin]

On Tue, Apr 19, 2016 at 03:02:09PM +0000, Li, Liang Z wrote:
> > On Tue, 2016-04-19 at 22:34 +0800, Liang Li wrote:
> > > The free page bitmap will be sent to QEMU through virtio interface and
> > > used for live migration optimization.
> > > Drop the cache before building the free page bitmap can get more free
> > > pages. Whether dropping the cache is decided by user.
> > >
> > 
> > How do you prevent the guest from using those recently-freed pages for
> > something else, between when you build the bitmap and the live migration
> > completes?
> 
> Because the dirty page logging is enabled before building the bitmap, there 
> is no need
> to prevent the guest from using the recently-freed pages ...
> 
> Liang

Well one point of telling host that page is free is so that
it can mark it clean even if it was dirty previously.
So I think you must pass the pages to guest under the lock.
This will allow host optimizations such as marking these
pages MADV_DONTNEED or MADV_FREE.
Otherwise it's all too tied up to a specific usecase -
you aren't telling host that a page is free, you are telling it
that a page was free in the past.

-- 
MST