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

[Dr. David Alan Gilbert]

* Michael S. Tsirkin (mst@xxxxxxxxxx) wrote:
> 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.

But doing it under lock sounds pretty expensive, especially given
how long the userspace side is going to take to work through the bitmap
and device what to do.

Dave

> 
> -- 
> MST
--
Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK