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

[Michael S. Tsirkin]

On Fri, Apr 22, 2016 at 10:48:38AM +0100, Dr. David Alan Gilbert wrote:
> * 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

We need to make it as fast as we can since the VCPU is stopped on exit
anyway. This just means e.g. sizing the bitmap reasonably -
don't always try to fit all memory in a single bitmap.

Really, if the page can in fact be in use when you tell host it's free,
then it's rather hard to explain what does it mean from host/guest
interface point of view.

It probably can be defined but the interface seems very complex.

Let's start with a simple thing instead unless it can be shown
that there's a performance problem.


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