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

[Li, Liang Z]

> 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.

Then we should pause the whole VM when using the bitmap, too expensive?

> 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.
> 

How about rename the interface to a more appropriate name other than 'free page' ?

Liang.
> 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