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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 20, 2016 at 01:41:24AM +0000, Li, Liang Z wrote:
> > Cc: Rik van Riel; viro@zeniv.linux.org.uk; linux-kernel@vger.kernel.org;
> > quintela@redhat.com; amit.shah@redhat.com; pbonzini@redhat.com;
> > dgilbert@redhat.com; linux-mm@kvack.org; kvm@vger.kernel.org; qemu-
> > devel@nongnu.org; agraf@suse.de; borntraeger@de.ibm.com
> > Subject: Re: [PATCH kernel 1/2] mm: add the related functions to build the
> > free page bitmap
> > 
> > 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.
> 
> Thanks! You mean save the free page bitmap in host pages?

No, I literally mean don't release &zone->lock before you pass
the list of pages to host.

> > 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.
> > 
> 
> Then we should prevent the guest from using those recently-freed pages, 
> before doing the MADV_DONTNEED or MADV_FREE, or the pages in the
> free page bitmap may be not free any more. In which case we will do something
> like this? Balloon?
> 
> Liang
> 

Wouldn't keeping &zone->lock make sure these pages aren't used?


> > --
> > MST