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

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Date:	Tue, 19 Apr 2016 19:15:00 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Li, Liang Z" <liang.z.li@...el.com>
Cc:	Rik van Riel <riel@...hat.com>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"quintela@...hat.com" <quintela@...hat.com>,
	"amit.shah@...hat.com" <amit.shah@...hat.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"dgilbert@...hat.com" <dgilbert@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
	"agraf@...e.de" <agraf@...e.de>,
	"borntraeger@...ibm.com" <borntraeger@...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.
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