linux-kernel - RE: [Qemu-devel] [RFC qemu 4/4] migration: filter out guest's free pages in ram bulk stage

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Date:	Fri, 4 Mar 2016 02:43:49 +0000
From:	"Li, Liang Z" <liang.z.li@...el.com>
To:	"Daniel P. Berrange" <berrange@...hat.com>
CC:	"quintela@...hat.com" <quintela@...hat.com>,
	"amit.shah@...hat.com" <amit.shah@...hat.com>,
	"qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ehabkost@...hat.com" <ehabkost@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"mst@...hat.com" <mst@...hat.com>,
	"dgilbert@...hat.com" <dgilbert@...hat.com>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"rth@...ddle.net" <rth@...ddle.net>
Subject: RE: [Qemu-devel] [RFC qemu 4/4] migration: filter out guest's free
 pages in ram bulk stage

> On Thu, Mar 03, 2016 at 06:44:28PM +0800, Liang Li wrote:
> > Get the free pages information through virtio and filter out the free
> > pages in the ram bulk stage. This can significantly reduce the total
> > live migration time as well as network traffic.
> >
> > Signed-off-by: Liang Li <liang.z.li@...el.com>
> > ---
> >  migration/ram.c | 52
> > ++++++++++++++++++++++++++++++++++++++++++++++------
> >  1 file changed, 46 insertions(+), 6 deletions(-)
> 
> > @@ -1945,6 +1971,20 @@ static int ram_save_setup(QEMUFile *f, void
> *opaque)
> >                                              DIRTY_MEMORY_MIGRATION);
> >      }
> >      memory_global_dirty_log_start();
> > +
> > +    if (balloon_free_pages_support() &&
> > +        balloon_get_free_pages(migration_bitmap_rcu->free_pages_bmap,
> > +                               &free_pages_count) == 0) {
> > +        qemu_mutex_unlock_iothread();
> > +        while (balloon_get_free_pages(migration_bitmap_rcu-
> >free_pages_bmap,
> > +                                      &free_pages_count) == 0) {
> > +            usleep(1000);
> > +        }
> > +        qemu_mutex_lock_iothread();
> > +
> > +        filter_out_guest_free_pages(migration_bitmap_rcu-
> >free_pages_bmap);
> > +    }
> 
> IIUC, this code is synchronous wrt to the guest OS balloon drive. ie it is asking
> the geust for free pages and waiting for a response. If the guest OS has
> crashed this is going to mean QEMU waits forever and thus migration won't
> complete. Similarly you need to consider that the guest OS may be malicious
> and simply never respond.
> 
> So if the migration code is going to use the guest balloon driver to get info
> about free pages it has to be done in an asynchronous manner so that
> migration can never be stalled by a slow/crashed/malicious guest driver.
> 
> Regards,
> Daniel

Really,  thanks a lot!

Liang