From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:49340) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YIhP2-00007d-Pz for qemu-devel@nongnu.org; Tue, 03 Feb 2015 12:36:45 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YIhOs-0000UK-UL for qemu-devel@nongnu.org; Tue, 03 Feb 2015 12:36:44 -0500 Received: from mx6-phx2.redhat.com ([209.132.183.39]:53373) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YIhOs-0000U0-Ng for qemu-devel@nongnu.org; Tue, 03 Feb 2015 12:36:34 -0500 Date: Tue, 3 Feb 2015 12:36:32 -0500 (EST) From: Pankaj Gupta Message-ID: <1032377914.4158867.1422984992735.JavaMail.zimbra@redhat.com> In-Reply-To: <20150203170925.GK2332@work-vm> References: <20150203170925.GK2332@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] balloon vs postcopy migrate List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Dr. David Alan Gilbert" Cc: Amit Shah , aarcange@redhat.com, Luiz Capitulino , qemu-devel@nongnu.org, gal@redhat.com > Hi, > Andrea pointed out there is a risk that a guest inflating its > balloon during a postcopy migrate could cause us problems, and > I wanted to see what the best way of avoiding the problem was. > > Guests inflating there balloon cause an madvise(MADV_DONTNEED) on > the host, marking pages as not present, that will potentially trigger > a userfault, that we are using in postcopy to detect pages that need > to be fetched from the source. > > In theory, at the moment guests *should* only ask for a balloon > inflation if they've been asked to do so by the host; however there > are no guards for that, and it's been suggested giving the > guest more freedom might be a good idea anyway. > > My alternatives seem to be: > 1) Stop servicing the message queue from the guest so > that we just don't notice the inflate messages until > afterwards. (Easy for Qemu, not sure how the guests > will like an unserviced queue). > > 2) I could keep servicing the queue and ignore the messages > (Easy for everyone, not very nice in actual used memory - > does it cause any long term problems other than that?) > > 3) I could keep servicing the queue but put the messages > in a list somewhere that replay after migrate has finished. > (That list sounds bounded only in a very large way?) > > Thoughts? Can we have some global flag somewhere when Post copy is ON/active. And we can ignore or defer only inflate/ballon messages/commands while servicing the commands with some warnings. Just my thought on logic. Not sure if I am missing some background here. > > Dave > > -- > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > >