From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37507) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aqm5l-0006By-G0 for qemu-devel@nongnu.org; Thu, 14 Apr 2016 14:34:18 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aqm5i-00071N-Aq for qemu-devel@nongnu.org; Thu, 14 Apr 2016 14:34:13 -0400 Received: from mx1.redhat.com ([209.132.183.28]:59353) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aqm5i-00071G-5j for qemu-devel@nongnu.org; Thu, 14 Apr 2016 14:34:10 -0400 Date: Thu, 14 Apr 2016 19:34:05 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20160414183404.GB21766@work-vm> References: <1460548364-27469-1-git-send-email-thuth@redhat.com> <20160413145835-mutt-send-email-mst@redhat.com> <570E5D05.2030507@redhat.com> <20160414114757.GE2252@work-vm> <570F8AD5.4040805@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <570F8AD5.4040805@redhat.com> Subject: Re: [Qemu-devel] [PATCH] hw/virtio/balloon: Fixes for different host page sizes List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Thomas Huth Cc: "Michael S. Tsirkin" , qemu-devel@nongnu.org, dgibson@redhat.com, wehuang@redhat.com, drjones@redhat.com, amit.shah@redhat.com, jitendra.kolhe@hpe.com * Thomas Huth (thuth@redhat.com) wrote: > On 14.04.2016 13:47, Dr. David Alan Gilbert wrote: > > * Thomas Huth (thuth@redhat.com) wrote: > > > >> That would mean a regression compared to what we have today. Currently, > >> the ballooning is working OK for 64k guests on a 64k ppc host - rather > >> by chance than on purpose, but it's working. The guest is always sending > >> all the 4k fragments of a 64k page, and QEMU is trying to call madvise() > >> for every one of them, but the kernel is ignoring madvise() on > >> non-64k-aligned addresses, so we end up with a situation where the > >> madvise() frees a whole 64k page which is also declared as free by the > >> guest. > > > > I wouldn't worry about migrating your fragmenet map; but I wonder if it > > needs to be that complex - does the guest normally do something more sane > > like do the 4k pages in order and so you've just got to track the last > > page it tried rather than having a full map? > > That's maybe a little bit easier and might work for well-known Linux > guests, but IMHO it's even more a hack than my approach: If the Linux > driver one day is switched to send the pages in the opposite order, or > if somebody tries to run a non-wellknown (i.e. non-Linux) guest, this > does not work at all anymore. True. > > A side question is whether the behaviour that's seen by virtio_ballon_handle_output > > is always actually the full 64k page; it calls balloon_page once > > for each message/element - but if all of those elements add back up to the full > > page, perhaps it makes more sense to reassemble it there? > > That might work for 64k page size guests ... but for 4k guests, I think > you'll have a hard time to reassemble a page there more easily than with > my current approach. Or do you have a clever algorithm in mind that > could do the job well there? No, i didn't; I just have an ulterior motive which is trying to do as few madvise's as possible, and while virtio_balloon_handle_output sees potentially quite a few requests at once, balloon_page is stuck down there at the bottom without any idea of whether there are any more coming. Dave > Thomas > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK