From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx165.postini.com [74.125.245.165]) by kanga.kvack.org (Postfix) with SMTP id 2AB856B0044 for ; Tue, 14 Aug 2012 16:53:29 -0400 (EDT) Date: Tue, 14 Aug 2012 23:54:26 +0300 From: "Michael S. Tsirkin" Subject: Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages Message-ID: <20120814205426.GA29162@redhat.com> References: <20120813084123.GF14081@redhat.com> <20120814182244.GB13338@t510.redhat.com> <20120814195139.GA28870@redhat.com> <20120814195916.GC28870@redhat.com> <20120814200830.GD22133@t510.redhat.com> <20120814202401.GB28990@redhat.com> <20120814202949.GF22133@t510.redhat.com> <20120814204906.GD28990@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120814204906.GD28990@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Rafael Aquini Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Rusty Russell , Rik van Riel , Mel Gorman , Andi Kleen , Andrew Morton , Konrad Rzeszutek Wilk , Minchan Kim On Tue, Aug 14, 2012 at 11:49:06PM +0300, Michael S. Tsirkin wrote: > On Tue, Aug 14, 2012 at 05:29:50PM -0300, Rafael Aquini wrote: > > On Tue, Aug 14, 2012 at 11:24:01PM +0300, Michael S. Tsirkin wrote: > > > On Tue, Aug 14, 2012 at 05:08:31PM -0300, Rafael Aquini wrote: > > > > On Tue, Aug 14, 2012 at 10:59:16PM +0300, Michael S. Tsirkin wrote: > > > > > > > > What if there is more than one balloon device? > > > > > > > > > > > > > > Is it possible to load this driver twice, or are you foreseeing a future case > > > > > > > where this driver will be able to manage several distinct memory balloons for > > > > > > > the same guest? > > > > > > > > > > > > > > > > > > > Second. > > > > > > It is easy to create several balloons they are just > > > > > > pci devices. > > > > > > > > > > > > > > > > > > > > and it might not be too important to make it work but > > > > > at least would be nice not to have a crash in this > > > > > setup. > > > > > > > > > Fair enough. For now, as I believe it's safe to assume we are only inflating one > > > > balloon per guest, I'd like to propose this as a future enhancement. Sounds > > > > good? > > > > > > > > > > Since guest crashes when it's not the case, no it doesn't, sorry :(. > > > > > Ok, but right now this driver only takes care of 1 balloon per guest, > > It does? Are you sure? There is no global state as far as I can see. So > I can create 2 devices and driver will happily create two instances, > each one can be inflated/deflated independently. > > > so how > > could this approach crash it? > > Add device. inflate. Add another device. inflate. deflate. unplug. > Now you have pointer to freed memory and when mm touches > page from first device, you ge use after free. > > > Your point is a good thing to be on a to-do list for future enhancements, but > > it's not a dealbreaker for the present balloon driver implementation, IMHO. > > > > Yes it looks like a dealbreaker to me. To clarify, the global state that this patch adds, is ugly even if we didn't support multiple balloons yet. So I don't think I can accept such a patch. Rusty has a final word here, maybe he thinks differently. > -- > MST -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org