From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx151.postini.com [74.125.245.151]) by kanga.kvack.org (Postfix) with SMTP id F3B336B0071 for ; Wed, 15 Aug 2012 06:00:17 -0400 (EDT) Date: Wed, 15 Aug 2012 13:01:08 +0300 From: "Michael S. Tsirkin" Subject: Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages Message-ID: <20120815100108.GA1999@redhat.com> References: <20120813084123.GF14081@redhat.com> <20120814182244.GB13338@t510.redhat.com> <20120814195139.GA28870@redhat.com> <20120814201113.GE22133@t510.redhat.com> <20120815090528.GH4052@csn.ul.ie> <20120815092528.GA29214@redhat.com> <20120815094839.GJ4052@csn.ul.ie> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120815094839.GJ4052@csn.ul.ie> Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman Cc: Rafael Aquini , linux-mm@kvack.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Rusty Russell , Rik van Riel , Andi Kleen , Andrew Morton , Konrad Rzeszutek Wilk , Minchan Kim On Wed, Aug 15, 2012 at 10:48:39AM +0100, Mel Gorman wrote: > On Wed, Aug 15, 2012 at 12:25:28PM +0300, Michael S. Tsirkin wrote: > > On Wed, Aug 15, 2012 at 10:05:28AM +0100, Mel Gorman wrote: > > > On Tue, Aug 14, 2012 at 05:11:13PM -0300, Rafael Aquini wrote: > > > > On Tue, Aug 14, 2012 at 10:51:39PM +0300, Michael S. Tsirkin wrote: > > > > > What I think you should do is use rcu for access. > > > > > And here sync rcu before freeing. > > > > > Maybe an overkill but at least a documented synchronization > > > > > primitive, and it is very light weight. > > > > > > > > > > > > > I liked your suggestion on barriers, as well. > > > > > > > > > > I have not thought about this as deeply as I shouold but is simply rechecking > > > the mapping under the pages_lock to make sure the page is still a balloon > > > page an option? i.e. use pages_lock to stabilise page->mapping. > > > > To clarify, are you concerned about cost of rcu_read_lock > > for non balloon pages? > > > > Not as such, but given the choice between introducing RCU locking and > rechecking page->mapping under a spinlock I would choose the latter as it > is more straight-forward. OK but checking it how? page->mapping == balloon_mapping does not scale to multiple balloons, so I hoped we can switch to page->mapping->flags & BALLOON_MAPPING or some such, but this means we dereference it outside the lock ... We will also need to add some API to set/clear mapping so that driver does not need to poke in mm internals, but that's easy. > -- > Mel Gorman > SUSE Labs -- 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