From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 23 Jan 2008 04:52:47 -0600 From: Robin Holt Subject: Re: [kvm-devel] [PATCH] export notifier #1 Message-ID: <20080123105246.GG26420@sgi.com> References: <478F9C9C.7070500@qumranet.com> <20080117193252.GC24131@v2.random> <20080121125204.GJ6970@v2.random> <4795F9D2.1050503@qumranet.com> <20080122144332.GE7331@v2.random> <20080122200858.GB15848@v2.random> <20080122223139.GD15848@v2.random> <479716AD.5070708@qumranet.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <479716AD.5070708@qumranet.com> Sender: owner-linux-mm@kvack.org Return-Path: To: Avi Kivity Cc: Christoph Lameter , Andrea Arcangeli , Izik Eidus , Andrew Morton , Nick Piggin , kvm-devel@lists.sourceforge.net, Benjamin Herrenschmidt , steiner@sgi.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, daniel.blueman@quadrics.com, holt@sgi.com, Hugh Dickins List-ID: On Wed, Jan 23, 2008 at 12:27:57PM +0200, Avi Kivity wrote: >> The approach with the export notifier is page based not based on the >> mm_struct. We only need a single page count for a page that is exported to >> a number of remote instances of linux. The page count is dropped when all >> the remote instances have unmapped the page. > > That won't work for kvm. If we have a hundred virtual machines, that means > 99 no-op notifications. But 100 callouts holding spinlocks will not work for our implementation and even if the callouts are made with spinlocks released, we would very strongly prefer a single callout which messages the range to the other side. > Also, our rmap key for finding the spte is keyed on (mm, va). I imagine > most RDMA cards are similar. For our RDMA rmap, it is based upon physical address. >> There is only the need to walk twice for pages that are marked Exported. >> And the double walk is only necessary if the exporter does not have its >> own rmap. The cross partition thing that we are doing has such an rmap and >> its a matter of walking the exporters rmap to clear out the external >> references and then we walk the local rmaps. All once. >> > > The problem is that external mmus need a reverse mapping structure to > locate their ptes. We can't expand struct page so we need to base it on mm > + va. Our rmap takes a physical address and turns it into mm+va. > Can they wait on that bit? PageLocked(page) should work, right? We already have a backoff mechanism so we expect to be able to adapt it to include a PageLocked(page) check. Thanks, Robin -- 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