From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:47491) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UN4eG-0006UV-VE for qemu-devel@nongnu.org; Tue, 02 Apr 2013 13:05:33 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UN4eC-0000Vv-BR for qemu-devel@nongnu.org; Tue, 02 Apr 2013 13:05:28 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49844) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UN4eC-0000Vj-2X for qemu-devel@nongnu.org; Tue, 02 Apr 2013 13:05:24 -0400 Date: Tue, 2 Apr 2013 20:05:22 +0300 From: "Michael S. Tsirkin" Message-ID: <20130402170521.GA30295@redhat.com> References: <20130324155153.GA8597@redhat.com> <20130402155104.GA27382@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCHv2] rdma: add a new IB_ACCESS_GIFT flag List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Roland Dreier Cc: qemu-devel@nongnu.org, "linux-rdma@vger.kernel.org" , Yishai Hadas , LKML , "Michael R. Hines" , Hal Rosenstock , Jason Gunthorpe , Sean Hefty , Christoph Lameter On Tue, Apr 02, 2013 at 09:57:38AM -0700, Roland Dreier wrote: > On Tue, Apr 2, 2013 at 8:51 AM, Michael S. Tsirkin wrote: > >> At the moment registering an MR breaks COW. This breaks memory > >> overcommit for users such as KVM: we have a lot of COW pages, e.g. > >> instances of the zero page or pages shared using KSM. > >> > >> If the application does not care that adapter sees stale data (for > >> example, it tracks writes reregisters and resends), it can use a new > >> IBV_ACCESS_GIFT flag to prevent registration from breaking COW. > >> > >> The semantics are similar to that of SPLICE_F_GIFT thus the name. > >> > >> Signed-off-by: Michael S. Tsirkin > > > > Roland, Michael is yet to test this but could you please > > confirm whether this looks acceptable to you? > > The patch itself is reasonable I guess, given the needs of this particular app. > > I'm not particularly happy with the name of the flag. The analogy > with SPLICE_F_GIFT doesn't seem particularly strong and I'm not > convinced even the splice flag name is very understandable. But in > the RDMA case there's not really any sense in which we're "gifting" > memory to the adapter -- we're just telling the library "please don't > trigger copy-on-write" and it doesn't seem particularly easy for users > to understand that from the flag name. > > - R. The point really is that any writes by application won't be seen until re-registration, right? OK, what's a better name? IBV_ACCESS_NON_COHERENT? Please tell me what is preferable and we'll go ahead with it. -- MST