From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael R. Hines" Subject: Re: [PATCHv2] rdma: add a new IB_ACCESS_GIFT flag Date: Tue, 02 Apr 2013 18:17:09 -0400 Message-ID: <515B58E5.9070808@linux.vnet.ibm.com> References: <20130324155153.GA8597@redhat.com> <20130402155104.GA27382@redhat.com> <20130402170521.GA30295@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130402170521.GA30295@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org Sender: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org To: "Michael S. Tsirkin" Cc: Roland Dreier , qemu-devel@nongnu.org, "linux-rdma@vger.kernel.org" , Yishai Hadas , LKML , Hal Rosenstock , Jason Gunthorpe , Sean Hefty , Christoph Lameter List-Id: linux-rdma@vger.kernel.org I'm getting around to it, Michael, I promise =). Just came back from vacation. I have to re-build the ib_ucm kernel module from the original SUSE kernel that I'm using along before I can test it...... The machines I'm using are slightly tied up with other things, so its taking me a little time to prepare to apply the patch and test the new kernel module... - Michael On 04/02/2013 01:05 PM, Michael S. Tsirkin wrote: > 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. >