linux-kernel - Re: [PATCHv2] rdma: add a new IB_ACCESS

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Message-ID: <20130403160748.GA24874@redhat.com>
Date:	Wed, 3 Apr 2013 19:07:48 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Roland Dreier <roland@...nel.org>
Cc:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	"Michael R. Hines" <mrhines@...ux.vnet.ibm.com>,
	Sean Hefty <sean.hefty@...el.com>,
	Hal Rosenstock <hal.rosenstock@...il.com>,
	Yishai Hadas <yishaih@...lanox.com>,
	Christoph Lameter <cl@...ux.com>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, qemu-devel@...gnu.org
Subject: Re: [PATCHv2] rdma: add a new IB_ACCESS_GIFT flag

On Tue, Apr 02, 2013 at 08:05:21PM +0300, 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 <mst@...hat.com> 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 <mst@...hat.com>
> > >
> > > 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.

Um. ping? We are at -rc5 and things need to fall into place
if we are to have it in 3.10 ...

> -- 
> MST
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