Re: [PATCHv2] rdma: add a new IB_ACCESS_GIFT flag

Michael S. Tsirkin
2013-04-03T16:08:53Z

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@redhat.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@redhat.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