netdev - Re: [net-next][PATCH v2 2/2] rds: add sysctl for rds support of On-Demand-Paging

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Message-ID: <20190502061800.GL7676@mtr-leonro.mtl.com>
Date:   Thu, 2 May 2019 09:18:00 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Santosh Shilimkar <santosh.shilimkar@...cle.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [net-next][PATCH v2 2/2] rds: add sysctl for rds support of
 On-Demand-Paging

On Wed, May 01, 2019 at 10:54:50AM -0700, Santosh Shilimkar wrote:
> On 5/1/2019 12:45 AM, Leon Romanovsky wrote:
> > On Mon, Apr 29, 2019 at 04:37:20PM -0700, Santosh Shilimkar wrote:
> > > RDS doesn't support RDMA on memory apertures that require On Demand
> > > Paging (ODP), such as FS DAX memory. A sysctl is added to indicate
> > > whether RDMA requiring ODP is supported.
> > >
> > > Reviewed-by: Håkon Bugge <haakon.bugge@...cle.com>
> > > Reviewed-tested-by: Zhu Yanjun <yanjun.zhu@...cle.com>
> > > Signed-off-by: Hans Westgaard Ry <hans.westgaard.ry@...cle.com>
> > > Signed-off-by: Santosh Shilimkar <santosh.shilimkar@...cle.com>
> > > ---
> > >   net/rds/ib.h        | 1 +
> > >   net/rds/ib_sysctl.c | 8 ++++++++
> > >   2 files changed, 9 insertions(+)
> >
> > This sysctl is not needed at all
> >
> Its needed for application to check the support of the ODP support
> feature which in progress. Failing the RDS_GET_MR was just one path
> and we also support inline MR registration along with message request.
>
> Basically application runs on different kernel versions and to be
> portable, it will check if underneath RDS support ODP and then only
> use RDMA. If not it will fallback to buffer copy mode. Hope
> it clarifies.

Using ODP sysctl to determine if to use RDMA or not, looks like very
problematic approach. How old applications will work in such case
without knowledge of such sysctl?
How new applications will distinguish between ODP is not supported, but
RDMA works?

Thanks

>
>
> Regards,
> Santosh