On Wed, May 01, 2019 at 10:54:50AM -0700, Santosh Shilimkar wrote:

On 5/1/2019 12:45 AM, Leon Romanovsky wrote:

quoted

On Mon, Apr 29, 2019 at 04:37:20PM -0700, Santosh Shilimkar wrote:

quoted

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 <redacted>
Reviewed-tested-by: Zhu Yanjun [off-list ref]
Signed-off-by: Hans Westgaard Ry <redacted>
Signed-off-by: Santosh Shilimkar <redacted>
---
  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