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

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Message-ID: <6560f4e5-8ded-6fb3-dd2b-d4733633addc@oracle.com>
Date:   Thu, 2 May 2019 10:59:58 -0700
From:   Santosh Shilimkar <santosh.shilimkar@...cle.com>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        Moni Shoua <monis@...lanox.com>
Subject: Re: [net-next][PATCH v2 2/2] rds: add sysctl for rds support of
 On-Demand-Paging



On 5/1/2019 11:18 PM, Leon Romanovsky wrote:
> 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?
> 
Actually this is not ODP sysctl but really whether RDS supports
RDMA on fs_dax memory or not. I had different name for sysctl but
in internal review it got changed.

Ignoring the name of the sysctl, here is the application logic.
- If fs_dax sysctl path doesn't exist, no RDMA on FS DAX memory(this
will cover all the older kernels, which doesn't have this patch)
- If fs_dax sysctl path exist and its value is 0, no RDMA on FS
DAX. This will cover kernels which this patch but don't have
actual support for ODP based registration.
- If fs_dax sysctl path exist and its value is 1, RDMA can be
issued on FS DAX memory. This sysctl will be updated to value 1
once the support gets added.

Hope it clarifies better now.

Regards,
Santosh