linux-kernel - Re: [RFC PATCH rdma-next 08/10] RDMA/rxe: Implement flush execution in responder side

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Message-ID: <20220112131242.GL6467@ziepe.ca>
Date:   Wed, 12 Jan 2022 09:12:42 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "lizhijian@...itsu.com" <lizhijian@...itsu.com>
Cc:     "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "zyjzyj2000@...il.com" <zyjzyj2000@...il.com>,
        "aharonl@...dia.com" <aharonl@...dia.com>,
        "leon@...nel.org" <leon@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mbloch@...dia.com" <mbloch@...dia.com>,
        "liangwenpeng@...wei.com" <liangwenpeng@...wei.com>,
        "yangx.jy@...itsu.com" <yangx.jy@...itsu.com>,
        "rpearsonhpe@...il.com" <rpearsonhpe@...il.com>,
        "y-goto@...itsu.com" <y-goto@...itsu.com>
Subject: Re: [RFC PATCH rdma-next 08/10] RDMA/rxe: Implement flush execution
 in responder side

On Wed, Jan 12, 2022 at 09:50:38AM +0000, lizhijian@...itsu.com wrote:
> 
> 
> On 12/01/2022 04:48, Jason Gunthorpe wrote:
> > On Tue, Jan 11, 2022 at 05:34:36AM +0000, lizhijian@...itsu.com wrote:
> >
> >> Yes, that's true. that's because only pmem has ability to persist data.
> >> So do you mean we don't need to prevent user to create/register a persistent
> >> access flag to a non-pmem MR? it would be a bit confusing if so.
> > Since these extensions seem to have a mode that is unrelated to
> > persistent memory,
> I can only agree with part of them, since the extensions also say that:
> 
> oA19-1: Responder shall successfully respond on FLUSH operation only
> after providing the placement guarantees, as specified in the packet, of
> preceding memory updates (for example: RDMA WRITE, Atomics and
> ATOMIC WRITE) towards the memory region.
> 
> it mentions *shall successfully respond on FLUSH operation only
> after providing the placement guarantees*. If users request a
> persistent placement to a non-pmem MR without errors,  from view
> of the users, they will think of their request had been *successfully responded*
> that doesn't reflect the true(data was persisted).

The "placement guarentees" should obviously be variable depending on
the type of memory being targeted.

> Further more, If we have a checking during the new MR creating/registering,
> user who registers this MR can know if the target MR supports persistent access flag.
> Then they can tell this information to the request side so that request side can
> request a valid placement type later. that is similar behavior with current librpma.

Then you can't use ATOMIC_WRITE with non-nvdimm memory, which is
nonsense

Jason