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To:     Jason Gunthorpe <jgg@ziepe.ca>
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From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <142badec-f6f5-e471-698e-8a386aae3c2b@deltatee.com>
Date:   Wed, 29 Sep 2021 17:52:32 -0600
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Subject: Re: [PATCH v3 00/20] Userspace P2PDMA with O_DIRECT NVMe devices
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On 2021-09-29 5:36 p.m., Jason Gunthorpe wrote:
> On Wed, Sep 29, 2021 at 05:28:38PM -0600, Logan Gunthorpe wrote:
>>
>>
>> On 2021-09-29 5:21 p.m., Jason Gunthorpe wrote:
>>> On Wed, Sep 29, 2021 at 03:50:02PM -0600, Logan Gunthorpe wrote:
>>>>
>>>>
>>>> On 2021-09-28 2:02 p.m., Jason Gunthorpe wrote:
>>>>> On Thu, Sep 16, 2021 at 05:40:40PM -0600, Logan Gunthorpe wrote:
>>>>>> Hi,
>>>>>>
>>>>>> This patchset continues my work to add userspace P2PDMA access using
>>>>>> O_DIRECT NVMe devices. My last posting[1] just included the first 13
>>>>>> patches in this series, but the early P2PDMA cleanup and map_sg error
>>>>>> changes from that series have been merged into v5.15-rc1. To address
>>>>>> concerns that that series did not add any new functionality, I've added
>>>>>> back the userspcae functionality from the original RFC[2] (but improved
>>>>>> based on the original feedback).
>>>>>
>>>>> I really think this is the best series yet, it really looks nice
>>>>> overall. I know the sg flag was a bit of a debate at the start, but it
>>>>> serves an undeniable purpose and the resulting standard DMA APIs 'just
>>>>> working' is really clean.
>>>>
>>>> Actually, so far, nobody has said anything negative about using the SG flag.
>>>>
>>>>> There is more possible here, we could also pass the new GUP flag in the
>>>>> ib_umem code..
>>>>
>>>> Yes, that would be very useful.
>>>
>>> You might actually prefer to do that then the bio changes to get the
>>> infrastructur merged as it seems less "core"
>>
>> I'm a little bit more concerned about my patch set growing too large.
>> It's already at 20 patches and I think I'll need to add a couple more
>> based on the feedback you've already provided. So I'm leaning toward
>> pushing more functionality as future work.
> 
> I mean you could postpone the three block related patches and use a
> single ib_umem patch instead as the consumer.

I think that's not a very compelling use case given the only provider of
these VMAs is an NVMe block device. My patch set enables a real world
use (copying data between NVMe devices P2P through the CMB with O_DIRECT).

Being able to read or write a CMB with RDMA and only RDMA is not very
compelling.

Logan