linux-kernel - Re: [PATCH v3 00/20] Userspace P2PDMA with O

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Date:   Wed, 29 Sep 2021 17:52:32 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Martin Oliveira <martin.oliveira@...eticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@...il.com>
Subject: Re: [PATCH v3 00/20] Userspace P2PDMA with O_DIRECT NVMe devices



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