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To:     Jason Gunthorpe <jgg@mellanox.com>, Christoph Hellwig <hch@lst.de>
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From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <e4fd743f-61cb-a443-bc53-9a1c036ebe8c@deltatee.com>
Date:   Thu, 31 Jan 2019 12:19:31 -0700
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Subject: Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device
 vma
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On 2019-01-31 12:02 p.m., Jason Gunthorpe wrote:
> I still think the right direction is to build on what Logan has done -
> realize that he created a DMA-only SGL - make that a formal type of
> the kernel and provide the right set of APIs to work with this type,
> without being forced to expose struct page.

> Basically invert the API flow - the DMA map would be done close to
> GUP, not buried in the driver. This absolutely doesn't work for every
> flow we have, but it does enable the ones that people seem to care
> about when talking about P2P.
> It also does present a path to solve some cases of the O_DIRECT
> problems if the block stack can develop some way to know if an IO will
> go down a DMA-only IO path or not... This seems less challenging that
> auditing every SGL user for iomem safety??


The DMA-only SGL will work for some use cases, but I think it's going to
be a challenge for others. We care most about NVMe and, therefore, the
block layer.

Given my understanding of the block layer, and it's queuing
infrastructure, I don't think having a DMA-only IO path makes sense. I
think it has to be the same path, but with a special DMA-only bio; and
endpoints would have to indicate support for that bio. I can't say I
have a deep enough understanding of the block layer to know how possible
that would be.

Logan