linux-kernel - Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device vma

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Date:   Thu, 31 Jan 2019 09:05:01 +0100
From:   Christoph Hellwig <hch@....de>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        Christoph Hellwig <hch@....de>,
        Jerome Glisse <jglisse@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christian Koenig <christian.koenig@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <jroedel@...e.de>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device
 vma

On Wed, Jan 30, 2019 at 08:44:20PM +0000, Jason Gunthorpe wrote:
> Not really, for MRs most drivers care about DMA addresses only. The
> only reason struct page ever gets involved is because it is part of
> the GUP, SGL and dma_map family of APIs.

And the only way you get the DMA address is through the dma mapping
APIs.  Which except for the little oddball dma_map_resource expect
a struct page in some form.  And dma_map_resource isn't really up
to speed for full blown P2P.

Now we could and maybe eventually should change all this.  But that
is a pre-requisitive for doing anything more fancy, and not something
to be hacked around.

> O_DIRECT seems to be the justification for struct page, but nobody is
> signing up to make O_DIRECT have the required special GUP/SGL/P2P flow
> that would be needed to *actually* make that work - so it really isn't
> a justification today.

O_DIRECT is just the messenger.  Anything using GUP will need a struct
page, which is all our interfaces that do I/O directly to user pages.