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

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Date:   Wed, 30 Jan 2019 09:02:08 +0100
From:   Christoph Hellwig <hch@....de>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        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>,
        Christoph Hellwig <hch@....de>,
        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 Tue, Jan 29, 2019 at 08:58:35PM +0000, Jason Gunthorpe wrote:
> On Tue, Jan 29, 2019 at 01:39:49PM -0700, Logan Gunthorpe wrote:
> 
> > implement the mapping. And I don't think we should have 'special' vma's
> > for this (though we may need something to ensure we don't get mapping
> > requests mixed with different types of pages...).
> 
> I think Jerome explained the point here is to have a 'special vma'
> rather than a 'special struct page' as, really, we don't need a
> struct page at all to make this work.
> 
> If I recall your earlier attempts at adding struct page for BAR
> memory, it ran aground on issues related to O_DIRECT/sgls, etc, etc.

Struct page is what makes O_DIRECT work, using sgls or biovecs, etc on
it work.  Without struct page none of the above can work at all.  That
is why we use struct page for backing BARs in the existing P2P code.
Not that I'm a particular fan of creating struct page for this device
memory, but without major invasive surgery to large parts of the kernel
it is the only way to make it work.