linux-kernel - Re: Interacting with coherent memory on external devices

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Message-ID: <alpine.DEB.2.11.1504271148240.29735@gentwo.org>
Date:	Mon, 27 Apr 2015 11:51:51 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Jerome Glisse <j.glisse@...il.com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	jglisse@...hat.com, mgorman@...e.de, aarcange@...hat.com,
	riel@...hat.com, airlied@...hat.com,
	aneesh.kumar@...ux.vnet.ibm.com,
	Cameron Buschardt <cabuschardt@...dia.com>,
	Mark Hairgrove <mhairgrove@...dia.com>,
	Geoffrey Gerfin <ggerfin@...dia.com>,
	John McKenna <jmckenna@...dia.com>, akpm@...ux-foundation.org
Subject: Re: Interacting with coherent memory on external devices

On Mon, 27 Apr 2015, Jerome Glisse wrote:

> > Well lets avoid that. Access to device memory comparable to what the
> > drivers do today by establishing page table mappings or a generalization
> > of DAX approaches would be the most straightforward way of implementing it
> > and would build based on existing functionality. Page migration currently
> > does not work with driver mappings or DAX because there is no struct page
> > that would allow the lockdown of the page. That may require either
> > continued work on the DAX with page structs approach or new developments
> > in the page migration logic comparable to the get_user_page() alternative
> > of simply creating a scatter gather table to just submit a couple of
> > memory ranges to the I/O subsystem thereby avoiding page structs.
>
> What you refuse to see is that DAX is geared toward filesystem and as such
> rely on special mapping. There is a reason why dax.c is in fs/ and not mm/
> and i keep pointing out we do not want our mecanism to be perceive as fs
> from userspace point of view. We want to be below the fs, at the mm level
> where we could really do thing transparently no matter what kind of memory
> we are talking about (anonymous, file mapped, share).

Ok that is why I mentioned the device memory mappings that are currently
used for this purpose. You could generalize the DAX approach (which I
understand as providing rw mappings to memory outside of the memory
managed by the kernel and not as a fs specific thing).

We can drop the DAX name and just talk about mapping to external memory if
that confuses the issue.

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