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

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Message-ID: <20150425112039.GH5561@linux.vnet.ibm.com>
Date:	Sat, 25 Apr 2015 04:20:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jerome Glisse <j.glisse@...il.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	Austin S Hemmelgarn <ahferroin7@...il.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 Fri, Apr 24, 2015 at 11:09:36AM -0400, Jerome Glisse wrote:
> On Fri, Apr 24, 2015 at 07:57:38AM -0700, Paul E. McKenney wrote:
> > On Fri, Apr 24, 2015 at 09:12:07AM -0500, Christoph Lameter wrote:
> > > On Thu, 23 Apr 2015, Paul E. McKenney wrote:
> > > 
> > > >
> > > > DAX
> > > >
> > > > 	DAX is a mechanism for providing direct-memory access to
> > > > 	high-speed non-volatile (AKA "persistent") memory.  Good
> > > > 	introductions to DAX may be found in the following LWN
> > > > 	articles:
> > > 
> > > DAX is a mechanism to access memory not managed by the kernel and is the
> > > successor to XIP. It just happens to be needed for persistent memory.
> > > Fundamentally any driver can provide an MMAPPed interface to allow access
> > > to a devices memory.
> > 
> > I will take another look, but others in this thread have called out
> > difficulties with DAX's filesystem nature.
> 
> Do not waste your time on that this is not what we want. Christoph here
> is more than stuborn and fails to see the world.

Well, we do need to make sure that we are correctly representing DAX's
capabilities.  It is a hot topic, and others will probably also suggest
that it be used.  That said, at the moment, I don't see how it would help,
given the need to migrate memory.  Perhaps Boas Harrosh's patch set to
allow struct pages to be associated might help?  But from what I can see,
a fair amount of other functionality would still be required either way.

I am updating the DAX section a bit, but I don't claim that it is complete.

							Thanx, Paul

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