Date: Sat, 25 Apr 2015 04:20:39 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Jerome Glisse <j.glisse@gmail.com>
Cc: Christoph Lameter <cl@linux.com>,
        Austin S Hemmelgarn <ahferroin7@gmail.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, jglisse@redhat.com,
        mgorman@suse.de, aarcange@redhat.com, riel@redhat.com,
        airlied@redhat.com, aneesh.kumar@linux.vnet.ibm.com,
        Cameron Buschardt <cabuschardt@nvidia.com>,
        Mark Hairgrove <mhairgrove@nvidia.com>,
        Geoffrey Gerfin <ggerfin@nvidia.com>,
        John McKenna <jmckenna@nvidia.com>, akpm@linux-foundation.org
Subject: Re: Interacting with coherent memory on external devices
Message-ID: <20150425112039.GH5561@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
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 <alpine.DEB.2.11.1504230921020.32297@gentwo.org>
 <55390EE1.8020304@gmail.com>
 <20150423193339.GR5561@linux.vnet.ibm.com>
 <alpine.DEB.2.11.1504240909350.7582@gentwo.org>
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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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