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

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Message-ID: <alpine.DEB.2.11.1504271411060.30615@gentwo.org>
Date:	Mon, 27 Apr 2015 14:26:04 -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:

> > We can drop the DAX name and just talk about mapping to external memory if
> > that confuses the issue.
>
> DAX is for direct access block layer (X is for the cool name factor)
> there is zero code inside DAX that would be usefull to us. Because it
> is all about filesystem and short circuiting the pagecache. So DAX is
> _not_ about providing rw mappings to non regular memory, it is about
> allowing to directly map _filesystem backing storage_ into a process.

Its about directly mapping memory outside of regular kernel
management via a block device into user space. That you can put a
filesystem on top is one possible use case. You can provide a block
device to map the memory of the coprocessor and then configure the memory
space to have the same layout on the coprocessor as well as the linux
process.

> Moreover DAX is not about managing that persistent memory, all the
> management is done inside the fs (ext4, xfs, ...) in the same way as
> for non persistent memory. While in our case we want to manage the
> memory as a runtime resources that is allocated to process the same
> way regular system memory is managed.

I repeatedly said that. So you would have a block device that would be
used to mmap portions of the special memory into a process.

> So current DAX code have nothing of value for our usecase nor what we
> propose will have anyvalue for DAX. Unless they decide to go down the
> struct page road for persistent memory (which from last discussion i
> heard was not there plan, i am pretty sure they entirely dismissed
> that idea for now).

DAX is about directly accessing memory. It is made for the purpose of
serving as a block device for a filesystem right now but it can easily be
used as a way to map any external memory into a processes space using the
abstraction of a block device. But then you can do that with any device
driver using VM_PFNMAP or VM_MIXEDMAP. Maybe we better use that term
instead. Guess I have repeated myself 6 times or so now? I am stopping
with this one.

> My point is that this is 2 differents non overlapping problems, and
> thus mandate 2 differents solution.

Well confusion abounds since so much other stuff has ben attached to DAX
devices.

Lets drop the DAX term and use VM_PFNMAP or VM_MIXEDMAP instead. MIXEDMAP
is the mechanism that DAX relies on in the VM.

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