Re: [LSF/MM TOPIC] Un-addressable device memory and block/fs implications

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On Wed, Dec 14, 2016 at 07:15:15AM +1100, Dave Chinner wrote:
> On Tue, Dec 13, 2016 at 01:15:11PM -0500, Jerome Glisse wrote:
> > I would like to discuss un-addressable device memory in the context of
> > filesystem and block device. Specificaly how to handle write-back, read,
> > ... when a filesystem page is migrated to device memory that CPU can not
> > access.
> 
> You mean pmem that is DAX-capable that suddenly, without warning,
> becomes non-DAX capable?
> 
> If you are not talking about pmem and DAX, then exactly what does
> "when a filesystem page is migrated to device memory that CPU can
> not access" mean? What "filesystem page" are we talking about that
> can get migrated from main RAM to something the CPU can't access?

I am talking about GPU, FPGA, ... any PCIE device that have fast on
board memory that can not be expose transparently to the CPU. I am
reusing ZONE_DEVICE for this, you can see HMM patchset on linux-mm
https://lwn.net/Articles/706856/

So in my case i am only considering non DAX/PMEM filesystem ie any
"regular" filesystem back by a "regular" block device. I want to be
able to migrate mmaped area of such filesystem to device memory while
the device is actively using that memory.

>From kernel point of view such memory is almost like any other, it
has a struct page and most of the mm code is non the wiser, nor need
to be about it. CPU access trigger a migration back to regular CPU
accessible page.

But for thing like writeback i want to be able to do writeback with-
out having to migrate page back first. So that data can stay on the
device while writeback is happening.

Jérôme
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