Re: Interacting with coherent memory on external devices

[Christoph Lameter <cl@linux.com>]

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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