Re: [RFC 0/4] RFC - Coherent Device Memory (Not for inclusion)

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Fri, 2017-05-05 at 13:48 -0400, Jerome Glisse wrote:
> Well there is _no_ migration issues with HMM (anonymous or file back
> pages). What you don't get is thing like lru or numa balancing but i
> believe you do not want either of those anyway.

We don't want them in the specific case of GPUs today for various
reasons related more to how they are used and specific implementation
shortcomings, so matter of policy.

However, I don't think they are necessarily to be excluded in the grand
scheme of things of coherent accelerators with local memory.

So my gut feeling (but we can agree to disagree, in the end, what we
need is *a* workable solution to enable these things, which ever it is
that wins), is that we are better off simply treating them as normal
numa nodes, and adding more policy tunables where needed, if possible
with some of these being set to reasonable defaults by the driver
itself to account for implementation shortcomings.

Now, if Michal and Mel strongly prefer the approach based on HMM, we
can make it work as well I believe. It feels less "natural" and more
convoluted. That's it.

This is by no mean a criticism of HMM btw :-) HMM still is a critical
part of getting the non-coherent devices working properly, and which
ever representation we use for the memory on the coherent ones, we will
also use parts of HMM infrastructure for driver directed migration
anyway.

Cheers,
Ben.

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