Re: [RFC summary] Enable Coherent Device Memory

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

On Wed, 2017-05-17 at 11:58 +0100, Mel Gorman wrote:
> The race is a non-issue unless for some reason you decide to hot-add the node
> when the machine is already heavily loaded and under memory pressure. Do it
> near boot time and no CPU-local allocation is going to hit it. In itself,
> special casing the core VM is overkill.
> 
> If you decide to use ZONE_MOVABLE and take the remote hit penalty of page
> tables, then you can also migrate all the pages away after the onlining
> and isolation is complete if it's a serious concern in practice.
> 
> > Unless we have a way to create a node without actually making it
> > available for allocations, so we get a chance to establish policies for
> > it, then "online" it ?
> > 
> 
> Conceivably, that could be done although again it's somewhat overkill
> as the race only applies if hot-adding CDM under heavy memory pressure
> sufficient to overflow to a very remote node.

I wouldn't dismiss the problem that readily. It might by ok for our
initial customer needs but long run, there's a lot of demand for SR-IOV 
GPUs and pass-through.

It's not far fetched to have GPU being dynamically added/removed from
partitions based on usage, which means possibly under significant
pressure.

That said, this can be solved later if needed.

Cheers,
Ben.

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