Re: [PATCH part2 v2 0/8] Arrange hotpluggable memory as ZONE_MOVABLE

[Ingo Molnar]

* H. Peter Anvin <hpa@xxxxxxxxx> wrote:

> On 10/14/2013 01:37 PM, Yinghai Lu wrote:
> >>
> >> Optimizing NUMA boot just requires moving the heavy lifting to
> >> appropriate NUMA nodes.  It doesn't require that early boot phase
> >> should strictly follow NUMA node boundaries.
> > 
> > At end of day, I like to see all numa system (ram/cpu/pci) could have
> > non boot nodes to be hot-removed logically. with any boot command
> > line.
> > 
> 
> I don't think that is realistic without hardware support, simply because 
> all it takes is a single page of kernel locked memory to prevent a page 
> from being removed.  The only realistic way around that, I believe, is 
> to remove the identity-mapping in the kernel, but it still has all kinds 
> of funnies involving devices and DMA.

We played with virtual kernel memory a decade ago and it's doable. The 
only complication was DMA from the kernel stack - that was done with some 
really broken old ISA drivers IIRC. Those should be a distant memory, in 
terms of practical impact.

So if anyone can implement it using huge pages, with a really fast __va() 
and __pa() implementation, then it might be possible. But that's a pretty 
major surgery on x86.

Thanks,

	Ingo
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