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

* H. Peter Anvin <hpa@zytor.com> 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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