linux-kernel - Re: [PATCH part2 v2 0/8] Arrange hotpluggable memory as ZONE

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Message-ID: <20131015065040.GB24584@gmail.com>
Date:	Tue, 15 Oct 2013 08:50:40 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Yinghai Lu <yinghai@...nel.org>, Tejun Heo <tj@...nel.org>,
	Zhang Yanfei <zhangyanfei.yes@...il.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	Toshi Kani <toshi.kani@...com>, Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH part2 v2 0/8] Arrange hotpluggable memory as ZONE_MOVABLE


* H. Peter Anvin <hpa@...or.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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