linux-kernel - Re: [PATCH part5 0/7] Arrange hotpluggable memory as ZONE

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Message-ID: <20130812180758.GA8288@mtj.dyndns.org>
Date:	Mon, 12 Aug 2013 14:07:58 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Tang Chen <imtangchen@...il.com>
Cc:	Tang Chen <tangchen@...fujitsu.com>, robert.moore@...el.com,
	lv.zheng@...el.com, rjw@...k.pl, lenb@...nel.org,
	tglx@...utronix.de, mingo@...e.hu, hpa@...or.com,
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	jiang.liu@...wei.com, wency@...fujitsu.com, laijs@...fujitsu.com,
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	yanghy@...fujitsu.com, x86@...nel.org, linux-doc@...r.kernel.org,
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	linux-acpi@...r.kernel.org
Subject: Re: [PATCH part5 0/7] Arrange hotpluggable memory as ZONE_MOVABLE.

Hey,

On Tue, Aug 13, 2013 at 01:01:09AM +0800, Tang Chen wrote:
> Sorry for the misunderstanding.
> 
> I was trying to answer your question: "Why can't the kenrel allocate
> hotpluggable memory opportunistic ?".

I've used the wrong word, I was meaning best-effort, which is the only
thing we can do anyway given that we have no control over where the
kernel image is linked in relation to NUMA nodes.

> If the kernel has any opportunity to allocate hotpluggable memory in
> SRAT, then the kernel should tell users which memory is hotpluggable.
> 
> But in what way ?  I think node is the best for now. But a node could
> have a lot of memory. If the kernel uses only a little memory, we will
> lose the whole movable node, which I don't want to do.
> 
> So, I don't want to allow the kenrel allocating hotpluggable memory
> opportunistic.

What I was saying was that the kernel should try !hotpluggable memory
first then fall back to hotpluggable memory instead of failing boot as
nothing really is worse than failing to boot.

> >Short of being able to remap memory under the kernel, I don't think
> >this can be very generic and as a compromise trying to keep as many
> >hotpluggable nodes as possible doesn't sound too bad.
> 
> I think making one of the node hotpluggable is better. But OK, it is
> no big deal. There won't be such machine in reality, I think. :)

Hmmm... but allocating close to kernel image will keep the number of
nodes which are made un-removeable via permanent allocation to
minimum.  In most configurations that I can recall, I don't think we'd
lose anything really and the code will be much simpler and generic.
It seems like a good trade-off to me given that we need to report
which nodes are hot unpluggable no matter what.

Thanks.

-- 
tejun
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