linux-kernel - Re: [PATCH 0/3] Early use of boot service memory

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Date:	Fri, 15 Nov 2013 09:07:38 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...nel.org>,
	jerry.hoemann@...com, Pekka Enberg <penberg@...nel.org>,
	Rob Landley <rob@...dley.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	x86 maintainers <x86@...nel.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"list@...ederm.org:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	"list@...ederm.org:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
	linux-efi@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] Early use of boot service memory

On Thu, Nov 14, 2013 at 10:59:05PM -0800, H. Peter Anvin wrote:
> On 11/14/2013 10:55 PM, Yinghai Lu wrote:
> > 
> > Why just asking distros to append ",high" in their installation
> > program for 64bit by default?
> > 
> [...]
> > 
> > What is hpa's suggestion?
> > 
> 
> Pretty much what you just said ;)

I think crashkernel=X,high is not a good default choice for distros. 
Reserving memory high reserves 72MB (or more) low memory for swiotlb. We
work hard to keep crashkernel memory amount low and currently reserve
128M by default. Now suddenly our total memory reservation will shoot
to 200 MB if we choose ,high option. That's jump of more than 50%. It
is not needed.

We can do dumping operation successfully in *less* reserved memory by
reserving memory below 4G. And hence crashkernel=,high is not a good
default.

Instead, crashkernel=X is a good default if we are ready to change
semantics a bit. If sufficient crashkernel memory is not available
in low memory area, look for it above 4G. This incurs penalty of
72M *only* if it has to and not by default on most of the systems.

And this should solve jerry's problem too on *latest* kernels. For
older kernels, we don't have ,high support. So using that is not
an option. (until and unless somebody is ready to backport everything
needed to boot old kernel above 4G).

Thanks
Vivek
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