linux-kernel - Re: [PATCH 3/3] x86, crash: Allocate enough low-mem when crashkernel=high

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Message-ID: <20150126121142.GG30345@8bytes.org>
Date:	Mon, 26 Jan 2015 13:11:42 +0100
From:	Joerg Roedel <joro@...tes.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 3/3] x86, crash: Allocate enough low-mem when
 crashkernel=high

On Fri, Jan 23, 2015 at 06:02:43PM +0100, Borislav Petkov wrote:
> On Tue, Jan 06, 2015 at 03:51:14PM +0100, Joerg Roedel wrote:
> > From: Joerg Roedel <jroedel@...e.de>
> > 
> > When the crashkernel is loaded above 4GiB in memory the
> > first kernel only allocates 72MiB of low-memory for the DMA
> > requirements of the second kernel. On systems with many
> > devices this is not enough and causes device driver
> > initialization errors and failed crash dumps. Set this
> > default value to 256MiB to make sure there is enough memory
> 
> This upper limit of 256 looks arbitrary. Are we going to raise it a
> couple of years from now if it becomes insufficient then?

Yes, it is arbitrary. I am open for suggestions on what might be a
proper value to satisfy most systems.

> It probably won't be easy but is there some more reliable way to
> allocate enough memory for DMA on a say per-system basis or whatever...?
> Probably not but let me ask it anyway.

Well, there is no easy way. But we could collect information from the
loaded drivers on boot about how many dma-memory they allocate and base
our allocation on that. Or we solve it in user-space by some more
cleverness in creating the kernel command-line for crashkernel=high.

But besides that, I think the first two patches of this set make sense
anyway. I understand that the third one is debatable.


	Joerg

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