linux-kernel - Re: [PATCH v2] Do not reserve crashkernel high memory if crashkernel low memory reserving failed

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Date:	Tue, 21 Jul 2015 16:19:32 +0800
From:	Dave Young <dyoung@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Baoquan He <bhe@...hat.com>, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, bp@...e.de, akpm@...ux-foundation.org,
	jkosina@...e.cz, vgoyal@...hat.com, linux-kernel@...r.kernel.org,
	yinghai@...nel.org, Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH v2] Do not reserve crashkernel high memory if crashkernel
 low memory reserving failed

On 07/21/15 at 09:38am, Ingo Molnar wrote:
> 
> * Dave Young <dyoung@...hat.com> wrote:
> 
> > Hi, Baoquan
> > 
> > The interface was introduced by Yinghai, ccing him.
> 
> Also, why was this syntax introduced in the first place? Why should the user 
> care??

The history is like below, I might miss something, Yinghai/HPA/Vivek may correct
and provide more background:

First commit is below:
commit 7d41a8a4a2b2438621a9159477bff36a11d79a42
Author: Yinghai Lu <yinghai@...nel.org>
Date:   Thu Jan 24 12:20:10 2013 -0800

    x86, kdump: Remove crashkernel range find limit for 64bit

Previously kdump reservation is limited to <896M, above commit remove the limitation.
Then because of kdump kernel need access low mem for swiotlb buffer, thus there's
commit below:

commit 0212f9159694be61c6bc52e925fa76643e0c1abf
Author: Yinghai Lu <yinghai@...nel.org>
Date:   Thu Jan 24 12:20:11 2013 -0800

    x86: Add Crash kernel low reservation

Later, Vivek found old kexec-tools does not work, then Yinghai introduced new param
crashkernel_high=:

commit 55a20ee7804ab64ac90bcdd4e2868a42829e2784
Author: Yinghai Lu <yinghai@...nel.org>
Date:   Mon Apr 15 22:23:47 2013 -0700

    x86, kdump: Retore crashkernel= to allocate under 896M

Finally it becomes crashkernel=x,high:
commit adbc742bf78695bb98c79d18c558b61571748b99
Author: Yinghai Lu <yinghai@...nel.org>
Date:   Mon Apr 15 22:23:48 2013 -0700

    x86, kdump: Change crashkernel_high/low= to crashkernel=,high/low


> 
> We should only have a single crashkernel option, to enable it - and everything 
> else should be figured out by the kernel, automatically.

Presonally I do not current complicated params either, I hope we can have an
elegant interface, but seems there's no better way to resolve the low memory
only requirement for software tlb.

> 
> Any other sub-options just paper over some fragility elsewhere and make the 
> feature harder to use, hence more fragile.

Agree, it is becoming worse..

Thanks
Dave
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