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

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Date:	Tue, 28 Jul 2015 17:19:56 +0800
From:	Baoquan He <bhe@...hat.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Dave Young <dyoung@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Vivek Goyal <vgoyal@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] Do not reserve crashkernel high memory if crashkernel
 low memory reserving failed

On 07/27/15 at 07:45pm, Yinghai Lu wrote:
> On Mon, Jul 27, 2015 at 5:52 PM, Baoquan He <bhe@...hat.com> wrote:
> > On 07/22/15 at 04:47pm, Yinghai Lu wrote:
> >
> > Sorry for late reply. This problem is reported by customers. They usulay
> > don't like to make these things public. While for those systems with
> > good hard iommu support, it could also fail to initialize hw iommu and
> > then use swiotlb again, E.g in kdump kernel case. You can see in
> > intel_iommu_init() swiotlb is assigned to 0 only if intel iommu (namely vt-d)
> > is initialized successfully. There's possibility that hw iommu
> > initialization will fail, in this case kdump kernel will fail to boot
> > if no any low memory is given. So we can't make assumption that system
> > can boot always well without low memory.
> 
> When we had crashkernel=,high working with auto low=40M.
> all system with crashkernel=,high worked.
> Now come one model (assume it is 16 socket system), and it
> could use crashkernel=,high crashkernel=256M,low. So it forces
> all auto_low be 256M.

Previously the default low memory was 64+8M (64M is for swiotlb). That
value was changed since people complained their crash dumping failed
because of insufficient low memory unexpectedly, almost 200M low memory
is needed on their system when crashkernel=, high is specified. 
I think it makes sense to set a default low memory size to make systems
work, specify a exact low memory size to make it better (for memory
usage optimizing).
> 
> That is ugly. If the customer does not want to make the log public to make
> us to find good solution, why should we care about it ?
> why not just let them carry the "crashkernel=256M,low" all the way?

Now companies can provide big servers with tens of Tera bytes physical
memory, I am not sure if this will leak information of their products. I
will ask if a boot log is pasted here. They found the old default
72M low memory is not enough since they believe the old default low
memory and then crash dump failed.

> 
> Yinghai
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