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

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Date:	Wed, 22 Jul 2015 16:41:00 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Baoquan He <bhe@...hat.com>, 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 Wed, Jul 22, 2015 at 3:11 AM, Joerg Roedel <joro@...tes.org> wrote:
> On Tue, Jul 21, 2015 at 12:22:53PM -0700, Yinghai Lu wrote:
>> On Tue, Jul 21, 2015 at 1:58 AM, Baoquan He <bhe@...hat.com> wrote:
>>
>> > Maybe system which don't need low memory is rare, only for testing?
>>
>> No, it is not rare.
>>
>> All recent intel based systems with iommu support does not need low.
>
> All Intel-IOMMU systems have the iommu disabled by default (at least
> that is the default in most distros). So low memory is definitly needed
> by those systems too.

Do those systems need crashkernel=,high?

Do you mean BIOS have that disabled with not exposing DMAR table ?

kernel for RHEL 6 and RHEL7 have them enabled.
Also opensuse kernel have that enabled too.


>
>> that reserve 256M low always. and those 256M get wasted.
>>
>> That commit should only be used to workaround some systems that
>> have partial iommu support.
>
> We currently lack the infrastructure for that, but I am happy to review
> patches. How about letting subsystems announce their need for low
> crash-kernel memory and allocate based on that?
>
> The subsystems (like iommu or swiotlb code, for example) could even
> announce how much memory they need and we base our allocation on that.

That would be hard, as we don't know if second kernel could take what
kernel parameters.
user could disable iommu etc from command kernel for second kernel.

Yinghai
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