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

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Message-ID: <CAE9FiQWY-7fJ1AoAsD1b6O6ExgKriSC+DaeTO8NUuKZ_Jhre8w@mail.gmail.com>
Date:	Wed, 10 Jun 2015 10:48:14 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Baoquan He <bhe@...hat.com>, Joerg Roedel <joro@...tes.org>
Cc:	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] x86/crash: Allocate enough low memory when crashkernel=high

On Wed, Jun 10, 2015 at 12:04 AM, Baoquan He <bhe@...hat.com> wrote:
>
> In fact in this case, it doesn't matter how much memory the system has
> since most of them is above 4G and only 72M is reserved for dma/swiotlb
> in kdump kernel. And it doesn't matter much how many pci/pcie cards the
> system has since it matters how greedy those devices expects dma
> allocation.
>
> Before Joerg posted this patchset v1, our customer complained about
> this too. That means this is not individual case. I think it makes sense
> to increase the default low-memory to make kdump succeed, it's
> definitely better than letting user get a failed kdump and then know
> they need specify more low-memory manually. If user clearly know their
> system doesn't need so much low-memory, specifying crashkernel=xx,low
> works for them.

Now intel 4socket or 8socket system mostly have [2G,4G) for mmio low.
also I saw one system had [1.5G, 4G) for mmio.

So under 4G low ram is tight, with changing default to 256M is just lazy
and punishing the other system that does not need that.
those systems do not need to have "crashkernel=xx,low" before.

should have other smart way to detect and update the default value.
even dmi blacklist way is better than just changing default value.

Thanks

Yinghai
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