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Date:   Thu, 24 May 2018 08:57:08 +0200
From:   Petr Tesarik <ptesarik@suse.cz>
To:     Dave Young <dyoung@redhat.com>
Cc:     dzickus@redhat.com, Neil Horman <nhorman@redhat.com>,
        Tony Luck <tony.luck@intel.com>, bhe@redhat.com,
        Michael Ellerman <mpe@ellerman.id.au>,
        kexec@lists.infradead.org, linux-kernel@vger.kernel.org,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Hari Bathini <hbathini@linux.vnet.ibm.com>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Vivek Goyal <vgoyal@redhat.com>
Subject: Re: [PATCH] kdump: add default crashkernel reserve kernel config
 options
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On Thu, 24 May 2018 09:49:05 +0800
Dave Young <dyoung@redhat.com> wrote:

> Hi Petr,
> 
> On 05/23/18 at 10:22pm, Petr Tesarik wrote:
>[...]
> > In short, if one size fits none, what good is it to hardcode that "one
> > size" into the kernel image?  
> 
> I agreed with all the things that we can not know the exact memory
> requirement for 100% use cases.  But that does not means this is useless
> it is still useful for common use cases of no special and memory hog
> requirements as I mentioned in another reply it can simplify the kdump
> deployment for those people who do not need the special setup.

I still tend to disagree. This "common-case" reservation depends on
things that are defined by user space. It surely does not make it
easier to build a distribution kernel. Today, I get bug reports that
the number calculated and added to the boot loader configuration by the
installer is inaccurate. If I put a fixed number into a kernel config
option, I will start getting bugs that this number is incorrect (for
some systems).

> For example, if this is a workstation I just want to break into a shell
> to collect some panic info, then I just need a very minimal initrd, then
> the Kconfig will work just fine.

What is "a very minimal initrd"? Last time I had to make a significant
adjustment to the estimation for openSUSE, this was caused by growing
user-space requirements (systemd in this case, but I don't want to
start flamewars on that topic, please).

Anyway, if you want to improve the "common case", then look how IBM
tries to solve it for firmware-assisted dump (fadump) on powerpc:

https://patchwork.ozlabs.org/patch/905026/

The main idea is:

> Instead of setting aside a significant chunk of memory nobody can use,
> [...] reserve a significant chunk of memory that the kernel is prevented
> from using [...], but applications are free to use it.

That works great, because user space pages are filtered out in the
common case, so they can be used freely by the panic kernel.

Just my two cents,
Petr T