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Date:   Thu, 24 May 2018 15:31:05 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Petr Tesarik <ptesarik@suse.cz>
Cc:     Dave Young <dyoung@redhat.com>, dzickus@redhat.com,
        Neil Horman <nhorman@redhat.com>,
        Tony Luck <tony.luck@intel.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
Message-ID: <20180524073105.GF24627@MiWiFi-R3L-srv>
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On 05/24/18 at 08:57am, Petr Tesarik wrote:
> 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.

This seems a good idea, just makedumpfile need be adjusted since it allows
user to decide if dump user space data or not.