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Date:   Thu, 24 May 2018 15:39:15 +0800
From:   Dave Young <dyoung@redhat.com>
To:     Petr Tesarik <ptesarik@suse.cz>
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
Message-ID: <20180524073915.GB1940@dhcp-128-65.nay.redhat.com>
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On 05/24/18 at 03:26pm, Dave Young wrote:
> 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).
> 
> The value is a best effort, it will never be 100% correct.  We did not
> guarantee that.   The kernel config option value is just up to user.
> So I'm thinking it as a good to have benefit.

I means this patch is not trying to force add a fixed value for crashkernel
in kernel code. It provides another way one can use on kernel build time
the value just works.

> 
> > 
> > > 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).
> 
> Still I think we have agreement and same feeling about the userspace
> memory requirement.   I think although it is hard, we have been still
> trying to shrink the initramfs memory use.
> 
> Besides of distribution use,  why people can not use some minimal
> initrd?  For example only a basic shell and some necessary tools and
> basic storage eg. raw disks supported, and he/she can just collect the
> panic infomation by himself in a shell.
> 
> > 
> > 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.
> 
> Good suggestion. I have been reading that posts already at the same time before I saw
> this reply from you :)
> 
> That could be a good idea and worth to discuss more.  I cced Hari
> already in the thread. Hari, is it possible for you to extend your
> idea to general use, ie. shared by both kdump and fadump?  Anyway I
> think that is another topic we can discuss separately.
> 
> > 
> > Just my two cents,
> > Petr T
> 
> Thanks
> Dave