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Date:   Fri, 25 May 2018 06:59:43 +0200
From:   Petr Tesarik <ptesarik@suse.cz>
To:     ebiederm@xmission.com (Eric W. Biederman)
Cc:     Dave Young <dyoung@redhat.com>, 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>,
        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
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V Thu, 24 May 2018 11:34:05 -0500
ebiederm@xmission.com (Eric W. Biederman) napsáno:

> Petr Tesarik <ptesarik@suse.cz> writes:
> 
> 2> 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.  
> 
> They absolutely can not be used in the kdump case.
> 
> The kdump requirement is that they are pages no-one initiates any I/O
> to.  To avoid the problem of devices doing DMA as the new kernel starts
> and runs.

Good point. This means that memory reserved for this purpose would also
have to be excluded from allocations that may be eventually used for
DMA transfers.

>  Secondarily to avoid problems with cpus that refused to halt.

Let's face it - if some CPUs refused to halt, all bets are off. The
code running on such a CPU can break many other things besides memory,
most importantly, it may meddle with the HW registers of crucial
devices in the system. To be less abstract, I have seen a failure to
stop a CPU in the crashed kernel a few times, and the panic kernel
could never successfully save anything; it always crashed at boot or a
little bit later.

Anyway, of course we would still have to keep the current method,
because user pages are not always filtered. For example, a major SUSE
account runs a database in user space and also inspects its data
structures in case of a system crash.

Petr T