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Date: Tue, 8 Jun 2021 14:14:10 -0700
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] crash_core, vmcoreinfo: Append 'SECTION_SIZE_BITS' to
 vmcoreinfo
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To: Baoquan He <bhe@redhat.com>
Cc: Pingfan Liu <kernelfans@gmail.com>, Bhupesh Sharma <bhupesh.sharma@linaro.org>, Kazuhito Hagio <k-hagio@ab.jp.nec.com>, Dave Young <dyoung@redhat.com>, Boris Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, James Morse <james.morse@arm.com>, Mark Rutland <mark.rutland@arm.com>, Will Deacon <will@kernel.org>, Catalin Marinas <catalin.marinas@arm.com>, Michael Ellerman <mpe@ellerman.id.au>, Paul Mackerras <paulus@samba.org>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Dave Anderson <anderson@redhat.com>, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org

On Tue, 8 Jun 2021 22:24:32 +0800 Baoquan He <bhe@redhat.com> wrote:

> On 06/08/21 at 06:33am, Pingfan Liu wrote:
> > As mentioned in kernel commit 1d50e5d0c505 ("crash_core, vmcoreinfo:
> > Append 'MAX_PHYSMEM_BITS' to vmcoreinfo"), SECTION_SIZE_BITS in the
> > formula:
> >     #define SECTIONS_SHIFT    (MAX_PHYSMEM_BITS - SECTION_SIZE_BITS)
> > 
> > Besides SECTIONS_SHIFT, SECTION_SIZE_BITS is also used to calculate
> > PAGES_PER_SECTION in makedumpfile just like kernel.
> > 
> > Unfortunately, this arch-dependent macro SECTION_SIZE_BITS changes, e.g.
> > recently in kernel commit f0b13ee23241 ("arm64/sparsemem: reduce
> > SECTION_SIZE_BITS"). But user space wants a stable interface to get this
> > info. Such info is impossible to be deduced from a crashdump vmcore.
> > Hence append SECTION_SIZE_BITS to vmcoreinfo.
> 
> ...
>
> Add the discussion of the original thread in kexec ML for reference:
> http://lists.infradead.org/pipermail/kexec/2021-June/022676.html

I added a Link: for this.
 
> This looks good to me.
> 
> Acked-by: Baoquan He <bhe@redhat.com>

I'm thinking we should backport this at least to Fixes:f0b13ee23241. 
But perhaps it's simpler to just backport it as far as possible, so I
added a bare cc:stable with no Fixes:.  Thoughts?

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