linux-kernel - Re: [PATCH] crash_core, vmcoreinfo: Append 'SECTION_SIZE

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Date:   Tue, 8 Jun 2021 14:14:10 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     Pingfan Liu <kernelfans@...il.com>,
        Bhupesh Sharma <bhupesh.sharma@...aro.org>,
        Kazuhito Hagio <k-hagio@...jp.nec.com>,
        Dave Young <dyoung@...hat.com>, Boris Petkov <bp@...en8.de>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        James Morse <james.morse@....com>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Dave Anderson <anderson@...hat.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        kexec@...ts.infradead.org, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] crash_core, vmcoreinfo: Append 'SECTION_SIZE_BITS' to
 vmcoreinfo

On Tue, 8 Jun 2021 22:24:32 +0800 Baoquan He <bhe@...hat.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@...hat.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?