linux-kernel - Re: [PATCH v17 03/10] x86: kdump: use macro CRASH_ADDR_LOW_MAX in functions reserve

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Message-ID: <YbseAX6X1VHUF12f@zn.tnic>
Date:   Thu, 16 Dec 2021 12:07:45 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
Cc:     Baoquan He <bhe@...hat.com>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Dave Young <dyoung@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        kexec@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        linux-doc@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        Feng Zhou <zhoufeng.zf@...edance.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Chen Zhou <dingguo.cz@...group.com>
Subject: Re: [PATCH v17 03/10] x86: kdump: use macro CRASH_ADDR_LOW_MAX in
 functions reserve_crashkernel()

On Thu, Dec 16, 2021 at 10:46:12AM +0800, Leizhen (ThunderTown) wrote:
> The original value (1ULL << 32) is inaccurate

I keep asking *why*?

> and it enlarged the CRASH_ADDR_LOW upper limit.

$ git grep -E "CRASH_ADDR_LOW\W"
$

I have no clue what you mean here.

> This is because when the memory is allocated from the low end, the
> address cannot exceed CRASH_ADDR_LOW_MAX, see "if (!high)" branch.

> If
> the memory is allocated from the high end, 'crash_base' is greater than or
> equal to (1ULL << 32), and naturally, it is greater than CRASH_ADDR_LOW_MAX.
> 
> I think I should update the description, thanks.

I think you should explain why is (1ULL << 32) wrong.

It came from:

  eb6db83d1059 ("x86/setup: Do not reserve crashkernel high memory if low reservation failed")

which simply frees the high memory portion when the low reservation
fails. And the test for that is, is crash base > 4G. So that makes
perfect sense to me.

So your change is a NOP on 64-bit and it is a NOP on 32-bit by virtue of
the _low() variant always returning 0 on 32-bit.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette