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

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Date:   Thu, 16 Dec 2021 22:11:15 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Borislav Petkov <bp@...en8.de>,
        Zhen Lei <thunder.leizhen@...wei.com>
Cc:     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 12/16/21 at 11:55am, Borislav Petkov wrote:
> On Thu, Dec 16, 2021 at 09:10:40AM +0800, Baoquan He wrote:
> > reserve_crashkernel_low() always return 0 on x86_32, so the not equivalent
> > transformation for x86_32 doesn't matter, I think.
> 
> That is, of course, very obvious... not!
> 
> Why is that function parsing crashkernel=XM, and crashkernel=X,high,
> then it attempts some low memory reservation too? Why isn't
> crashkernel=Y,low parsed there too?
> 
> I guess this alludes to why:
> 
>         crashkernel=size[KMG],low
>                         [KNL, X86-64] range under 4G. When crashkernel=X,high
>                         is passed, kernel could allocate physical memory region
>                         above 4G, that cause second kernel crash on system
>                         that require some amount of low memory, e.g. swiotlb
>                         requires at least 64M+32K low memory, also enough extra
>                         low memory is needed to make sure DMA buffers for 32-bit
>                         devices won't run out.
> 
> So, before this is going anywhere, I'd like to see this function
> documented properly. I see Documentation/admin-guide/kdump/kdump.rst
> explains the crashkernel= options too so you can refer to it in the
> comments so that when someone looks at that code, someone can follow why
> it is doing what it is doing.
> 
> Then, as a future work, all that parsing of crashkernel= cmdline options
> should be concentrated at the beginning and once it is clear what the
> user requests, the reservations should be done.

Totally agree we should refactor code to make reserve_crashkernel()
clearer on logic and readibility. In this patchset, we can rewrite the
kernel-doc of reserve_crashkernel() to add more words to explain. As for
the code refactoring, it can be done in another patchset.

> 
> As it is, reserve_crashkernel() is pretty unwieldy and hard to read.