linux-kernel - Re: [PATCH v17 02/10] x86: kdump: make the lower bound of crash kernel reservation consistent

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Date:   Wed, 15 Dec 2021 19:45:46 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Baoquan He <bhe@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>
CC:     Borislav Petkov <bp@...en8.de>,
        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>, 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 02/10] x86: kdump: make the lower bound of crash
 kernel reservation consistent



On 2021/12/15 19:16, Baoquan He wrote:
> On 12/15/21 at 11:01am, Catalin Marinas wrote:
>> On Wed, Dec 15, 2021 at 11:42:19AM +0800, Baoquan He wrote:
>>> On 12/14/21 at 07:24pm, Catalin Marinas wrote:
>>>> On Tue, Dec 14, 2021 at 08:07:58PM +0100, Borislav Petkov wrote:
>>>>> On Fri, Dec 10, 2021 at 02:55:25PM +0800, Zhen Lei wrote:
>>>>>> From: Chen Zhou <chenzhou10@...wei.com>
>>>>>>
>>>>>> The lower bounds of crash kernel reservation and crash kernel low
>>>>>> reservation are different, use the consistent value CRASH_ALIGN.
>>>>>
>>>>> A big WHY is missing here to explain why the lower bound of the
>>>>> allocation range needs to be 16M and why was 0 wrong?
>>>>
>>>> I asked the same here:
>>>>
>>>> https://lore.kernel.org/r/20210224143547.GB28965@arm.com
>>>>
>>>> IIRC Baoquan said that there is a 1MB reserved for x86 anyway in the
>>>> lower part, so that's equivalent in practice to starting from
>>>> CRASH_ALIGN.
>>>
>>> Yeah, even for i386, there's area reserved by BIOS inside low 1M.
>>> Considering the existing alignment CRASH_ALIGN which is 16M, we
>>> definitely have no chance to get memory starting from 0. So starting
>>> from 16M can skip the useless memblock searching, and make the
>>> crashkernel low reservation consisten with crashkernel reservation on
>>> allocation code.
>>
>> That's the x86 assumption. Is it valid for other architectures once the
>> code has been made generic in patch 6? It should be ok for arm64, RAM
>> tends to start from higher up but other architectures may start using
>> this common code.
> 
> Good point. I didn't think of this from generic code side, then let's
> keep it as 0.
> 
>>
>> If you want to keep the same semantics as before, just leave it as 0.
>> It's not that the additional lower bound makes the search slower.
> 
> Agree.

OK, I will drop this patch.

> 
> .
>