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

Leizhen (ThunderTown)
2021-12-15T11:46:05Z

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@huawei.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.

>
> .
>