Re: [PATCH v7 1/4] x86: kdump: move reserve_crashkernel_low() into crash_core.c

[Chen Zhou]

Hi Dave/James,

On 2020/1/17 11:58, Dave Young wrote:
> On 01/16/20 at 03:17pm, James Morse wrote:
>> Hi guys,
>>
>> On 28/12/2019 09:32, Dave Young wrote:
>>> On 12/27/19 at 07:04pm, Chen Zhou wrote:
>>>> On 2019/12/27 13:54, Dave Young wrote:
>>>>> On 12/23/19 at 11:23pm, Chen Zhou wrote:
>>>>>> In preparation for supporting reserve_crashkernel_low in arm64 as
>>>>>> x86_64 does, move reserve_crashkernel_low() into kernel/crash_core.c.
>>>>>>
>>>>>> Note, in arm64, we reserve low memory if and only if crashkernel=X,low
>>>>>> is specified. Different with x86_64, don't set low memory automatically.
>>>>>
>>>>> Do you have any reason for the difference?  I'd expect we have same
>>>>> logic if possible and remove some of the ifdefs.
>>>>
>>>> In x86_64, if we reserve crashkernel above 4G, then we call reserve_crashkernel_low()
>>>> to reserve low memory.
>>>>
>>>> In arm64, to simplify, we call reserve_crashkernel_low() at the beginning of reserve_crashkernel()
>>>> and then relax the arm64_dma32_phys_limit if reserve_crashkernel_low() allocated something.
>>>> In this case, if reserve crashkernel below 4G there will be 256M low memory set automatically
>>>> and this needs extra considerations.
>>
>>> Sorry that I did not read the old thread details and thought that is
>>> arch dependent.  But rethink about that, it would be better that we can
>>> have same semantic about crashkernel parameters across arches.  If we
>>> make them different then it causes confusion, especially for
>>> distributions.
>>
>> Surely distros also want one crashkernel* string they can use on all platforms without
>> having to detect the kernel version, platform or changeable memory layout...
>>
>>
>>> OTOH, I thought if we reserve high memory then the low memory should be
>>> needed.  There might be some exceptions, but I do not know the exact
>>> one,
>>
>>> can we make the behavior same, and special case those systems which
>>> do not need low memory reservation.
>>
>> Its tricky to work out which systems are the 'normal' ones.
>>
>> We don't have a fixed memory layout for arm64. Some systems have no memory below 4G.
>> Others have no memory above 4G.
>>
>> Chen Zhou's machine has some memory below 4G, but its too precious to reserve a large
>> chunk for kdump. Without any memory below 4G some of the drivers won't work.
>>
>> I don't see what distros can set as their default for all platforms if high/low are
>> mutually exclusive with the 'crashkernel=' in use today. How did x86 navigate this, ... or
>> was it so long ago?
> 
> It is very rare for such machine without any low memory in X86, at least
> from what I know,  so the current way just works fine.
> 
> Since arm64 is quite different, I would agree with current way
> proposed in the patch, but a question is, for those arm64 systems how can
> admin know if low crashkernel memory is needed or not?  and just skip the
> low reservation for machine with high memory installed only?

Specified size low memory is for crash dump kernel devices.
I think admin should know if there are devices needing low memory in crash dump kernel.

James, any suggestions?

Thanks,
Chen Zhou

> 
>>
>> No one else has reported a problem with the existing placement logic, hence treating this
>> 'low' thing as the 'in addition' special case.
>>
>>
>>>> previous discusses:
>>>> 	https://lkml.org/lkml/2019/6/5/670
>>>> 	https://lkml.org/lkml/2019/6/13/229
>>>
>>> Another concern from James:
>>> "
>>> With both crashk_low_res and crashk_res, we end up with two entries in /proc/iomem called
>>> "Crash kernel". Because its sorted by address, and kexec-tools stops searching when it
>>> find "Crash kernel", you are always going to get the kernel placed in the lower portion.
>>> "
>>>
>>> The kexec-tools code is iterating all "Crash kernel" ranges and add them
>>> in an array.  In X86 code, it uses the higher range to locate memory.
>>
>> Then my hurried reading of what the user-space code does was wrong!
>>
>> If kexec-tools places the kernel in the low region, there may not be enough memory left
>> for whatever purpose it was reserved for. This was the motivation for giving it a
>> different name.
> 
> Agreed,  it is still a potential problem though.  Say we have both low
> and high reserved.  Kdump kernel boots up, the kernel and drivers,
> applications will use memory, I'm not sure if there is a memory
> allocation policy to let them all use high mem first..  Anyway that is
> beyond the kexec-tools and resource name.
> 
> Thanks
> Dave
> 
> 
> .
>