Re: [PATCH v11 3/5] arm64: kdump: reimplement crashkernel=X

[chenzhou]

Hi Catalin,


On 2020/9/3 1:09, Catalin Marinas wrote:
> On Sat, Aug 01, 2020 at 09:08:54PM +0800, Chen Zhou wrote:
>> There are following issues in arm64 kdump:
>> 1. We use crashkernel=X to reserve crashkernel below 4G, which
>> will fail when there is no enough low memory.
>> 2. If reserving crashkernel above 4G, in this case, crash dump
>> kernel will boot failure because there is no low memory available
>> for allocation.
>> 3. Since commit 1a8e1cef7603 ("arm64: use both ZONE_DMA and ZONE_DMA32"),
>> if the memory reserved for crash dump kernel falled in ZONE_DMA32,
>> the devices in crash dump kernel need to use ZONE_DMA will alloc
>> fail.
>>
>> To solve these issues, change the behavior of crashkernel=X.
>> crashkernel=X tries low allocation in ZONE_DMA, and fall back to
>> high allocation if it fails.
>>
>> If requized size X is too large and leads to very little free memory
>> in ZONE_DMA after low allocation, the system may not work normally.
>> So add a threshold and go for high allocation directly if the required
>> size is too large. The value of threshold is set as the half of
>> the low memory.
>>
>> If crash_base is outside ZONE_DMA, try to allocate at least 256M in
>> ZONE_DMA automatically. "crashkernel=Y,low" can be used to allocate
>> specified size low memory.
> Except for the threshold to keep zone ZONE_DMA memory,
> reserve_crashkernel() looks very close to the x86 version. Shall we try
> to make this generic as well? In the first instance, you could avoid the
> threshold check if it takes an explicit ",high" option.
Ok, i will try to do this.

I look into the function reserve_crashkernel() of x86 and found the start address is
CRASH_ALIGN in function memblock_find_in_range(), which is different with arm64.

I don't figure out why is CRASH_ALIGN in x86, is there any specific reason?

Thanks,
Chen Zhou