Re: [PATCH v3 1/2] arm64: kdump: Provide default size when crashkernel=Y,low is not specified

["Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>]

On 2022/11/8 1:18, Catalin Marinas wrote:
> On Mon, Jul 11, 2022 at 05:03:18PM +0800, Zhen Lei wrote:
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index 2522b11e593f239..65a2c3a22a4b57d 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -843,7 +843,7 @@
>>  			available.
>>  			It will be ignored if crashkernel=X is specified.
>>  	crashkernel=size[KMG],low
>> -			[KNL, X86-64] range under 4G. When crashkernel=X,high
>> +			[KNL, X86-64, ARM64] 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
>> @@ -857,12 +857,6 @@
>>  			It will be ignored when crashkernel=X,high is not used
>>  			or memory reserved is below 4G.
>>  
>> -			[KNL, ARM64] range in low memory.
>> -			This one lets the user specify a low range in the
>> -			DMA zone for the crash dump kernel.
>> -			It will be ignored when crashkernel=X,high is not used
>> -			or memory reserved is located in the DMA zones.
>> -
>>  	cryptomgr.notests
>>  			[KNL] Disable crypto self-tests
>>  
>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> index 339ee84e5a61a0b..5390f361208ccf7 100644
>> --- a/arch/arm64/mm/init.c
>> +++ b/arch/arm64/mm/init.c
>> @@ -96,6 +96,14 @@ phys_addr_t __ro_after_init arm64_dma_phys_limit = PHYS_MASK + 1;
>>  #define CRASH_ADDR_LOW_MAX		arm64_dma_phys_limit
>>  #define CRASH_ADDR_HIGH_MAX		(PHYS_MASK + 1)
>>  
>> +/*
>> + * This is an empirical value in x86_64 and taken here directly. Please
>> + * refer to the code comment in reserve_crashkernel_low() of x86_64 for more
>> + * details.
>> + */
>> +#define DEFAULT_CRASH_KERNEL_LOW_SIZE	\
>> +	max(swiotlb_size_or_default() + (8UL << 20), 256UL << 20)
>> +
>>  static int __init reserve_crashkernel_low(unsigned long long low_size)
>>  {
>>  	unsigned long long low_base;
>> @@ -147,7 +155,9 @@ static void __init reserve_crashkernel(void)
>>  		 * is not allowed.
>>  		 */
>>  		ret = parse_crashkernel_low(cmdline, 0, &crash_low_size, &crash_base);
>> -		if (ret && (ret != -ENOENT))
>> +		if (ret == -ENOENT)
>> +			crash_low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
>> +		else if (ret)
>>  			return;
> 
> BTW, since we want a default low allocation, I think we should change
> the checking logic slightly. Currently we have:
> 
> 	if ((crash_base >= CRASH_ADDR_LOW_MAX) &&
> 	     crash_low_size && reserve_crashkernel_low(crash_low_size)) {
> 		...
> 
> If crash_base is just below CRASH_ADDR_LOW_MAX, we deem it sufficient
> but a crashkernel trying to allocate 64MB of swiotlb may fail. So maybe
> change this to crash_base >= CRASH_ADDR_LOW_MAX - crash_low_size.

The equal sign needs to be removed.

The situation should be the allocation of "crashkernel=X,high".

This possibility is too small, the high memory is unlikely to be that small.
memblock_phys_alloc_range() always search for memory block from high addresses
to low addresses. In the initial phase, high-end memory is not fragmented.

Of course, the modification can make people look more reassuring. OK, I'll
update it.

> 

-- 
Regards,
  Zhen Lei

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