Re: [PATCH v23 3/6] arm64: kdump: Reimplement crashkernel=X

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

On 2022/5/6 1:01, Catalin Marinas wrote:
> On Thu, May 05, 2022 at 05:18:42PM +0800, Zhen Lei wrote:
>> From: Chen Zhou <chenzhou10@huawei.com>
>>
>> There are following issues in arm64 kdump:
>> 1. We use crashkernel=X to reserve crashkernel in DMA zone, which
>> will fail when there is not enough low memory.
>> 2. If reserving crashkernel above DMA zone, in this case, crash dump
>> kernel will fail to boot because there is no low memory available
>> for allocation.
>>
>> To solve these issues, introduce crashkernel=X,[high,low].
>> The "crashkernel=X,high" is used to select a region above DMA zone, and
>> the "crashkernel=Y,low" is used to allocate specified size low memory.
> 
> Thanks for posting the simplified version, though the discussion with
> Baoquan is still ongoing. AFAICT there is no fallback if crashkernel=
> fails. The advantage with this series is cleaner code, we set the limits
> during parsing and don't have to adjust them if some of the first
> allocation failed.

Yes, I'm currently implementing it in the simplest version, providing only
the most basic functions. Because the conclusions of this part of the discussion
are clear. I think I can send the fallback, default low size, and mapping optimization
patches separately after this basic version is merged. These three functions can
be discussed separately.

> 
>> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
>> index 51863f1448c6989..11406f3e1443168 100644
>> --- a/arch/arm64/mm/init.c
>> +++ b/arch/arm64/mm/init.c
>> @@ -90,6 +90,32 @@ phys_addr_t __ro_after_init arm64_dma_phys_limit;
>>  phys_addr_t __ro_after_init arm64_dma_phys_limit = PHYS_MASK + 1;
>>  #endif
>>  
>> +/* Current arm64 boot protocol requires 2MB alignment */
>> +#define CRASH_ALIGN			SZ_2M
>> +
>> +#define CRASH_ADDR_LOW_MAX		arm64_dma_phys_limit
>> +#define CRASH_ADDR_HIGH_MAX		memblock.current_limit
> 
> Better use memblock_get_current_limit() if you need to or just
> MEMBLOCK_ALLOC_ANYWHERE, memblock.current_limit is just a memblock
> internal. But I think we can go for (PHYS_MASK + 1) if you need
> something other than MEMBLOCK_ALLOC_ANYWHERE, memblock knows what to
> allocate anyway.

Yes, it would be better to use (PHYS_MASK + 1).

> 
>> +static int __init reserve_crashkernel_low(unsigned long long low_size)
>> +{
>> +	unsigned long long low_base;
>> +
>> +	low_base = memblock_phys_alloc_range(low_size, CRASH_ALIGN, 0, CRASH_ADDR_LOW_MAX);
>> +	if (!low_base) {
>> +		pr_err("cannot allocate crashkernel low memory (size:0x%llx).\n", low_size);
>> +		return -ENOMEM;
>> +	}
>> +
>> +	pr_info("crashkernel low memory reserved: 0x%08llx - 0x%08llx (%lld MB)\n",
>> +		low_base, low_base + low_size, low_size >> 20);
>> +
>> +	crashk_low_res.start = low_base;
>> +	crashk_low_res.end   = low_base + low_size - 1;
>> +	insert_resource(&iomem_resource, &crashk_low_res);
>> +
>> +	return 0;
>> +}
>> +
>>  /*
>>   * reserve_crashkernel() - reserves memory for crash kernel
>>   *
>> @@ -100,17 +126,32 @@ phys_addr_t __ro_after_init arm64_dma_phys_limit = PHYS_MASK + 1;
>>  static void __init reserve_crashkernel(void)
>>  {
>>  	unsigned long long crash_base, crash_size;
>> -	unsigned long long crash_max = arm64_dma_phys_limit;
>> +	unsigned long long crash_low_size = 0;
>> +	unsigned long long crash_max = CRASH_ADDR_LOW_MAX;
>> +	char *cmdline = boot_command_line;
>>  	int ret;
>>  
>>  	if (!IS_ENABLED(CONFIG_KEXEC_CORE))
>>  		return;
>>  
>> -	ret = parse_crashkernel(boot_command_line, memblock_phys_mem_size(),
>> +	/* crashkernel=X[@offset] */
>> +	ret = parse_crashkernel(cmdline, memblock_phys_mem_size(),
>>  				&crash_size, &crash_base);
>> -	/* no crashkernel= or invalid value specified */
>> -	if (ret || !crash_size)
>> -		return;
>> +	if (ret || !crash_size) {
> 
> I think we should check for ret == -ENOENT only. If the crashkernel=
> exists but is malformed or the size is 0, we shouldn't bother with
> high/low at all.

That's right.

> 
>> +		ret = parse_crashkernel_high(cmdline, 0, &crash_size, &crash_base);
>> +		if (ret || !crash_size)
>> +			return;
>> +
>> +		/*
>> +		 * crashkernel=Y,low can be specified or not, but invalid value
>> +		 * is not allowed.
>> +		 */
>> +		ret = parse_crashkernel_low(cmdline, 0, &crash_low_size, &crash_base);
>> +		if (ret && (ret != -ENOENT))
>> +			return;
>> +
>> +		crash_max = CRASH_ADDR_HIGH_MAX;
>> +	}
>>  
>>  	crash_size = PAGE_ALIGN(crash_size);
>>  
>> @@ -118,8 +159,7 @@ static void __init reserve_crashkernel(void)
>>  	if (crash_base)
>>  		crash_max = crash_base + crash_size;
>>  
>> -	/* Current arm64 boot protocol requires 2MB alignment */
>> -	crash_base = memblock_phys_alloc_range(crash_size, SZ_2M,
>> +	crash_base = memblock_phys_alloc_range(crash_size, CRASH_ALIGN,
>>  					       crash_base, crash_max);
>>  	if (!crash_base) {
>>  		pr_warn("cannot allocate crashkernel (size:0x%llx)\n",
> 
> I personally like this but let's see how the other thread goes. I guess

Me too. This fallback complicates code logic more than just a little.
I'm not sure why someone would rather add fallback than change the bootup
options to crashkernel=X,[high|low]. Perhaps fallback to high/low is a better
compatible and extended mode when crashkernel=X fails to reserve memory. And
the code logic will be much clearer.

//parse crashkernel=X		//To simplify the discussion, Ignore [@offset]
crash_base = memblock_phys_alloc_range()
if (!crash_base || /* crashkernel=X is not specified */) {
	//parse crashkernel=X,[high,low]
	//reserve high/low memory
}

So that, the following three modes are supported:
1) crashkernel=X[@offset]
2) crashkernel=X,high crashkernel=X,low
3) crashkernel=X[@offset] crashkernel=X,high [crashkernel=Y,low]

For case 3), try "crashkernel=X[@offset]" first, if it can not work, fallback
to "crashkernel=X,high crashkernel=X,low". This looks better than the old "crashkernel=X"
fallback ---- Select a region under 4G first, and fall back to reserve region above 4G.

Note: when the X of crashkernel=X and crashkernel=X,high are the same, It's equivalent
to the old "crashkernel=X" fallback.

> if we want a fallback, it would come just before the check the above:
> 
> 	if (!crash_base && crash_max != CRASH_ADDR_HIGH_MAX) {
> 		/* attempt high allocation with default low */
> 		if (!crash_low_size)
> 			crash_low_size = some default;
> 		crash_max = CRASH_ADDR_LOW_MAX;

crash_max = CRASH_ADDR_HIGH_MAX; We should fallback to high memory now.

> 		crash_base = memblock_phys_alloc_range();
> 	}
> 
> Well, I guess we end up with your earlier proposal but I think I
> understand it better now ;).
> 

-- 
Regards,
  Zhen Lei