Re: [PATCH v3 2/3] crash: Fix x86_32 crash memory reserve dead loop bug at high

[Jinjie Ruan]

On 2024/7/18 19:14, Baoquan He wrote:
> On 07/18/24 at 11:54am, Jinjie Ruan wrote:
> 
> I don't fully catch the subject, what does the 'dead loop bug at high'
> mean?

It means alloc at [CRASH_ADDR_LOW_MAX, CRASH_ADDR_HIGH_MAX] repeatedly
which corresponds to the crashkernel parameter of the "high".

> 
>> On x86_32 Qemu machine with 1GB memory, the cmdline "crashkernel=512M" will
>> also cause system stall as below:
>>
>> 	ACPI: Reserving FACP table memory at [mem 0x3ffe18b8-0x3ffe192b]
>> 	ACPI: Reserving DSDT table memory at [mem 0x3ffe0040-0x3ffe18b7]
>> 	ACPI: Reserving FACS table memory at [mem 0x3ffe0000-0x3ffe003f]
>> 	ACPI: Reserving APIC table memory at [mem 0x3ffe192c-0x3ffe19bb]
>> 	ACPI: Reserving HPET table memory at [mem 0x3ffe19bc-0x3ffe19f3]
>> 	ACPI: Reserving WAET table memory at [mem 0x3ffe19f4-0x3ffe1a1b]
>> 	143MB HIGHMEM available.
>> 	879MB LOWMEM available.
>> 	  mapped low ram: 0 - 36ffe000
>> 	  low ram: 0 - 36ffe000
>> 	  (stall here)
>>
>> The reason is that the CRASH_ADDR_LOW_MAX is equal to CRASH_ADDR_HIGH_MAX
>> on x86_32, the first "low" crash kernel memory reservation for 512M fails,
>> then it go into the "retry" loop and never came out as below (consider
>> CRASH_ADDR_LOW_MAX = CRASH_ADDR_HIGH_MAX = 512M):
>>
>> -> reserve_crashkernel_generic() and high is false
>>    -> alloc at [0, 0x20000000] fail
>>       -> alloc at [0x20000000, 0x20000000] fail and repeatedly
>>       (because CRASH_ADDR_LOW_MAX = CRASH_ADDR_HIGH_MAX).
>>
>> Fix it by skipping meaningless calls of memblock_phys_alloc_range() with
>> `start = end`
>>
>> After this patch, the retry dead loop is avoided and print below info:
>> 	cannot allocate crashkernel (size:0x20000000)
>>
>> And apply generic crashkernel reservation to 32bit system will be ready.
>>
>> Fixes: 9c08a2a139fe ("x86: kdump: use generic interface to simplify crashkernel reservation code")
>> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
>> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
>> Tested-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
> 
> Also the tag issues, please update.
> 
> Other than above concerns, the patch looks good to me.

Thank you for your review, I'll fix it.

> 
>> ---
>> v3:
>> - Fix it as Baoquan suggested.
>> - Update the commit message.
>> ---
>>  kernel/crash_reserve.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/crash_reserve.c b/kernel/crash_reserve.c
>> index c5213f123e19..dacc268429e2 100644
>> --- a/kernel/crash_reserve.c
>> +++ b/kernel/crash_reserve.c
>> @@ -414,7 +414,8 @@ void __init reserve_crashkernel_generic(char *cmdline,
>>  			search_end = CRASH_ADDR_HIGH_MAX;
>>  			search_base = CRASH_ADDR_LOW_MAX;
>>  			crash_low_size = DEFAULT_CRASH_KERNEL_LOW_SIZE;
>> -			goto retry;
>> +			if (search_base != search_end)
>> +				goto retry;
>>  		}
>>  
>>  		/*
>> -- 
>> 2.34.1
>>
> 
>