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Subject: Re: [PATCH v11 5/5] kdump: update Documentation about crashkernel
To:     Dave Young <dyoung@redhat.com>
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From:   chenzhou <chenzhou10@huawei.com>
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Date:   Mon, 10 Aug 2020 11:28:22 +0800
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On 2020/8/8 18:02, Dave Young wrote:
> On 08/01/20 at 09:08pm, Chen Zhou wrote:
>> Now the behavior of crashkernel=X has been changed, which 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 well.
>> So add a threshold and go for high allocation directly if the required
>> size is too large. The threshold is set as the half of 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. For non-RPi4 platforms, change ZONE_DMA
>> memtioned above to ZONE_DMA32.
>>
>> So update the Documentation.
>>
>> Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
>> ---
>>  Documentation/admin-guide/kdump/kdump.rst     | 21 ++++++++++++++++---
>>  .../admin-guide/kernel-parameters.txt         | 11 ++++++++--
>>  2 files changed, 27 insertions(+), 5 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
>> index 2da65fef2a1c..4b58f97351d5 100644
>> --- a/Documentation/admin-guide/kdump/kdump.rst
>> +++ b/Documentation/admin-guide/kdump/kdump.rst
>> @@ -299,7 +299,15 @@ Boot into System Kernel
>>     "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
>>     starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
>>  
>> -   On x86 and x86_64, use "crashkernel=64M@16M".
>> +   On x86 use "crashkernel=64M@16M".
>> +
>> +   On x86_64, use "crashkernel=X" to select a region under 4G first, and
>> +   fall back to reserve region above 4G.
>> +   We can also use "crashkernel=X,high" to select a region above 4G, which
>> +   also tries to allocate at least 256M below 4G automatically and
>> +   "crashkernel=Y,low" can be used to allocate specified size low memory.
>> +   Use "crashkernel=Y@X" if you really have to reserve memory from specified
>> +   start address X.
>>  
>>     On ppc64, use "crashkernel=128M@32M".
>>  
>> @@ -316,8 +324,15 @@ Boot into System Kernel
>>     kernel will automatically locate the crash kernel image within the
>>     first 512MB of RAM if X is not given.
>>  
>> -   On arm64, use "crashkernel=Y[@X]".  Note that the start address of
>> -   the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).
>> +   On arm64, use "crashkernel=X" to try low allocation in ZONE_DMA, and
>> +   fall back to high allocation if it fails. And go for high allocation
>> +   directly if the required size is too large. 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.
>> +   For non-RPi4 platforms, change ZONE_DMA memtioned above to ZONE_DMA32.
>> +   Use "crashkernel=Y@X" if you really have to reserve memory from
>> +   specified start address X. Note that the start address of the kernel,
>> +   X if explicitly specified, must be aligned to 2MiB (0x200000).
>>  
>>  Load the Dump-capture Kernel
>>  ============================
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index fb95fad81c79..d1b6016850d6 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -722,6 +722,10 @@
>>  			[KNL, x86_64] select a region under 4G first, and
>>  			fall back to reserve region above 4G when '@offset'
>>  			hasn't been specified.
>> +			[KNL, arm64] Try low allocation in ZONE_DMA, fall back
>> +			to high allocation if it fails when '@offset' hasn't been
>> +			specified. For non-RPi4 platforms, change ZONE_DMA to
>> +			ZONE_DMA32.
>>  			See Documentation/admin-guide/kdump/kdump.rst for further details.
>>  
>>  	crashkernel=range1:size1[,range2:size2,...][@offset]
>> @@ -746,13 +750,16 @@
>>  			requires at least 64M+32K low memory, also enough extra
>>  			low memory is needed to make sure DMA buffers for 32-bit
>>  			devices won't run out. Kernel would try to allocate at
>> -			at least 256M below 4G automatically.
>> +			least 256M below 4G automatically.
>>  			This one let user to specify own low range under 4G
>>  			for second kernel instead.
>>  			0: to disable low allocation.
>>  			It will be ignored when crashkernel=X,high is not used
>>  			or memory reserved is below 4G.
>> -
>> +			[KNL, arm64] range under 4G.
>> +			This one let user to specify a low range in ZONE_DMA for
>> +			crash dump kernel. For non-RPi4 platforms, change ZONE_DMA
>> +			to ZONE_DMA32.
>>  	cryptomgr.notests
>>  			[KNL] Disable crypto self-tests
>>  
>> -- 
>> 2.20.1
>>
> Hi Chen,
>
> Previously I remember we talked about to use similar logic as X86, but I
> remember you mentioned on some arm64 platform there could be no low
> memory at all.  Is this not a problem now for the fallback?  Just be
> curious, thanks for the update, for the common part looks good.
Hi Dave,

Did you mean this discuss: https://lkml.org/lkml/2019/12/27/122?
This is about the different implementation instead of no low memory in arm64.

On arm64 platform, if there is no low memory, system will boot fail.

Thanks,
Chen Zhou
>
> Acked-by: Dave Young <dyoung@redhat.com>
>
> Thanks
> Dave
>
>
> .
>