Re: [PATCH v11 5/5] kdump: update Documentation about crashkernel

[Dave Young <dyoung@redhat.com>]

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.

Acked-by: Dave Young <dyoung@redhat.com>

Thanks
Dave