Re: [PATCH v12 9/9] kdump: update Documentation about crashkernel

Catalin Marinas
2020-10-05T17:19:45Z

On Mon, Sep 07, 2020 at 09:47:45PM +0800, Chen Zhou wrote:
> diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
> index 2da65fef2a1c..549611abc581 100644
> --- a/Documentation/admin-guide/kdump/kdump.rst
> +++ b/Documentation/admin-guide/kdump/kdump.rst
[...]
> @@ -316,8 +325,18 @@ 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 DMA zone, and
> + fall back to high allocation if it fails. And go for high allocation
> + directly if the required size is too large.
> + We can also use "crashkernel=X,high" to select a high region above
> + DMA zone, which also tries to allocate at least 256M low memory in
> + DMA zone automatically.
> + "crashkernel=Y,low" can be used to allocate specified size low memory
> + in DMA zone.
> + For non-RPi4 platforms, change DMA zone memtioned above to DMA32 zone.

I don't think we should mention non-RPi4 explicitly here. I don't even
understand what the suggestion is since the only way is to disable
ZONE_DMA in the kernel config. I'd just stick to ZONE_DMA description
here.

> + 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 a1068742a6df..f7df572d8f64 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -727,6 +727,10 @@
> [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 DMA zone, fall back
> + to high allocation if it fails when '@offset' hasn't been
> + specified. For non-RPi4 platforms, change DMA zone to
> + DMA32 zone.

Same here, unclear what "change DMA zone to DMA32 zone" means.

> See Documentation/admin-guide/kdump/kdump.rst for further details.
>
> crashkernel=range1:size1[,range2:size2,...][@offset]
> @@ -743,6 +747,8 @@
> Otherwise memory region will be allocated below 4G, if
> available.
> It will be ignored if crashkernel=X is specified.
> + [KNL, arm64] range in high memory.
> + Allow kernel to allocate physical memory region from top.
> crashkernel=size[KMG],low
> [KNL, X86-64] range under 4G. When crashkernel=X,high
> is passed, kernel could allocate physical memory region
> @@ -751,13 +757,16 @@
> 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 in low memory.
> + This one let user to specify a low range in DMA zone for
> + crash dump kernel. For non-RPi4 platforms, change DMA zone
> + to DMA32 zone.

And again here.

--
Catalin