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

Dave Young
2020-08-08T10:02:59Z

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