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SubjectRe: [PATCH v12 0/9] support reserving crashkernel above 4G on arm64 kdump
On Mon, Oct 05, 2020 at 11:12:10PM +0530, Bhupesh Sharma wrote:
> I think my earlier email with the test results on this series bounced
> off the mailing list server (for some weird reason), but I still see
> several issues with this patchset. I will add specific issues in the
> review comments for each patch again, but overall, with a crashkernel
> size of say 786M, I see the following issue:
> # cat /proc/cmdline
> BOOT_IMAGE=(hd7,gpt2)/vmlinuz-5.9.0-rc7+ root=<..snip..><..snip..> crashkernel=786M
> I see two regions of size 786M and 256M reserved in low and high
> regions respectively, So we reserve a total of 1042M of memory, which
> is an incorrect behaviour:
> # dmesg | grep -i crash
> [ 0.000000] Reserving 256MB of low memory at 2816MB for crashkernel (System low RAM: 768MB)
> [ 0.000000] Reserving 786MB of memory at 654158MB for crashkernel (System RAM: 130816MB)
> [ 0.000000] Kernel command line: BOOT_IMAGE=(hd2,gpt2)/vmlinuz-5.9.0-rc7+ root=/dev/mapper/rhel_ampere--hr330a--03-root ro crashkernel=786M cma=1024M
> # cat /proc/iomem | grep -i crash
> b0000000-bfffffff : Crash kernel (low)
> bfcbe00000-bffcffffff : Crash kernel

As Chen said, that's the intended behaviour and how x86 works. The
requested 768M goes in the high range if there's not enough low memory
and an additional buffer for swiotlb is allocated, hence the low 256M.

We could (as an additional patch), subtract the 256M from the high
allocation so that you'd get a low 256M and a high 512M, not sure it's
worth it. Note that with a "crashkernel=768M,high" option, you still get
the additional low 256M, otherwise the crashkernel won't be able to
boot as there's no memory in ZONE_DMA. In the explicit ",high" request
case, I'm not sure subtracted the 256M is more intuitive.

In 5.11, we also hope to fix the ZONE_DMA layout for non-RPi4 platforms
to cover the entire 32-bit address space (i.e. identical to the current

> IMO, we should test this feature more before including this in 5.11

Definitely. That's one of the reasons we haven't queued it yet. So any
help with testing here is appreciated.



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