Re: [PATCH v12 0/9] support reserving crashkernel above 4G on arm64 kdump

[Bhupesh Sharma <bhsharma@redhat.com>]

Hi Catalin,

On Tue, Oct 6, 2020 at 11:30 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> 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..> rd.lvm.lv=<..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 rd.lvm.lv=rhel_ampere-hr330a-03/root rd.lvm.lv=rhel_ampere-hr330a-03/swap 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.

I understand, but why 256M (as low) for arm64? x86_64 setups usually
have more system memory available as compared to several commercially
available arm64 setups. So is the intent, just to keep the behavior
similar between arm64 and x86_64?

Should we have a CONFIG option / bootarg to help one select the max
'low_size'? Currently the ' low_size' value is calculated as:

    /*
         * two parts from kernel/dma/swiotlb.c:
         * -swiotlb size: user-specified with swiotlb= or default.
         *
         * -swiotlb overflow buffer: now hardcoded to 32k. We round it
         * to 8M for other buffers that may need to stay low too. Also
         * make sure we allocate enough extra low memory so that we
         * don't run out of DMA buffers for 32-bit devices.
         */
        low_size = max(swiotlb_size_or_default() + (8UL << 20), 256UL << 20);

Since many arm64 boards ship with swiotlb=0 (turned off) via kernel
bootargs, the low_size, still ends up being 256M in such cases,
whereas this 256M can be used for some other purposes - so should we
be limiting this to 64M and failing the crash kernel allocation
request (gracefully) otherwise?

> 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
> ZONE_DMA32).
>
> > 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.

Sure, I am running more checks on this series. I will be soon back
with more updates.

Regards,
Bhupesh


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