Re: [PATCH v22 5/9] arm64: kdump: Reimplement crashkernel=X

Catalin Marinas
2022-05-05T14:20:37Z

On Thu, May 05, 2022 at 11:00:19AM +0800, Baoquan He wrote:
> On 05/03/22 at 11:00pm, Catalin Marinas wrote:
> > So, to recap, IIUC you are fine with:
> >
> > crashkernel=Y - allocate within ZONE_DMA with fallback
> > above with a default in ZONE_DMA (like
> > x86, 256M or swiotlb size)
>
> Ack to this one.
>
>
> > crashkernel=Y,high - allocate from above ZONE_DMA
>
> Not exactly. If there's only ZONE_DMA, crashkernel,high will
> be reserved in ZONE_DMA, and crashkernel,low will be ignored.
> Other than this, ack.

Yes, that's fine.

> > crashkernel=Y,low - allocate within ZONE_DMA
>
> Ack to this one.
> >
> > 'crashkernel' overrides the high and low while the latter two can be
> > passed independently.
>
> crashkernel=,high can be passed independently, then a crashkernel=,low
> is needed implicitly. If people don't want crashkernel=,low
> explicitly, crashkernel=0,low need be specified.

I find this complicating the interface. I don't know the background to
the x86 implementation but we diverge already on arm64 since we talk
about ZONE_DMA rather than 4G limit (though for most platforms these
would be the same).

I guess we could restate the difference between crashkernel= and
crashkernel=,high as the hint to go for allocation above ZONE_DMA first.

> An independent crashkernel=,low makes no sense. Crashkernel=,low
> should be paird with crashkernel=,high.

You could argue that crashkernel=,low gives the current crashkernel=
behaviour, i.e. either all within ZONE_DMA or fail to allocate. So it
may have some value on its own.

> My personal opinion according to the existed senmantics on x86.
> Otherwise, the guidance of crashkernel= |,high|,low reservation
> will be complicated to write.

It's more that I find the current semantics unnecessarily confusing. But
even reading the x86_64 text it's not that clear. For example the
default low allocation for crashkernel= and crashkernel=,high is only
mentioned in the crashkernel=,low description.

--
Catalin