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

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Apr 29, 2022 at 04:25:37PM +0800, Leizhen (ThunderTown) wrote:
> On 2022/4/29 16:02, Leizhen (ThunderTown) wrote:
> > On 2022/4/29 11:24, Baoquan He wrote:
> >> On 04/28/22 at 05:33pm, Leizhen (ThunderTown) wrote:
> >>> On 2022/4/28 11:52, Baoquan He wrote:
> >>>> On 04/28/22 at 11:40am, Baoquan He wrote:
> >>>>> On 04/27/22 at 05:04pm, Catalin Marinas wrote:
> >>>>>> There will be some difference as the 4G limit doesn't always hold for
> >>>>>> arm64 (though it's true in most cases). Anyway, we can probably simplify
> >>>>>> things a bit while following the documented behaviour:
> >>>>>>
> >>>>>> 	crashkernel=Y		- current behaviour within ZONE_DMA
> >>>>>> 	crashkernel=Y,high	- allocate from above ZONE_DMA
> >>>>>> 	crashkernel=Y,low	- allocate within ZONE_DMA
[...]
> >>>>> Sorry to interrupt. Seems the ,high ,low and fallback are main concerns
> >>>>> about this version. And I have the same concerns about them which comes
> >>>>> from below points:
> >>>>> 1) we may need to take best effort to keep ,high, ,low behaviour
> >>>>> consistent on all ARCHes. Otherwise user/admin may be confused when they
> >>>>> deploy/configure kdump on different machines of different ARCHes in the
> >>>>> same LAB. I think we should try to avoid the confusion.

I guess by all arches you mean just x86 here. Since the code is not
generic, all arches do their own stuff.

> > OK, I plan to remove optimization, fallback and default low size, to follow the
> > suggestion of Catalin first. But there's one minor point of contention.
> > 
> > 1)    Both "crashkernel=X,high" and "crashkernel=X,low" must be present.
> > 2)    Both "crashkernel=X,high" and "crashkernel=X,low" are present.
> >    or
> >       Allow "crashkernel=X,high" to be present alone. Unlike x86, the default low size is zero.
> > 
> > I prefer 2), how about you?

(2) works for me as well. We keep these simple as "expert" options and
allow crashkernel= to fall back to 'high' if not sufficient memory in
ZONE_DMA. That would be a slight change from the current behaviour but,
as Zhen Lei said, with the old tools it's just moving the error around,
the crashkernel wouldn't be available in either case.

> >>>>> 2) Fallback behaviour is important to our distros. The reason is we will
> >>>>> provide default value with crashkernel=xxxM along kernel of distros. In
> >>>>> this case, we hope the reservation will succeed by all means. The ,high
> >>>>> and ,low is an option if customer likes to take with expertise.

OK, that's good feedback.

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)
	crashkernel=Y,high	- allocate from above ZONE_DMA
	crashkernel=Y,low	- allocate within ZONE_DMA

'crashkernel' overrides the high and low while the latter two can be
passed independently.

> >>>>> After going through arm64 memory init code, I got below summary about
> >>>>> arm64_dma_phys_limit which is the first zone's upper limit. I think we
> >>>>> can make use of it to facilitate to simplify code.
> >>>>> ================================================================================
> >>>>>                         DMA                      DMA32                    NORMAL
> >>>>> 1)Raspberry Pi4         0~1G                     3G~4G                    (above 4G)
> >>>>> 2)Normal machine        0~4G                     0                        (above 4G)
> >>>>> 3)Special machine       (above 4G)~MAX
> >>>>> 4)No DMA|DMA32                                                            (above 4G)~MAX
> >>>
> >>> arm64_memblock_init()
> >>> 	reserve_crashkernel()        ---------------   0a30c53573b0 ("arm64: mm: Move reserve_crashkernel() into mem_init()")
> >> We don't need different code for this place of reservation as you are
> >> doing in this patchset, since arm64_dma_phys_limit is initialized as 
> >> below. In fact, in arm64_memblock_init(), we have made memblock ready,
> >> we can initialize arm64_dma_phys_limit as memblock_end_of_DRAM(). And if
> >> memblock_start_of_DRAM() is bigger than 4G, we possibly can call
> >> reserve_crashkernel() here too.
> > 
> > Yes. Maybe all the devices in this environment are 64-bit. One way I
> > know of allowing 32-bit devices to access high memory without SMMU
> > is: Set a fixed value for the upper 32 bits. In this case, the DMA
> > zone should be [phys_start, phys_start + 4G).

We decided that this case doesn't really exists for arm64 platforms (no
need for special ZONE_DMA).

> I just read the message of commit 791ab8b2e3 ("arm64: Ignore any DMA
> offsets in the max_zone_phys() calculation")
> 
>     Currently, the kernel assumes that if RAM starts above 32-bit (or
>     zone_bits), there is still a ZONE_DMA/DMA32 at the bottom of the RAM and
>     such constrained devices have a hardwired DMA offset. In practice, we
>     haven't noticed any such hardware so let's assume that we can expand
>     ZONE_DMA32 to the available memory if no RAM below 4GB. Similarly,
>     ZONE_DMA is expanded to the 4GB limit if no RAM addressable by
>     zone_bits.

I think the above log is slightly confusing. If the DRAM starts above
4G, ZONE_DMA goes to the end of DRAM. If the DRAM starts below 4G but
above the zone_bits for ZONE_DMA as specified in DT/ACPI, it pushes
ZONE_DMA to 4G. I don't remember why we did this last part, maybe in
case we get incorrect firmware tables, otherwise we could have extended
ZONE_DMA to end of DRAM.

Zhen Lei, if we agreed on the crashkernel behaviour, could you please
post a series that does the above parsing allocation? Ignore the
optimisations, we can look at them afterwards.

Thanks.

-- 
Catalin