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

[Leizhen (ThunderTown)]

On 2022/5/4 6:00, Catalin Marinas wrote:
> 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

Okay, so I'll follow 2) to update the code.

> 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.

OK, I've changed the code before the festival, and I'll test it today.

> 
> Thanks.
> 

-- 
Regards,
  Zhen Lei