Re: [PATCH -next] crash: Fix riscv64 crash memory reserve dead loop

[Jinjie Ruan <ruanjinjie@huawei.com>]

On 2024/8/7 3:34, Catalin Marinas wrote:
> On Tue, Aug 06, 2024 at 08:10:30PM +0100, Catalin Marinas wrote:
>> On Fri, Aug 02, 2024 at 06:11:01PM +0800, Baoquan He wrote:
>>> On 08/02/24 at 05:01pm, Jinjie Ruan wrote:
>>>> On RISCV64 Qemu machine with 512MB memory, cmdline "crashkernel=500M,high"
>>>> will cause system stall as below:
>>>>
>>>> 	 Zone ranges:
>>>> 	   DMA32    [mem 0x0000000080000000-0x000000009fffffff]
>>>> 	   Normal   empty
>>>> 	 Movable zone start for each node
>>>> 	 Early memory node ranges
>>>> 	   node   0: [mem 0x0000000080000000-0x000000008005ffff]
>>>> 	   node   0: [mem 0x0000000080060000-0x000000009fffffff]
>>>> 	 Initmem setup node 0 [mem 0x0000000080000000-0x000000009fffffff]
>>>> 	(stall here)
>>>>
>>>> commit 5d99cadf1568 ("crash: fix x86_32 crash memory reserve dead loop
>>>> bug") fix this on 32-bit architecture. However, the problem is not
>>>> completely solved. If `CRASH_ADDR_LOW_MAX = CRASH_ADDR_HIGH_MAX` on 64-bit
>>>> architecture, for example, when system memory is equal to
>>>> CRASH_ADDR_LOW_MAX on RISCV64, the following infinite loop will also occur:
>>>
>>> Interesting, I didn't expect risc-v defining them like these.
>>>
>>> #define CRASH_ADDR_LOW_MAX              dma32_phys_limit
>>> #define CRASH_ADDR_HIGH_MAX             memblock_end_of_DRAM()
>>
>> arm64 defines the high limit as PHYS_MASK+1, it doesn't need to be
>> dynamic and x86 does something similar (SZ_64T). Not sure why the
>> generic code and riscv define it like this.
>>
>>>> 	-> reserve_crashkernel_generic() and high is true
>>>> 	   -> alloc at [CRASH_ADDR_LOW_MAX, CRASH_ADDR_HIGH_MAX] fail
>>>> 	      -> alloc at [0, CRASH_ADDR_LOW_MAX] fail and repeatedly
>>>> 	         (because CRASH_ADDR_LOW_MAX = CRASH_ADDR_HIGH_MAX).
>>>>
>>>> Before refactor in commit 9c08a2a139fe ("x86: kdump: use generic interface
>>>> to simplify crashkernel reservation code"), x86 do not try to reserve crash
>>>> memory at low if it fails to alloc above high 4G. However before refator in
>>>> commit fdc268232dbba ("arm64: kdump: use generic interface to simplify
>>>> crashkernel reservation"), arm64 try to reserve crash memory at low if it
>>>> fails above high 4G. For 64-bit systems, this attempt is less beneficial
>>>> than the opposite, remove it to fix this bug and align with native x86
>>>> implementation.
>>>
>>> And I don't like the idea crashkernel=,high failure will fallback to
>>> attempt in low area, so this looks good to me.
>>
>> Well, I kind of liked this behaviour. One can specify ,high as a
>> preference rather than forcing a range. The arm64 land has different
>> platforms with some constrained memory layouts. Such fallback works well
>> as a default command line option shipped with distros without having to
>> guess the SoC memory layout.
> 
> I haven't tried but it's possible that this patch also breaks those
> arm64 platforms with all RAM above 4GB when CRASH_ADDR_LOW_MAX is
> memblock_end_of_DRAM(). Here all memory would be low and in the absence
> of no fallback, it fails to allocate.
> 
> So, my strong preference would be to re-instate the current behaviour
> and work around the infinite loop in a different way.

Hi, baoquan, What's your opinion?

Only this patch should be re-instate or all the 3 dead loop fix patch?

> 
> Thanks.
>