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

[Jinjie Ruan]

On 2024/8/7 3:10, Catalin Marinas wrote:
> To Jinjie, if you make generic changes that affect other architectures,
> please either cc the individual lists/maintainers or at least cross-post
> to linux-arch. I don't follow lkml, there's just too much traffic there.

Sorry, I forgot to Cc to the other architectures.

> 
> 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.
> 
> Something like below should fix the issue as well (untested):

I tested it on QEMU and it is good to fix this dead loop problem.

> 
> diff --git a/kernel/crash_reserve.c b/kernel/crash_reserve.c
> index d3b4cd12bdd1..ae92d6745ef4 100644
> --- a/kernel/crash_reserve.c
> +++ b/kernel/crash_reserve.c
> @@ -420,7 +420,8 @@ void __init reserve_crashkernel_generic(char *cmdline,
>  		 * For crashkernel=size[KMG],high, if the first attempt was
>  		 * for high memory, fall back to low memory.
>  		 */
> -		if (high && search_end == CRASH_ADDR_HIGH_MAX) {
> +		if (high && search_end == CRASH_ADDR_HIGH_MAX &&
> +		    CRASH_ADDR_LOW_MAX < CRASH_ADDR_HIGH_MAX) {
>  			search_end = CRASH_ADDR_LOW_MAX;
>  			search_base = 0;
>  			goto retry;
>