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

[Baoquan He <bhe@redhat.com>]

On 08/08/24 at 03:56pm, Jinjie Ruan wrote:
> 
> 
> 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?

I am not sure which way Catalin suggested to take. 

Hi Catalin,

Could you say more words about your preference so that Jinjie can
proceed accordingly?

Thanks
Baoquan


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