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

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

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.

Thanks.

-- 
Catalin