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

[Catalin Marinas]

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.

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):

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;

-- 
Catalin