Re: [PATCH 1/5] mm: compaction: push watermark into compaction_suitable() callers

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Mar 21, 2025 at 02:21:20PM +0800, kernel test robot wrote:
> commit: 6304be90cf5460f33b031e1e19cbe7ffdcbc9f66 ("[PATCH 1/5] mm: compaction: push watermark into compaction_suitable() callers")
> url: https://github.com/intel-lab-lkp/linux/commits/Johannes-Weiner/mm-compaction-push-watermark-into-compaction_suitable-callers/20250314-050839
> base: https://git.kernel.org/cgit/linux/kernel/git/akpm/mm.git mm-everything
> patch link: https://lore.kernel.org/all/20250313210647.1314586-2-hannes@cmpxchg.org/
> patch subject: [PATCH 1/5] mm: compaction: push watermark into compaction_suitable() callers

> test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G

> [   24.321289][   T36] BUG: unable to handle page fault for address: ffff88844000c5f8
> [   24.322631][   T36] #PF: supervisor read access in kernel mode
> [   24.323577][   T36] #PF: error_code(0x0000) - not-present page
> [   24.324482][   T36] PGD 3a01067 P4D 3a01067 PUD 0
> [   24.325301][   T36] Oops: Oops: 0000 [#1] PREEMPT SMP PTI
> [   24.326157][   T36] CPU: 1 UID: 0 PID: 36 Comm: kcompactd0 Not tainted 6.14.0-rc6-00559-g6304be90cf54 #1
> [   24.327631][   T36] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> [ 24.329194][ T36] RIP: 0010:__zone_watermark_ok (mm/page_alloc.c:3256) 
> [ 24.330125][ T36] Code: 84 c0 78 14 4c 8b 97 48 06 00 00 45 31 db 4d 85 d2 4d 0f 4f da 4c 01 de 49 29 f1 41 f7 c0 38 02 00 00 0f 85 92 00 00 00 48 98 <48> 03 54 c7 38 49 39 d1 7e 7e b0 01 85 c9 74 7a 83 f9 0a 7f 73 48
> All code
> ========
>    0:	84 c0                	test   %al,%al
>    2:	78 14                	js     0x18
>    4:	4c 8b 97 48 06 00 00 	mov    0x648(%rdi),%r10
>    b:	45 31 db             	xor    %r11d,%r11d
>    e:	4d 85 d2             	test   %r10,%r10
>   11:	4d 0f 4f da          	cmovg  %r10,%r11
>   15:	4c 01 de             	add    %r11,%rsi
>   18:	49 29 f1             	sub    %rsi,%r9
>   1b:	41 f7 c0 38 02 00 00 	test   $0x238,%r8d
>   22:	0f 85 92 00 00 00    	jne    0xba
>   28:	48 98                	cltq
>   2a:*	48 03 54 c7 38       	add    0x38(%rdi,%rax,8),%rdx		<-- trapping instruction

That would be the zone->lowmem_reserve[highest_zoneidx] deref:

        long int                   lowmem_reserve[4];    /*  0x38  0x20 */

>   2f:	49 39 d1             	cmp    %rdx,%r9
>   32:	7e 7e                	jle    0xb2
>   34:	b0 01                	mov    $0x1,%al
>   36:	85 c9                	test   %ecx,%ecx
>   38:	74 7a                	je     0xb4
>   3a:	83 f9 0a             	cmp    $0xa,%ecx
>   3d:	7f 73                	jg     0xb2
>   3f:	48                   	rex.W
> 
> Code starting with the faulting instruction
> ===========================================
>    0:	48 03 54 c7 38       	add    0x38(%rdi,%rax,8),%rdx
>    5:	49 39 d1             	cmp    %rdx,%r9
>    8:	7e 7e                	jle    0x88
>    a:	b0 01                	mov    $0x1,%al
>    c:	85 c9                	test   %ecx,%ecx
>    e:	74 7a                	je     0x8a
>   10:	83 f9 0a             	cmp    $0xa,%ecx
>   13:	7f 73                	jg     0x88
>   15:	48                   	rex.W
> [   24.333001][   T36] RSP: 0018:ffffc90000137cd0 EFLAGS: 00010246
> [   24.334003][   T36] RAX: 00000000000036a8 RBX: 0000000000000001 RCX: 0000000000000000
> [   24.335270][   T36] RDX: 0000000000000006 RSI: 0000000000000000 RDI: ffff88843fff1080

and %rax and %rdx look like the swapped watermark and zoneidx (36a8 is
14k pages, or 54M, which matches a min watermark on a 16G system).

So this is the bug that Hugh fixed here:

https://lore.kernel.org/all/005ace8b-07fa-01d4-b54b-394a3e029c07@google.com/

It's resolved in the latest version of the patch in -mm.