Re: [BUG next-20260105] khugepaged: kernel BUG at ./include/linux/xarray.h:1441

[Calvin Owens <calvin@wbinvd.org>]

On Monday 01/05 at 18:24 +0000, Matthew Wilcox wrote:
> On Mon, Jan 05, 2026 at 09:24:46AM -0800, Calvin Owens wrote:
> > Hello all,
> > 
> > I'm hitting this BUG_ON consistently after boot on next-20260105:
> > 
> >     [   17.103774][   T40] node ffff888102d43918 offset 0 parent ffff888102d42920 shift 0 count 64 values 0 array ffff888107432e38 list ffff888102d43930 ffff888102d43930 marks 0 0 0
> >     [   17.108851][   T40] ------------[ cut here ]------------
> >     [   17.110476][   T40] kernel BUG at ./include/linux/xarray.h:1441!
> >     [   17.112321][   T40] Oops: invalid opcode: 0000 [#1] SMP PTI
> >     [   17.114177][   T40] CPU: 1 UID: 0 PID: 40 Comm: khugepaged Not tainted 6.19.0-rc4-next-20260105-x86-hardened-yugetlb #1 PREEMPTLAZY  a1685c863f468289fb411d1c9b04bc9e336fe015
> >     [   17.118934][   T40] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-20240910_120124-localhost 04/01/2014
> >     [   17.122375][   T40] RIP: 0010:hpage_collapse_scan_file+0x1b05/0x1b30
> 
> Could you run this through ./scripts/decode_stacktrace.sh ?  I'd like
> to see which caller of xas_lock() triggered this.

Something about the instruction decoding is broken with llvm, but it
still pulls the line numbers out:

{0}[calvinow@handel ~/git/linux] ./scripts/decode_stacktrace.sh vmlinux < ~/stack.txt
[   16.027866][   T58] kernel BUG at ./include/linux/xarray.h:1441!
[   16.030181][   T58] Oops: invalid opcode: 0000 [#1] SMP PTI
[   16.032294][   T58] CPU: 3 UID: 0 PID: 58 Comm: khugepaged Not tainted 6.19.0-rc4-next-20260105-x86-hardened-yugetlb #1 PREEMPTLAZY  9ffdba64952937e3f0143c6f15435a9a9e8ec86b
[   16.038092][   T58] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-20240910_120124-localhost 04/01/2014
[   16.042275][   T58] RIP: 0010:hpage_collapse_scan_file (./include/linux/xarray.h:1441 mm/khugepaged.c:2042 mm/khugepaged.c:2389)
[   16.044711][   T58] Code: e8 d5 b2 ff ff 48 83 c4 18 65 ff 0d da c8 8e 01 0f 85 5a fe ff ff e8 6f 1b d6 ff e9 50 fe ff ff 48 85 ff 74 05 e8 80 4b 69 00 <0f> 0b 48 85 ff 74 05 e8 74 4b 69 00 0f 0b 48 85 ff 74 05 e8 68 4b
objdump: error: '/tmp/tmp.oGg2U9eb6N.o': No such file or directory
objdump: error: '/tmp/tmp.oGg2U9eb6N.o': No such file or directory
All code
========

Code starting with the faulting instruction
===========================================
[   16.051928][   T58] RSP: 0018:ffffc900001ffca8 EFLAGS: 00010246
[   16.054183][   T58] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[   16.056731][   T58] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
[   16.058926][   T58] RBP: 0000000000000200 R08: 0000000000000000 R09: 0000000000000000
[   16.061226][   T58] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[   16.063446][   T58] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[   16.065658][   T58] FS:  0000000000000000(0000) GS:ffff8882b4ff0000(0000) knlGS:0000000000000000
[   16.068581][   T58] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   16.070783][   T58] CR2: 000076192552d16c CR3: 00000001398b6004 CR4: 0000000000170eb0
[   16.073466][   T58] Call Trace:
[   16.074591][   T58]  <TASK>
[   16.075560][   T58]  ? __cfi_workingset_update_node (mm/workingset.c:614)
[   16.077556][   T58]  ? khugepaged (mm/khugepaged.c:2473 mm/khugepaged.c:2572 mm/khugepaged.c:2628)
[   16.079077][   T58]  khugepaged (mm/khugepaged.c:2474 mm/khugepaged.c:2572 mm/khugepaged.c:2628)
[   16.080522][   T58]  ? __cfi_khugepaged (mm/khugepaged.c:2621)
[   16.082140][   T58]  kthread (kernel/kthread.c:469)
[   16.083529][   T58]  ? __cfi_kthread (kernel/kthread.c:412)
[   16.085063][   T58]  ret_from_fork (arch/x86/kernel/process.c:164)
[   16.086603][   T58]  ? __cfi_kthread (kernel/kthread.c:412)
[   16.088156][   T58]  ret_from_fork_asm (arch/x86/entry/entry_64.S:259)
[   16.089751][   T58]  </TASK>
[   16.090774][   T58] ---[ end trace 0000000000000000 ]---
[   16.092603][   T58] RIP: 0010:hpage_collapse_scan_file (./include/linux/xarray.h:1441 mm/khugepaged.c:2042 mm/khugepaged.c:2389)
[   16.094787][   T58] Code: e8 d5 b2 ff ff 48 83 c4 18 65 ff 0d da c8 8e 01 0f 85 5a fe ff ff e8 6f 1b d6 ff e9 50 fe ff ff 48 85 ff 74 05 e8 80 4b 69 00 <0f> 0b 48 85 ff 74 05 e8 74 4b 69 00 0f 0b 48 85 ff 74 05 e8 68 4b
objdump: error: '/tmp/tmp.iup3T2wP74.o': No such file or directory
objdump: error: '/tmp/tmp.iup3T2wP74.o': No such file or directory
All code
========

Code starting with the faulting instruction
===========================================
[   16.101366][   T58] RSP: 0018:ffffc900001ffca8 EFLAGS: 00010246
[   16.103417][   T58] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[   16.106095][   T58] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
[   16.108764][   T58] RBP: 0000000000000200 R08: 0000000000000000 R09: 0000000000000000
[   16.111474][   T58] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[   16.114304][   T58] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[   16.117128][   T58] FS:  0000000000000000(0000) GS:ffff8882b4ff0000(0000) knlGS:0000000000000000
[   16.120308][   T58] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   16.122638][   T58] CR2: 000076192552d16c CR3: 00000001398b6004 CR4: 0000000000170eb0

Thanks,
Calvin