Re: [syzbot] [mm?] kernel BUG in collapse_scan_file

[Lance Yang <lance.yang@linux.dev>]

Ccing Willy

IIUC, this is a dup of the earlier report[1], which I looked into back
in January. The root cause is the same: collapse_file() calls
xas_lock_irq() without resetting the xas state first, tripping the
XAS_INVALID() assertion:

#define xas_lock_irq(xas)	xa_lock_irq(XAS_INVALID(xas)->xa)

static inline struct xa_state *XAS_INVALID(struct xa_state *xas)
{
	XA_NODE_BUG_ON(xas->xa_node, xas_valid(xas));
	return xas;
}

Added by commit:

commit 43b00759f21b10142094d1ae5ff65cbb368953a3
Author: Matthew Wilcox (Oracle) <willy@infradead.org>
Date:   Sun Dec 14 10:53:31 2025 -0500

     XArray: Add extra debugging check to xas_lock and friends

     While tracking down a recent bug, we discovered somewhere that had
     forgotten to call xas_reset() before calling xas_lock().  Add a debug
     check to be sure that doesn't happen in future and fix all the 
places in
     the test suite which were carelessly doing just this.

     Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
     Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

I posted a HACK fix at the time[2], but David pointed out that Willy
had mentioned it likely needs more thought[3].

[1] 
https://lore.kernel.org/all/69757ea0.a00a0220.33ccc7.0017.GAE@google.com/
[2] https://lore.kernel.org/all/20260125121001.32733-1-lance.yang@linux.dev/
[3] 
https://lore.kernel.org/all/7bce9231-714c-424a-a4e3-dd42734fb767@kernel.org/


Thanks,
Lance


On 2026/3/19 15:20, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    95c541ddfb08 Add linux-next specific files for 20260316
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=15ccc216580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=ed431987028345c6
> dashboard link: https://syzkaller.appspot.com/bug?extid=8961cb270ae74b4129fb
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12f778da580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12cc006a580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/c40f27ad73d8/disk-95c541dd.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/bd811888f684/vmlinux-95c541dd.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/3b72363d7dbd/bzImage-95c541dd.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+8961cb270ae74b4129fb@syzkaller.appspotmail.com
> 
> node ffff88805d558b00 offset 0 parent ffff88805d558840 shift 0 count 3 values 0 array ffff88807a8195c0 list ffff88805d558b18 ffff88805d558b18 marks 0 0 0
> ------------[ cut here ]------------
> kernel BUG at ./include/linux/xarray.h:1441!
> Oops: invalid opcode: 0000 [#1] SMP KASAN PTI
> CPU: 0 UID: 0 PID: 6001 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
> RIP: 0010:XAS_INVALID include/linux/xarray.h:1441 [inline]
> RIP: 0010:collapse_file mm/khugepaged.c:2055 [inline]
> RIP: 0010:collapse_scan_file+0x4f98/0x5230 mm/khugepaged.c:2404
> Code: ff 4c 89 e7 48 c7 c6 60 b2 dc 8b e8 82 62 f1 fe 90 0f 0b 48 85 db 0f 84 03 01 00 00 e8 71 e5 8f ff 48 89 df e8 a9 20 7b 09 90 <0f> 0b e8 61 e5 8f ff 48 89 df 48 c7 c6 60 b2 dc 8b e8 52 62 f1 fe
> RSP: 0018:ffffc90003826e20 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff88805d558b00 RCX: a13f20bd39c5a100
> RDX: 0000000000000000 RSI: 0000000080000000 RDI: 0000000000000000
> RBP: ffffc90003827130 R08: ffffc90003826ba7 R09: 1ffff92000704d74
> R10: dffffc0000000000 R11: fffff52000704d75 R12: ffffea0001b678f0
> R13: dffffc0000000000 R14: 0000000000000000 R15: ffffc90003827010
> FS:  000055557e3c2500(0000) GS:ffff888125437000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000020000000b000 CR3: 000000007ac66000 CR4: 00000000003526f0
> Call Trace:
>   <TASK>
>   collapse_single_pmd+0x22b/0x4510 mm/khugepaged.c:2437
>   madvise_collapse+0x34c/0x820 mm/khugepaged.c:2859
>   madvise_vma_behavior+0x1094/0x4460 mm/madvise.c:1362
>   madvise_walk_vmas+0x573/0xae0 mm/madvise.c:1711
>   madvise_do_behavior+0x386/0x540 mm/madvise.c:1927
>   do_madvise+0x1fa/0x2e0 mm/madvise.c:2020
>   __do_sys_madvise mm/madvise.c:2029 [inline]
>   __se_sys_madvise mm/madvise.c:2027 [inline]
>   __x64_sys_madvise+0xa6/0xc0 mm/madvise.c:2027
>   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>   do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f90d419c799
> Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffd50711398 EFLAGS: 00000246 ORIG_RAX: 000000000000001c
> RAX: ffffffffffffffda RBX: 00007f90d4415fa0 RCX: 00007f90d419c799
> RDX: 0000000000000019 RSI: 0000000000600003 RDI: 0000200000000000
> RBP: 00007f90d4232c99 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f90d4415fac R14: 00007f90d4415fa0 R15: 00007f90d4415fa0
>   </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:XAS_INVALID include/linux/xarray.h:1441 [inline]
> RIP: 0010:collapse_file mm/khugepaged.c:2055 [inline]
> RIP: 0010:collapse_scan_file+0x4f98/0x5230 mm/khugepaged.c:2404
> Code: ff 4c 89 e7 48 c7 c6 60 b2 dc 8b e8 82 62 f1 fe 90 0f 0b 48 85 db 0f 84 03 01 00 00 e8 71 e5 8f ff 48 89 df e8 a9 20 7b 09 90 <0f> 0b e8 61 e5 8f ff 48 89 df 48 c7 c6 60 b2 dc 8b e8 52 62 f1 fe
> RSP: 0018:ffffc90003826e20 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff88805d558b00 RCX: a13f20bd39c5a100
> RDX: 0000000000000000 RSI: 0000000080000000 RDI: 0000000000000000
> RBP: ffffc90003827130 R08: ffffc90003826ba7 R09: 1ffff92000704d74
> R10: dffffc0000000000 R11: fffff52000704d75 R12: ffffea0001b678f0
> R13: dffffc0000000000 R14: 0000000000000000 R15: ffffc90003827010
> FS:  000055557e3c2500(0000) GS:ffff888125537000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f8156602000 CR3: 000000007ac66000 CR4: 00000000003526f0
> 
> 
> ---
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