linux-next test error: WARNING in collapse_file

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    a722f75b Add linux-next specific files for 20191022
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=126aea5b600000
kernel config:  https://syzkaller.appspot.com/x/.config?x=32434321999f01e9
dashboard link: https://syzkaller.appspot.com/bug?extid=667740df862911577d63
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+667740df862911577d63@xxxxxxxxxxxxxxxxxxxxxxxxx

------------[ cut here ]------------
WARNING: CPU: 1 PID: 1080 at mm/khugepaged.c:1643  
collapse_file+0x1f9d/0x4170 mm/khugepaged.c:1643
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 1080 Comm: khugepaged Not tainted 5.4.0-rc4-next-20191022 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x172/0x1f0 lib/dump_stack.c:113
 panic+0x2e3/0x75c kernel/panic.c:221
 __warn.cold+0x2f/0x35 kernel/panic.c:582
 report_bug+0x289/0x300 lib/bug.c:195
 fixup_bug arch/x86/kernel/traps.c:174 [inline]
 fixup_bug arch/x86/kernel/traps.c:169 [inline]
 do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:267
 do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:286
 invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1027
RIP: 0010:collapse_file+0x1f9d/0x4170 mm/khugepaged.c:1643
Code: a1 c0 ff 31 c9 ba 01 00 00 00 4c 89 fe 48 8b bd c0 fe ff ff e8 e4 e9  
ec ff e9 b4 fd ff ff 4c 8b bd 88 fe ff ff e8 93 a1 c0 ff <0f> 0b 4c 8b a3  
50 ff ff ff c7 85 80 fe ff ff 00 00 00 00 e9 05 f6
RSP: 0018:ffff8880a818fad0 EFLAGS: 00010293
RAX: ffff8880a7dd8440 RBX: ffff8880a818fc88 RCX: ffffffff81b2b688
RDX: 0000000000000000 RSI: ffffffff81b2b9ad RDI: 0000000000000001
RBP: ffff8880a818fcb0 R08: ffff8880a7dd8440 R09: fffff940004942b9
R10: fffff940004942b8 R11: ffffea00024a15c7 R12: 0000000000000001
R13: ffffea0002383a08 R14: 0000000000000000 R15: ffffea0002338000
 khugepaged_scan_file mm/khugepaged.c:1881 [inline]
 khugepaged_scan_mm_slot mm/khugepaged.c:1979 [inline]
 khugepaged_do_scan mm/khugepaged.c:2063 [inline]
 khugepaged+0x2da9/0x4360 mm/khugepaged.c:2108
 kthread+0x361/0x430 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxxx

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.