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SubjectWARNING in __mmdrop

syzbot found the following crash on:

HEAD commit: 6d21a41b Add linux-next specific files for 20190718
git tree: linux-next
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:

IMPORTANT: if you fix the bug, please add the following tag to the commit:

WARNING: CPU: 0 PID: 9142 at kernel/fork.c:677 __mmdrop+0x26a/0x320
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 9142 Comm: syz-executor.0 Not tainted 5.2.0-next-20190718 #41
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+0x2dc/0x755 /kernel/panic.c:219
__warn.cold+0x20/0x4c /kernel/panic.c:576
report_bug+0x263/0x2b0 /lib/bug.c:186
fixup_bug /arch/x86/kernel/traps.c:179 [inline]
fixup_bug /arch/x86/kernel/traps.c:174 [inline]
do_error_trap+0x11b/0x200 /arch/x86/kernel/traps.c:272
do_invalid_op+0x37/0x50 /arch/x86/kernel/traps.c:291
invalid_op+0x14/0x20 /arch/x86/entry/entry_64.S:1008
RIP: 0010:__mmdrop+0x26a/0x320 /kernel/fork.c:677
Code: 5f 5d c3 e8 18 7e 2f 00 4c 89 ef e8 60 d7 2b 00 eb d2 e8 09 7e 2f 00
0f 0b e8 02 7e 2f 00 0f 0b e9 fa fd ff ff e8 f6 7d 2f 00 <0f> 0b e9 2b fe
ff ff e8 ea 7d 2f 00 4c 89 e7 e8 a2 f7 67 00 e9 85
RSP: 0018:ffff8880a176fac0 EFLAGS: 00010293
RAX: ffff888098f02600 RBX: ffff888098f02600 RCX: ffffffff81430bab
RDX: 0000000000000000 RSI: ffffffff814306fa RDI: ffff888098f02a30
RBP: ffff8880a176fae8 R08: ffff888098f02600 R09: ffffed10148dd4ec
R10: ffffed10148dd4eb R11: ffff8880a46ea75f R12: ffff8880a46ea700
R13: ffff8880a46ea828 R14: ffff8880a46eac50 R15: 0000000000000000
mmdrop /./include/linux/sched/mm.h:49 [inline]
__mmput /kernel/fork.c:1074 [inline]
mmput+0x3f0/0x4d0 /kernel/fork.c:1085
exit_mm /kernel/exit.c:547 [inline]
do_exit+0x84e/0x2ea0 /kernel/exit.c:864
do_group_exit+0x135/0x360 /kernel/exit.c:981
get_signal+0x47c/0x2500 /kernel/signal.c:2728
do_signal+0x87/0x1700 /arch/x86/kernel/signal.c:815
exit_to_usermode_loop+0x286/0x380 /arch/x86/entry/common.c:159
prepare_exit_to_usermode /arch/x86/entry/common.c:194 [inline]
syscall_return_slowpath /arch/x86/entry/common.c:274 [inline]
do_syscall_64+0x5a9/0x6a0 /arch/x86/entry/common.c:299
RIP: 0033:0x459819
Code: Bad RIP value.
RSP: 002b:00007f75afccbc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: 0000000000000000 RBX: 0000000000000003 RCX: 0000000000459819
RDX: 00000000200023c0 RSI: 000000004028af11 RDI: 0000000000000003
RBP: 000000000075bfc8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f75afccc6d4
R13: 00000000004c4722 R14: 00000000004d87d0 R15: 00000000ffffffff
Kernel Offset: disabled
Rebooting in 86400 seconds..

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