linux-next test error: WARNING in __bio_associate_blkg

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    340ae71f9dd4 Add linux-next specific files for 20181221
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=11d95667400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=c190b602a5d2d731
dashboard link: https://syzkaller.appspot.com/bug?extid=a36a3ba92bea3b315c64
compiler:       gcc (GCC) 8.0.1 20180413 (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+a36a3ba92bea3b315c64@xxxxxxxxxxxxxxxxxxxxxxxxx

sshd (6102) used greatest stack depth: 15720 bytes left
IPVS: ftp: loaded support on port[0] = 21
WARNING: CPU: 1 PID: 1520 at include/linux/blk-cgroup.h:518  
blkg_tryget_closest include/linux/blk-cgroup.h:518 [inline]
WARNING: CPU: 1 PID: 1520 at include/linux/blk-cgroup.h:518  
__bio_associate_blkg.isra.36+0x80b/0x8f0 block/bio.c:2010
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 1520 Comm: kworker/u4:3 Not tainted 4.20.0-rc7-next-20181221  
#182
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: writeback wb_workfn (flush-8:0)
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1d3/0x2c6 lib/dump_stack.c:113
 panic+0x2ad/0x632 kernel/panic.c:214
 __warn.cold.8+0x20/0x4f kernel/panic.c:571
 report_bug+0x254/0x2d0 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:176 [inline]
 do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:269
 do_invalid_op+0x36/0x40 arch/x86/kernel/traps.c:288
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:973
RIP: 0010:blkg_tryget_closest include/linux/blk-cgroup.h:518 [inline]
RIP: 0010:__bio_associate_blkg.isra.36+0x80b/0x8f0 block/bio.c:2010
Code: 7b 50 48 89 f8 48 c1 e8 03 42 80 3c 20 00 0f 85 f1 00 00 00 48 8b 43  
50 4c 89 ef e8 bf c6 88 04 e9 a4 f9 ff ff e8 c5 ad 07 fe <0f> 0b e9 5c fa  
ff ff e8 b9 ad 07 fe e8 ab e2 88 fd e9 fe f9 ff ff
RSP: 0018:ffff8881d462d738 EFLAGS: 00010293
RAX: ffff8881d461e380 RBX: 0000000000000000 RCX: ffffffff83779576
RDX: 0000000000000000 RSI: ffffffff83779b1b RDI: 0000000000000005
RBP: ffff8881d462d8b8 R08: ffff8881d461e380 R09: ffffed103b5e5b77
R10: ffffed103b5e5b77 R11: ffff8881daf2dbbb R12: ffff8881cee19340
R13: ffff8881b3354a40 R14: ffff8881bdf4a218 R15: ffff8881cee19340
 bio_clone_blkg_association block/bio.c:2101 [inline]
 __bio_clone_fast+0x412/0x5e0 block/bio.c:612
 bio_clone_fast+0x42/0x110 block/bio.c:633
 bio_split+0x86/0x260 block/bio.c:1819
 blk_bio_segment_split block/blk-merge.c:231 [inline]
 blk_queue_split+0xa1e/0x2080 block/blk-merge.c:262
 blk_mq_make_request+0x2e7/0x1e30 block/blk-mq.c:1917
 generic_make_request+0x9a5/0x1890 block/blk-core.c:1087
 submit_bio+0xba/0x460 block/blk-core.c:1195
 ext4_io_submit+0x197/0x230 fs/ext4/page-io.c:356
 ext4_writepages+0x1c4b/0x4190 fs/ext4/inode.c:2911
 do_writepages+0x9a/0x1a0 mm/page-writeback.c:2335
 __writeback_single_inode+0x20a/0x1660 fs/fs-writeback.c:1316
 writeback_sb_inodes+0x71f/0x1210 fs/fs-writeback.c:1580
 __writeback_inodes_wb+0x1b9/0x340 fs/fs-writeback.c:1649
 wb_writeback+0xa73/0xfc0 fs/fs-writeback.c:1758
 wb_check_old_data_flush fs/fs-writeback.c:1860 [inline]
 wb_do_writeback fs/fs-writeback.c:1913 [inline]
 wb_workfn+0x1008/0x1790 fs/fs-writeback.c:1942
 process_one_work+0xc90/0x1c40 kernel/workqueue.c:2153
 worker_thread+0x17f/0x1390 kernel/workqueue.c:2296
 kthread+0x35a/0x440 kernel/kthread.c:246
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
Kernel Offset: disabled
Rebooting in 86400 seconds..


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