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Subjectworkqueue threads ->journal_info buggery
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Hello Tejun, 

I've hit the following problems under memory-heavy workload conditions:

First is a BUG_ON : J_ASSERT(journal_current_handle() == handle);

[ 64.261793] kernel BUG at fs/jbd2/transaction.c:1644!
[ 64.263894] invalid opcode: 0000 [#1] SMP
[ 64.266187] Modules linked in:
[ 64.267472] CPU: 1 PID: 542 Comm: kworker/u12:6 Not tainted 4.12.0-nbor #135
[ 64.269941] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
[ 64.272374] Workqueue: writeback wb_workfn (flush-254:0)
[ 64.273862] task: ffff88001c37b880 task.stack: ffff880018ac8000
[ 64.275580] RIP: 0010:jbd2_journal_stop+0x375/0x4d0
[ 64.276704] RSP: 0000:ffff880018acb990 EFLAGS: 00010286
[ 64.278708] RAX: ffff88001c37b880 RBX: ffff88001e83c000 RCX: ffff88001c4f8800
[ 64.280499] RDX: ffff88001e83c000 RSI: 0000000000000b26 RDI: ffff88001e83c000
[ 64.282262] RBP: ffff880018acba10 R08: ffff880019ec5888 R09: 0000000000000000
[ 64.284111] R10: 0000000000000000 R11: ffffffff81283f8f R12: ffff880018a1a140
[ 64.285553] R13: ffff88001c4f8800 R14: ffff88001c47d000 R15: ffff880018aa01f0
[ 64.286337] FS: 0000000000000000(0000) GS:ffff88001fc40000(0000) knlGS:0000000000000000
[ 64.287671] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 64.288568] CR2: 0000000000421ac0 CR3: 000000001ae83000 CR4: 00000000000006a0
[ 64.289468] Call Trace:
[ 64.289748] ? __ext4_journal_get_write_access+0x67/0xc0
[ 64.290330] ? ext4_writepages+0xec6/0x1200
[ 64.290786] __ext4_journal_stop+0x3c/0xa0
[ 64.291233] ext4_writepages+0x8b2/0x1200
[ 64.291682] ? writeback_sb_inodes+0x11f/0x5c0
[ 64.292174] do_writepages+0x1c/0x80
[ 64.292572] ? do_writepages+0x1c/0x80
[ 64.292985] __writeback_single_inode+0x61/0x760
[ 64.293575] writeback_sb_inodes+0x28d/0x5c0
[ 64.294192] __writeback_inodes_wb+0x92/0xc0
[ 64.294777] wb_writeback+0x3e9/0x560
[ 64.295241] wb_workfn+0x9a/0x5d0
[ 64.295977] ? wb_workfn+0x9a/0x5d0
[ 64.296788] ? process_one_work+0x15c/0x620
[ 64.297971] process_one_work+0x1d9/0x620
[ 64.298969] worker_thread+0x4e/0x3b0
[ 64.299684] kthread+0x113/0x150
[ 64.300287] ? process_one_work+0x620/0x620
[ 64.301145] ? kthread_create_on_node+0x40/0x40
[ 64.301953] ret_from_fork+0x2a/0x40
[ 64.302572] Code: dd ff 41 8b 45 60 85 c0 0f 84 29 fe ff ff 49 8d bd 00 01 00 00 31 c9 ba 01 00 00 00 be 03 00 00 00 e8 90 c1 dd ff e9 0c fe ff ff <0f> 0b 44 89 fe 4c 89 ef e8 ce 83 00 00 89 45 c4 e9 18 fe ff ff
[ 64.305997] RIP: jbd2_journal_stop+0x375/0x4d0 RSP: ffff880018acb990
[ 64.307037] ---[ end trace ec3f7cbd6e733faf ]---

I consulted with Jan his opinion is that this is due to ->journal_info
in workqueue threads gets modified while the work was running.

I've also hit this lockdep, Jan also said it's due to ->journal_info being modified, so
we have to start a new handle, thus causing the splat.

[ 64.153143] ============================================
[ 64.154787] WARNING: possible recursive locking detected
[ 64.156540] 4.12.0-nbor #135 Not tainted
[ 64.157704] --------------------------------------------
[ 64.159787] kworker/u12:6/542 is trying to acquire lock:
[ 64.160964] (jbd2_handle){++++-.}, at: [<ffffffff812bddf4>] start_this_handle+0x104/0x440
[ 64.163360]
[ 64.163360] but task is already holding lock:
[ 64.165240] (jbd2_handle){++++-.}, at: [<ffffffff812bddf4>] start_this_handle+0x104/0x440
[ 64.168034]
[ 64.168034] other info that might help us debug this:
[ 64.169969] Possible unsafe locking scenario:
[ 64.169969]
[ 64.172198] CPU0
[ 64.173047] ----
[ 64.173768] lock(jbd2_handle);
[ 64.174554] lock(jbd2_handle);
[ 64.175255]
[ 64.175255] *** DEADLOCK ***
[ 64.175255]
[ 64.176860] May be due to missing lock nesting notation
[ 64.176860]
[ 64.177932] 6 locks held by kworker/u12:6/542:
[ 64.179133] #0: ("writeback"){.+.+.+}, at: [<ffffffff810709ac>] process_one_work+0x15c/0x620
[ 64.181395] #1: ((&(&wb->dwork)->work)){+.+.+.}, at: [<ffffffff810709ac>] process_one_work+0x15c/0x620
[ 64.184053] #2: (&type->s_umount_key#27){++++.+}, at: [<ffffffff811d0d9b>] trylock_super+0x1b/0x50
[ 64.186218] #3: (&sbi->s_journal_flag_rwsem){.+.+.+}, at: [<ffffffff8115c41c>] do_writepages+0x1c/0x80
[ 64.188299] #4: (jbd2_handle){++++-.}, at: [<ffffffff812bddf4>] start_this_handle+0x104/0x440
[ 64.191933] #5: (&ei->i_data_sem){++++..}, at: [<ffffffff81281220>] ext4_map_blocks+0x130/0x5c0
[ 64.193975]
[ 64.193975] stack backtrace:
[ 64.194935] CPU: 1 PID: 542 Comm: kworker/u12:6 Not tainted 4.12.0-nbor #135
[ 64.196599] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
[ 64.198790] Workqueue: writeback wb_workfn (flush-254:0)
[ 64.200148] Call Trace:
[ 64.201020] dump_stack+0x85/0xc7
[ 64.202497] __lock_acquire+0x14b3/0x1790
[ 64.203815] lock_acquire+0xac/0x1e0
[ 64.204999] ? start_this_handle+0x134/0x440
[ 64.206552] ? lock_acquire+0xac/0x1e0
[ 64.208104] ? start_this_handle+0x104/0x440
[ 64.210158] start_this_handle+0x167/0x440
[ 64.212662] ? start_this_handle+0x104/0x440
[ 64.214370] ? rcu_read_lock_sched_held+0x72/0x80
[ 64.216280] ? kmem_cache_alloc+0x234/0x2a0
[ 64.217290] jbd2__journal_start+0xdb/0x2b0
[ 64.219336] ? ext4_dirty_inode+0x32/0x70
[ 64.220944] __ext4_journal_start_sb+0x89/0x1f0
[ 64.222357] ext4_dirty_inode+0x32/0x70
[ 64.224541] __mark_inode_dirty+0x79/0x690
[ 64.226472] ext4_da_update_reserve_space+0x1fe/0x270
[ 64.227906] ext4_ext_map_blocks+0xe62/0x17b0
[ 64.229897] ext4_map_blocks+0x154/0x5c0
[ 64.231572] ext4_writepages+0xac8/0x1200
[ 64.233013] ? writeback_sb_inodes+0x11f/0x5c0
[ 64.234501] do_writepages+0x1c/0x80
[ 64.236214] ? do_writepages+0x1c/0x80
[ 64.238178] __writeback_single_inode+0x61/0x760
[ 64.240088] writeback_sb_inodes+0x28d/0x5c0
[ 64.241429] __writeback_inodes_wb+0x92/0xc0
[ 64.243159] wb_writeback+0x3e9/0x560
[ 64.244948] wb_workfn+0x9a/0x5d0
[ 64.245627] ? wb_workfn+0x9a/0x5d0
[ 64.246341] ? process_one_work+0x15c/0x620
[ 64.248009] process_one_work+0x1d9/0x620
[ 64.248973] worker_thread+0x4e/0x3b0
[ 64.250521] kthread+0x113/0x150
[ 64.251950] ? process_one_work+0x620/0x620
[ 64.253650] ? kthread_create_on_node+0x40/0x40
[ 64.255605] ret_from_fork+0x2a/0x40
[ 64.257513] ------------[ cut here ]------------

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