GPF in wb_congested due to null bdi_writeback

[Nikolay Borisov]

Hello Tejun, 

So while running some fs tests I hit the following GPF. Btw the
warning taint flag was due to a debugging WARN_ON in btrfs 100 or so 
tests ago so is unrelated to this gpf: 

[ 4255.628110] general protection fault: 0000 [#1] SMP PTI
[ 4255.628303] Modules linked in:
[ 4255.628446] CPU: 4 PID: 58 Comm: kswapd0 Tainted: G        W        4.16.0-rc3-nbor #488
[ 4255.628666] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
[ 4255.628928] RIP: 0010:shrink_page_list+0x320/0x1180
[ 4255.629072] RSP: 0018:ffffc90000b2fb38 EFLAGS: 00010287
[ 4255.629220] RAX: 26c74ca226c74ca2 RBX: ffffea000444aea0 RCX: 0000000000000000
[ 4255.629394] RDX: 0000000000000000 RSI: 00000000ffffffff RDI: ffff880136761450
[ 4255.629568] RBP: ffffc90000b2fea0 R08: ffff880136761640 R09: 0000000000000000
[ 4255.629742] R10: 0000000000000000 R11: 0000000000000000 R12: ffffc90000b2fc68
[ 4255.629913] R13: ffffea000444ae80 R14: ffffc90000b2fba8 R15: 0000000000000001
[ 4255.630125] FS:  0000000000000000(0000) GS:ffff88013fd00000(0000) knlGS:0000000000000000
[ 4255.630339] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 4255.630494] CR2: 00007fb16b3955f8 CR3: 0000000135108000 CR4: 00000000000006a0
[ 4255.630667] Call Trace:
[ 4255.630790]  shrink_inactive_list+0x27b/0x800
[ 4255.630951]  shrink_node_memcg+0x3b0/0x7e0
[ 4255.631181]  ? mem_cgroup_iter+0xe3/0x730
[ 4255.631374]  ? mem_cgroup_iter+0xe3/0x730
[ 4255.631509]  ? shrink_node+0xcc/0x350
[ 4255.631651]  shrink_node+0xcc/0x350
[ 4255.631780]  kswapd+0x307/0x910
[ 4255.631913]  kthread+0x103/0x140
[ 4255.632033]  ? mem_cgroup_shrink_node+0x2f0/0x2f0
[ 4255.632201]  ? kthread_create_on_node+0x40/0x40
[ 4255.632348]  ret_from_fork+0x3a/0x50
[ 4255.632499] Code: 85 c0 74 59 49 8b 38 48 c7 c0 60 2f 16 82 48 85 ff 74 18 48 8b 47 28 48 3b 05 75 b6 0f 01 0f 84 42 0a 00 00 48 8b 80 28 01 00 00 <48> 8b 48 58 48 8b 51 20 48 85 d2 0f 84 69 04 00 00 4c 89 04 24 
[ 4255.633055] RIP: shrink_page_list+0x320/0x1180 RSP: ffffc90000b2fb38
[ 4255.633456] ---[ end trace 5c1558c67347a58d ]---
 
shrink_page_list+0x320/0x1180 is:
wb_congested at include/linux/backing-dev.h:170
 (inlined by) inode_congested at include/linux/backing-dev.h:456
 (inlined by) inode_write_congested at include/linux/backing-dev.h:468
 (inlined by) shrink_page_list at mm/vmscan.c:957

So the actual faulting code is in wb_congested's first line: 

struct backing_dev_info *bdi = wb->bdi;                                 

So this means wb_congested is called with a null bdi_writeback. 
This is the first time I've seen it so it's likely new. 
I haven't tried bisecting. FWIW I triggered it with xfstest 
generic/176 running on btrfs. But from the looks the filesystem 
wasn't a play here.