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SubjectGPF in wb_congested due to null bdi_writeback
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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.


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