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SubjectRe: GPF in wb_congested due to null bdi_writeback
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On 27.02.2018 18:05, Nikolay Borisov wrote:
> 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.
>

I should read more carefully - it's not due to null wb but rather
having garbage in rax. The actual (annotated) disassembly:
All code
========
0: 85 c0 test %eax,%eax
2: 74 59 je 0x5d
4: 49 8b 38 mov (%r8),%rdi
7: 48 c7 c0 60 2f 16 82 mov $0xffffffff82162f60,%rax
e: 48 85 ff test %rdi,%rdi
11: 74 18 je 0x2b
13: 48 8b 47 28 mov 0x28(%rdi),%rax ; rax = inode->i_sb (in inode_to_bdi )
17: 48 3b 05 75 b6 0f 01 cmp 0x10fb675(%rip),%rax # 0x10fb693
1e: 0f 84 42 0a 00 00 je 0xa66
24: 48 8b 80 28 01 00 00 mov 0x128(%rax),%rax ; rax = sb->s_bdi
2b:* 48 8b 48 58 mov 0x58(%rax),%rcx <-- trapping instruction
2f: 48 8b 51 20 mov 0x20(%rcx),%rdx
33: 48 85 d2 test %rdx,%rdx
36: 0f 84 69 04 00 00 je 0x4a5
3c: 4c 89 04 24 mov %r8,(%rsp)


SO somehow the inode's i_sb is bogus

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