Creation of pseudo items leads to (seemingly) duplicate inodes (BUG inside)

[Jan Schmidt <list.btrfs@jan-o-sch.net>]

Hi there,

while playing with snapshots for btrfs send, I also encountered
seemingly duplicate inodes, which are multiple
BTRFS_EMPTY_SUBVOL_DIR_OBJECTID objects within the same directory. I can
imagine software that feels uncomfortable, here.

Such objects are created by btrfs_lookup_dentry() in fs/btrfs/inode.c
when ENOENT is encountered.

The main purpose for this email is to ask: Is this going to stay like that?

David Sterba sent a related issue some days ago with effects seen on
dcache level (subject: "WARNING: at fs/dcache.c:1256
d_set_d_op+0xaa/0xc0() , nested snapshots").

I hit a bug when trying to rename(2) one of those objects, and I'm still
hitting it reproducably on 3.1-rc5. However, I cannot reproduce it with
the current for-linus branch (3.1-rc6). I don't see a commit I'd expect
to fix this, but it may be fixed:

--
Oct 17 13:28:26 oglaroon kernel: [266945.208573] btrfs failed to delete
reference to snap2, inode 257 parent 256
Oct 17 13:28:26 oglaroon kernel: [266945.208586] ------------[ cut here
]------------
Oct 17 13:28:26 oglaroon kernel: [266945.264688] kernel BUG at
fs/btrfs/inode.c:6907!
Oct 17 13:28:26 oglaroon kernel: [266945.320807] invalid opcode: 0000
[#1] SMP
Oct 17 13:28:26 oglaroon kernel: [266945.370907] CPU 2
Oct 17 13:28:26 oglaroon kernel: [266945.393831] Modules linked in:
btrfs mpt2sas scsi_transport_sas raid_class [last unloaded: btrfs]
Oct 17 13:28:26 oglaroon kernel: [266945.503578]
Oct 17 13:28:26 oglaroon kernel: [266945.522353] Pid: 29567, comm: mv
Not tainted 3.1.0-rc4+ #5 Supermicro X8SIL/X8SIL
Oct 17 13:28:26 oglaroon kernel: [266945.613011] RIP:
0010:[<ffffffffa06d906a>]  [<ffffffffa06d906a>] btrfs_rename+0x42a/0x770
[btrfs]
Oct 17 13:28:26 oglaroon kernel: [266945.720059] RSP:
0018:ffff88022f933c78  EFLAGS: 00010282
Oct 17 13:28:26 oglaroon kernel: [266945.784476] RAX: 00000000fffffffe
RBX: 000000003b2456a4 RCX: 0000000000000054
Oct 17 13:28:26 oglaroon kernel: [266945.870675] RDX: 0000000000000053
RSI: 000060fdc00042c0 RDI: ffff880234330000
Oct 17 13:28:26 oglaroon kernel: [266945.956874] RBP: ffff88022f933d48
R08: 0000000000000000 R09: 0000000000000001
Oct 17 13:28:26 oglaroon kernel: [266946.043072] R10: 0000000000000006
R11: 0000000000000000 R12: 000000004e9c1157
Oct 17 13:28:26 oglaroon kernel: [266946.129270] R13: ffff880234468368
R14: 0000000000000000 R15: ffff880234446d58
Oct 17 13:28:26 oglaroon kernel: [266946.215471] FS:
00007f3bf0b96700(0000) GS:ffff88023fc80000(0000) knlGS:0000000000000000
Oct 17 13:28:26 oglaroon kernel: [266946.313080] CS:  0010 DS: 0000 ES:
0000 CR0: 0000000080050033
Oct 17 13:28:26 oglaroon kernel: [266946.382681] CR2: 00007f3bf0072110
CR3: 000000015c05f000 CR4: 00000000000006e0
Oct 17 13:28:26 oglaroon kernel: [266946.468880] DR0: 0000000000000000
DR1: 0000000000000000 DR2: 0000000000000000
Oct 17 13:28:26 oglaroon kernel: [266946.555079] DR3: 0000000000000000
DR6: 00000000ffff0ff0 DR7: 0000000000000400
Oct 17 13:28:26 oglaroon kernel: [266946.641278] Process mv (pid: 29567,
threadinfo ffff88022f932000, task ffff880234330000)
Oct 17 13:28:26 oglaroon kernel: [266946.737849] Stack:
Oct 17 13:28:26 oglaroon kernel: [266946.762849]  000000000000000a
0000000000000006 00000000000000d0 0000000000000000
Oct 17 13:28:26 oglaroon kernel: [266946.852574]  ffff88022f933c01
0000000000000101 00ff8802346b4158 ffff8802349b81b8
Oct 17 13:28:26 oglaroon kernel: [266946.942299]  0000000000000000
ffff88022f9e7000 ffff88022f9e7000 ffff88019d4b6aa8
Oct 17 13:28:26 oglaroon kernel: [266947.032025] Call Trace:
Oct 17 13:28:26 oglaroon kernel: [266947.062215]  [<ffffffff81189ea2>]
vfs_rename+0x162/0x3e0
Oct 17 13:28:26 oglaroon kernel: [266947.126629]  [<ffffffff8118bfb9>]
sys_renameat+0x239/0x270
Oct 17 13:28:26 oglaroon kernel: [266947.193120]  [<ffffffff8187568c>] ?
do_page_fault+0x20c/0x450
Oct 17 13:28:26 oglaroon kernel: [266947.262722]  [<ffffffff818723ca>] ?
retint_swapgs+0xe/0x13
Oct 17 13:28:26 oglaroon kernel: [266947.329214]  [<ffffffff810c84dd>] ?
trace_hardirqs_on_caller+0xfd/0x190
Oct 17 13:28:26 oglaroon kernel: [266947.409185]  [<ffffffff8118c006>]
sys_rename+0x16/0x20
Oct 17 13:28:26 oglaroon kernel: [266947.471527]  [<ffffffff8187983b>]
system_call_fastpath+0x16/0x1b
Oct 17 13:28:26 oglaroon kernel: [266947.544240] Code: 68 ff ff ff 48 8b
4a 30 44 8b 4a 24 4c 8b 42 28 48 8b 55 90 4c 89 95 58 ff ff ff 44 88 9d
50 ff ff ff e8 ba c0 ff ff 85 c0 74 4a <0f> 0b eb fe 48 8b 45 80 48 8b
b8 20 01 00 00 88 95 48 ff ff ff
Oct 17 13:28:26 oglaroon kernel: [266947.777216] RIP
[<ffffffffa06d906a>] btrfs_rename+0x42a/0x770 [btrfs]
Oct 17 13:28:26 oglaroon kernel: [266947.856259]  RSP <ffff88022f933c78>
Oct 17 13:28:26 oglaroon kernel: [266947.899234] ---[ end trace
fd19520e3af48c17 ]---
--

Reproducer for the curious:

# mkfs.btrfs /dev/sdv2
# mount /dev/sdv2 /mnt
# btrfs subvol snap /mnt /mnt/snap1
# btrfs subvol snap /mnt /mnt/snap2
# btrfs subvol snap /mnt /mnt/snap3

When snap2 was created, there was a dir item for snap1, so this is no
surprise:

# ls -lai /mnt/snap2
total 8
256 dr-xr-xr-x 1 root root 20 Jan  1  1970 .
256 dr-xr-xr-x 1 root root 30 Jan  1  1970 ..
  2 drwxr-xr-x 1 root root  0 Oct 18 16:25 snap1

Inode 2 seems a bit strange, but stay tuned. When snap3 was created,
there were dir items for snap1 and snap2, so ... *drumroll*

# ls -lai /mnt/snap3
total 8
256 dr-xr-xr-x 1 root root 30 Jan  1  1970 .
256 dr-xr-xr-x 1 root root 30 Jan  1  1970 ..
  2 drwxr-xr-x 1 root root  0 Oct 18 16:26 snap1
  2 drwxr-xr-x 1 root root  0 Oct 18 16:26 snap2

-Jan