[4.1-rc7] btrfs related VM_BUG_ON in filemap.c

[Dave Jones]

Found this on serial console this morning. The machine had rebooted itself shortly
afterwards (surprising, given I don't have panic-on-oops or similar set).

	Dave

page:ffffea0002b0a040 count:4 mapcount:0 mapping:ffff8800abf76ad0 index:0x0
flags: 0x4000000000000806(error|referenced|private)
page dumped because: VM_BUG_ON_PAGE(!PageLocked(page))
------------[ cut here ]------------
kernel BUG at mm/filemap.c:745!
invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC 
CPU: 1 PID: 32187 Comm: trinity-c3 Not tainted 4.1.0-rc7-gelk-debug+ #4
task: ffff8800b6bd0a50 ti: ffff8800abf50000 task.ti: ffff8800abf50000
RIP: 0010:[<ffffffffb017292c>]  [<ffffffffb017292c>] unlock_page+0x7c/0x80
RSP: 0000:ffff8800abf53a58  EFLAGS: 00010292
RAX: 0000000000000036 RBX: 0000000000001000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffffb00c9e29 RDI: ffffffffb00c9a44
RBP: ffff8800abf53a58 R08: 0000000000000001 R09: 00000000000007f9
R10: 0000000000000478 R11: ffff8800bb20e848 R12: ffffea0002b0a040
R13: 0000000000000000 R14: 0000000000000fff R15: 0000000000000000
FS:  00007f9c1ea80700(0000) GS:ffff8800bf700000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f3ca086f850 CR3: 00000000a8ceb000 CR4: 00000000000007e0
DR0: 00007f1c6d7b0000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
Stack:
 ffff8800abf53b68 ffffffffc015c5ec 0000000000000fff 0000100800000008
 ffff8800abf76778 0000000000000000 ffff8800abf53ab8 0000000000000fff
 0000000000000000 ffff8800ac281000 ffff8800abf53c08 ffff8800abf76958
Call Trace:
 [<ffffffffc015c5ec>] __do_readpage+0x61c/0x7c0 [btrfs]
 [<ffffffffc0159873>] ? lock_extent_bits+0x83/0x2e0 [btrfs]
 [<ffffffffb00a6581>] ? get_parent_ip+0x11/0x50
 [<ffffffffc013fc20>] ? btrfs_real_readdir+0x5e0/0x5e0 [btrfs]
 [<ffffffffc015631a>] ? btrfs_lookup_ordered_extent+0x9a/0xd0 [btrfs]
 [<ffffffffc015c855>] __extent_read_full_page+0xc5/0xe0 [btrfs]
 [<ffffffffc013fc20>] ? btrfs_real_readdir+0x5e0/0x5e0 [btrfs]
 [<ffffffffc015d7b7>] extent_read_full_page+0x37/0x60 [btrfs]
 [<ffffffffc013cba5>] btrfs_readpage+0x25/0x30 [btrfs]
 [<ffffffffc014cdba>] prepare_uptodate_page+0x4a/0x90 [btrfs]
 [<ffffffffc014cf01>] prepare_pages+0x101/0x190 [btrfs]
 [<ffffffffc014da43>] __btrfs_buffered_write+0x1d3/0x650 [btrfs]
 [<ffffffffc0151653>] btrfs_file_write_iter+0x463/0x570 [btrfs]
 [<ffffffffb01d43d1>] __vfs_write+0xb1/0xf0
 [<ffffffffb01d4a59>] vfs_write+0xa9/0x1b0
 [<ffffffffb06d7c0c>] ? mutex_lock+0x2c/0x40
 [<ffffffffb01d5849>] SyS_write+0x49/0xb0
 [<ffffffffb01718f3>] ? context_tracking_user_enter+0x13/0x20
 [<ffffffffb0012865>] ? syscall_trace_leave+0x95/0x140
 [<ffffffffb06d9f17>] system_call_fastpath+0x12/0x6a
Code: 10 48 d3 ee 48 8d 0c b6 48 89 c6 48 8d 3c ca 31 d2 e8 59 ab f4 ff 5d c3 0f 1f 80 00 00 00 00 48 c7 c6 10 f4 a2 b0 e8 14 87 02 00 <0f> 0b 66 90 66 66 66 66 90 55 85 f6 48 89 e5 75 13 85 d2 74 3f 
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