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

[Dave Jones <davej@codemonkey.org.uk>]

On Wed, Jun 17, 2015 at 09:35:41AM -0400, Dave Jones wrote:

 > page:ffffea00027cc640 count:4 mapcount:0 mapping:ffff8800af11d8a0 index:0x0
 > flags: 0x4000000000000846(error|referenced|active|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: 5931 Comm: trinity-c5 Not tainted 4.1.0-rc8-gelk-debug+ #2
 > task: ffff8800b9ec0000 ti: ffff8800843ec000 task.ti: ffff8800843ec000
 > RIP: 0010:[<ffffffffb216ee5c>]  [<ffffffffb216ee5c>] unlock_page+0x7c/0x80
 > RSP: 0018:ffff8800843efa58  EFLAGS: 00010292
 > RAX: 0000000000000036 RBX: 0000000000001000 RCX: 0000000000000000
 > RDX: 0000000080000000 RSI: ffffffffb20c80c9 RDI: ffffffffb20c7ce4
 > RBP: ffff8800843efa58 R08: 0000000000000001 R09: 0000000000000d1d
 > R10: 000000000000037c R11: 0000000000000001 R12: ffffea00027cc640
 > R13: 0000000000000000 R14: 0000000000000fff R15: 0000000000000000
 > FS:  00007fc9c42b5700(0000) GS:ffff8800bf700000(0000) knlGS:0000000000000000
 > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 > CR2: 0000000000000008 CR3: 0000000050978000 CR4: 00000000000007e0
 > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 > DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
 > Stack:
 >  ffff8800843efb68 ffffffffc02d06ec 0000000000000fff 0000100800000008
 >  ffff8800af11d548 0000000000000000 ffff8800843efab8 0000000000000fff
 >  0000000000000000 ffff88009f319000 ffff8800843efc08 ffff8800af11d728
 > Call Trace:
 >  [<ffffffffc02d06ec>] __do_readpage+0x61c/0x7c0 [btrfs]
 >  [<ffffffffc02cd973>] ? lock_extent_bits+0x83/0x2e0 [btrfs]
 >  [<ffffffffb20a5001>] ? get_parent_ip+0x11/0x50
 >  [<ffffffffc02b3ca0>] ? btrfs_real_readdir+0x5e0/0x5e0 [btrfs]
 >  [<ffffffffc02ca41a>] ? btrfs_lookup_ordered_extent+0x9a/0xd0 [btrfs]
 >  [<ffffffffc02d0955>] __extent_read_full_page+0xc5/0xe0 [btrfs]
 >  [<ffffffffc02b3ca0>] ? btrfs_real_readdir+0x5e0/0x5e0 [btrfs]
 >  [<ffffffffc02d18b7>] extent_read_full_page+0x37/0x60 [btrfs]
 >  [<ffffffffc02b0c25>] btrfs_readpage+0x25/0x30 [btrfs]
 >  [<ffffffffc02c0e7a>] prepare_uptodate_page+0x4a/0x90 [btrfs]
 >  [<ffffffffc02c0fc1>] prepare_pages+0x101/0x190 [btrfs]
 >  [<ffffffffc02c1b03>] __btrfs_buffered_write+0x1d3/0x650 [btrfs]
 >  [<ffffffffc02c5713>] btrfs_file_write_iter+0x463/0x570 [btrfs]
 >  [<ffffffffb2045eea>] ? bad_area+0x4a/0x60
 >  [<ffffffffb21d05d1>] __vfs_write+0xb1/0xf0
 >  [<ffffffffb21d0c59>] vfs_write+0xa9/0x1b0
 >  [<ffffffffb21d1bd2>] SyS_pwrite64+0x72/0xb0
 >  [<ffffffffb20125d0>] ? syscall_trace_enter_phase2+0x220/0x260
 >  [<ffffffffb2012715>] ? syscall_trace_leave+0x95/0x140
 >  [<ffffffffb26d5b77>] tracesys_phase2+0x84/0x89
 > Code: 10 48 d3 ee 48 8d 0c b6 48 89 c6 48 8d 3c ca 31 d2 e8 29 ca f4 ff 5d c3 0f 1f 80 00 00 00 00 48 c7 c6 c0 ed a2 b2 e8 f4 84 02 00 <0f> 0b 66 90 66 66 66 66 90 55 85 f6 48 89 e5 75 13 85 d2 74 3f 
 > RIP  [<ffffffffb216ee5c>] unlock_page+0x7c/0x80

Still occasionally bumping into this.
The 'count:4 mapcount:0' is constant in every instance I've seen
so far. Could that be a clue ?

I've seen various page flags, but it's always !locked

Ideas on additional debugging I could add ?

	Dave