btrfs balance filling logs with "block rsv returned -28"

[66163.405869] btrfs: block rsv returned -28
[66163.405870] ------------[ cut here ]------------
[66163.405878] WARNING: at /home/apw/COD/linux/fs/btrfs/extent-tree.c:6297 use_block_rsv+0x198/0x1a0 [btrfs]()
[66163.405879] Hardware name: P5KC
[66163.405880] Modules linked in: snd_seq_dummy dm_mirror dm_region_hash dm_log ip6table_filter ip6_tables ebtable_nat ebtables ipt_MASQUERADE iptable_nat nf_nat_ipv4 nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 xt_state nf_conntrack dm_crypt ipt_REJECT xt_CHECKSUM iptable_mangle xt_tcpudp iptable_filter ip_tables x_tables bridge stp llc pci_stub vboxpci(OF) vboxnetadp(OF) vboxnetflt(OF) vboxdrv(OF) snd_hrtimer autofs4 snd_hda_codec_realtek hid_generic snd_hda_intel hid_a4tech snd_hda_codec usbhid nvidia(POF) hid snd_hwdep snd_pcm kvm_intel snd_seq_midi kvm dm_multipath snd_rawmidi snd_seq_midi_event snd_seq snd_timer snd_seq_device scsi_dh gpio_ich bnep rfcomm snd soundcore microcode lpc_ich bluetooth mac_hid parport_pc serio_raw asus_atk0110 ppdev snd_page_alloc nfsd nfs_acl auth_rpcgss nfs binfmt_misc fscache lockd hwmon_vid coretemp sunrpc lp parport reiserfs btrfs zlib_deflate libcrc32c raid10 raid456 async_pq async_xor xor async_memcpy async_raid6_recov raid6_pq async_tx raid0 multipath linear nbd raid1 atl1 8139too 8139cp
[66163.405924] Pid: 21980, comm: btrfs Tainted: PF       W  O 3.7.0-030700rc7-generic #201211252135
[66163.405925] Call Trace:
[66163.405929]  [<ffffffff810588cf>] warn_slowpath_common+0x7f/0xc0
[66163.405931]  [<ffffffff8105892a>] warn_slowpath_null+0x1a/0x20
[66163.405939]  [<ffffffffa00d0c38>] use_block_rsv+0x198/0x1a0 [btrfs]
[66163.405948]  [<ffffffffa00d3d3d>] btrfs_alloc_free_block+0x3d/0x220 [btrfs]
[66163.405956]  [<ffffffffa00c06d4>] ? __btrfs_cow_block+0x324/0x4f0 [btrfs]
[66163.405967]  [<ffffffffa0102fdc>] ? read_extent_buffer+0xbc/0x120 [btrfs]
[66163.405975]  [<ffffffffa00c1f9c>] ? comp_keys+0x2c/0x30 [btrfs]
[66163.405983]  [<ffffffffa00c04d2>] __btrfs_cow_block+0x122/0x4f0 [btrfs]
[66163.405992]  [<ffffffffa00dc330>] ? verify_parent_transid+0x170/0x170 [btrfs]
[66163.406000]  [<ffffffffa00c0a1c>] btrfs_cow_block+0xfc/0x220 [btrfs]
[66163.406011]  [<ffffffffa0127e4f>] do_relocation+0x46f/0x560 [btrfs]
[66163.406013]  [<ffffffff816cafde>] ? _raw_spin_lock+0xe/0x20
[66163.406021]  [<ffffffffa00c85ea>] ? block_rsv_add_bytes+0x5a/0x80 [btrfs]
[66163.406033]  [<ffffffffa0128184>] relocate_tree_block+0x244/0x280 [btrfs]
[66163.406044]  [<ffffffffa012c063>] relocate_tree_blocks+0x123/0x1a0 [btrfs]
[66163.406056]  [<ffffffffa012ceaa>] relocate_block_group+0x1fa/0x560 [btrfs]
[66163.406067]  [<ffffffffa012d3cd>] btrfs_relocate_block_group+0x1bd/0x2f0 [btrfs]
[66163.406078]  [<ffffffffa0106206>] btrfs_relocate_chunk.isra.53+0x56/0x3b0 [btrfs]
[66163.406090]  [<ffffffffa00fca79>] ? release_extent_buffer.isra.26+0x39/0x60 [btrfs]
[66163.406092]  [<ffffffff816cafde>] ? _raw_spin_lock+0xe/0x20
[66163.406103]  [<ffffffffa01021b7>] ? free_extent_buffer+0x37/0x90 [btrfs]
[66163.406114]  [<ffffffffa010a482>] __btrfs_balance+0x302/0x3e0 [btrfs]
[66163.406125]  [<ffffffffa010a87e>] btrfs_balance+0x31e/0x560 [btrfs]
[66163.406137]  [<ffffffffa0112de4>] btrfs_ioctl_balance+0x114/0x440 [btrfs]
[66163.406148]  [<ffffffffa0114968>] btrfs_ioctl+0x428/0x950 [btrfs]
[66163.406150]  [<ffffffff811a4c2a>] do_vfs_ioctl+0x8a/0x340
[66163.406152]  [<ffffffff811a4f71>] sys_ioctl+0x91/0xb0
[66163.406154]  [<ffffffff816cf589>] ? do_page_fault+0x39/0x50
[66163.406156]  [<ffffffff816d3edd>] system_call_fastpath+0x1a/0x1f
[66163.406158] ---[ end trace f99166bc7e8eaffd ]---