kernel 4.1.15 balance => oops. out of memory.

kernel 4.1.15

balancing a fs led to this oops.

after a reboot, the balance resumed without problem.

presumably there wasn't enough real memory available

David

 ------------[ cut here ]------------
 WARNING: CPU: 1 PID: 16560 at fs/btrfs/super.c:260 
__btrfs_abort_transaction+0x4b/0x120 [btrfs]()
 BTRFS: Transaction aborted (error -12)
 Modules linked in: ufs qnx4 hfsplus hfs minix ntfs vfat msdos fat jfs 
xfs libcrc32c dm_mod veth xt_multiport iptable_filter ip_tables x_tables nfsd auth_rpcgss oid_registry nfs_acl nfs lockd grace fscache sunrpc bridge stp llc ip_gre ip_tunnel gre fuse crct10dif_pclmul crc32_pclmul ppdev cirrus aesni_intel snd_pcsp aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd snd_pcm ttm snd_timer evdev psmouse snd soundcore serio_raw drm_kms_helper parport_pc 8250_fintek parport drm i2c_piix4 i2c_core acpi_cpufreq processor thermal_sys button ext4 crc16 mbcache jbd2 btrfs xor raid6_pq ata_generic xen_blkfront crc32c_intel floppy ata_piix libata scsi_mod ixgbevf
 CPU: 1 PID: 16560 Comm: apache2 Not tainted 4.1.15-dg1 #1
 Hardware name: Xen HVM domU, BIOS 4.2.amazon 12/07/2015
  0000000000000000 ffffffffa01a1af2 ffffffff81573596 ffff880029f13b08
  ffffffff81071221 ffff880139897388 00000000fffffff4 ffff88014ba71000
  ffffffffa01a061c 0000000000001854 ffffffff8107129a ffffffffa01a46e8
 Call Trace:
  [<ffffffff81573596>] ? dump_stack+0x40/0x50
  [<ffffffff81071221>] ? warn_slowpath_common+0x81/0xb0
  [<ffffffff8107129a>] ? warn_slowpath_fmt+0x4a/0x50
  [<ffffffffa00f4d8b>] ? __btrfs_abort_transaction+0x4b/0x120 [btrfs]
  [<ffffffffa013213a>] ? btrfs_add_link+0x2ea/0x350 [btrfs]
  [<ffffffffa01331e2>] ? btrfs_create+0x1a2/0x1e0 [btrfs]
  [<ffffffff811d663e>] ? generic_permission+0x13e/0x1a0
  [<ffffffff811d79d4>] ? vfs_create+0xc4/0x130
  [<ffffffff811dbe44>] ? do_last+0x1154/0x1210
  [<ffffffff810afe01>] ? __init_waitqueue_head+0x1/0x20
  [<ffffffff811dbf83>] ? path_openat+0x83/0x600
  [<ffffffff8118850e>] ? do_set_pte+0x9e/0xd0
  [<ffffffff8115c95e>] ? filemap_map_pages+0x23e/0x250
  [<ffffffff811ddce0>] ? do_filp_open+0x30/0xa0
  [<ffffffff811ea56f>] ? __alloc_fd+0x9f/0x130
  [<ffffffff811cb19a>] ? do_sys_open+0x12a/0x220
  [<ffffffff81579872>] ? system_call_fastpath+0x16/0x75
 ---[ end trace 388b9f3bfbb5b4e9 ]---
 BTRFS: error (device xvdf) in btrfs_add_link:6228: errno=-12 Out of memory
 BTRFS info (device xvdf): forced readonly