Suspend with btrfs rootfs will occasionally hang

[zhongfu]

When the kernel syncs filesystems after it's told to go to sleep with echo mem > /sys/power/state, it might hang due to btrfs-transaction attempting to perform I/O on the (now suspended) root cryptdevice.

dmesg:

[ 5073.146612] INFO: task btrfs-transacti:388 blocked for more than 120 seconds.
[ 5073.146621]       Tainted: G     U  W       4.10.0-15-generic #17-Ubuntu
[ 5073.146623] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 5073.146626] btrfs-transacti D    0   388      2 0x00000000
[ 5073.146630] Call Trace:
[ 5073.146639]  __schedule+0x233/0x6f0
[ 5073.146642]  schedule+0x36/0x80
[ 5073.146645]  schedule_timeout+0x22a/0x3f0
[ 5073.146691]  ? extent_write_cache_pages.isra.44.constprop.61+0x25f/0x410 [btrfs]
[ 5073.146696]  ? ktime_get+0x41/0xb0
[ 5073.146698]  io_schedule_timeout+0xa4/0x110
[ 5073.146703]  wait_on_page_bit_common+0x118/0x1d0
[ 5073.146706]  ? unlock_page+0x30/0x30
[ 5073.146709]  __filemap_fdatawait_range+0x114/0x190
[ 5073.146713]  filemap_fdatawait_range+0x14/0x30
[ 5073.146746]  btrfs_wait_ordered_range+0x6d/0x100 [btrfs]
[ 5073.146778]  __btrfs_wait_cache_io+0x49/0x1e0 [btrfs]
[ 5073.146808]  btrfs_wait_cache_io+0x2c/0x30 [btrfs]
[ 5073.146835]  btrfs_start_dirty_block_groups+0x197/0x440 [btrfs]
[ 5073.146863]  btrfs_commit_transaction+0x13b/0x910 [btrfs]
[ 5073.146890]  ? start_transaction+0x99/0x460 [btrfs]
[ 5073.146917]  transaction_kthread+0x1a6/0x1c0 [btrfs]
[ 5073.146921]  kthread+0x101/0x140
[ 5073.146946]  ? btrfs_cleanup_transaction+0x530/0x530 [btrfs]
[ 5073.146950]  ? kthread_create_on_node+0x60/0x60
[ 5073.146954]  ret_from_fork+0x2c/0x40

[zhongfu]

Perhaps we could fork echo mem > /sys/power/state, and run cryptsetup luksResume if that doesn't end within 5 (or so) seconds?

However, even if we unlock the LUKS device, we (probably) can't cancel sleep, so perhaps we can figure out how to wake up again automatically (then sync, do luksSuspend, and try to go to sleep again)

[zhongfu]

We could also stop the kernel from doing sys_sync() on suspend by setting CONFIG_SUSPEND_SKIP_SYNC=y, but that requires a kernel recompile, which might be a little overkill.