Comment # 2 on bug 1230631 from Martin Wilck

The btrfs-defrag process is reading from a pipe which is also connected to PID
22706 (fd 34):

> cat /proc/22770/stack 
[<0>] pipe_read+0x2fc/0x470
[<0>] vfs_read+0x2c0/0x2e0
[<0>] ksys_read+0x50/0xe0
[<0>] do_syscall_64+0x5b/0x80
[<0>] entry_SYSCALL_64_after_hwframe+0x7c/0xe6

# ls -l /proc/22770/fd/0
lr-x------ 1 root root 64 Sep 17 11:05 0 -> pipe:[6217340]

/proc/22706
l-wx------ 1 root root 64 Sep 17 11:05 34 -> pipe:[6217340]
/proc/22770
lr-x------ 1 root root 64 Sep 17 11:05 0 -> pipe:[6217340]