Hi, As I started experimenting with journaled raids: 1) can another (raid1) device or lvm mirror (using md underneath) be used formally as a write journal ? 2) for the testing purposes, are loopback devices expected to work ? I know I can successfully create both of the above (case scenario below), but any attempt to write to such md device ends with hung tasks and D states. kernel 5.1.3 mdadm 4.1 btrfs filesystem with /var subvolume (where losetup files were created with fallocate) What I did: - six 10gb files as backings for /dev/loop[0-5] - /dev/loop[4-5] - raid1 - /dev/loop[0-3] - raid5 Now at this point both raids work just fine, separately. The problem starts if I create raid1 as raid5's journal (doesn't matter whether w-t or w-b). Any attempt to write to a device like that instantly ends with D and respective traces in dmesg: May 22 14:39:37 hakai kernel: INFO: task dd:899 blocked for more than 245 seconds. May 22 14:39:37 hakai kernel: Not tainted 5.1.3-arch1-1-ARCH #1 May 22 14:39:37 hakai kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. May 22 14:39:37 hakai kernel: dd D 0 899 824 0x00000004 May 22 14:39:37 hakai kernel: Call Trace: May 22 14:39:37 hakai kernel: ? __schedule+0x30b/0x8b0 May 22 14:39:37 hakai kernel: ? raid5_unplug+0xb2/0x140 [raid456] May 22 14:39:37 hakai kernel: schedule+0x32/0x80 May 22 14:39:37 hakai kernel: io_schedule+0x12/0x40 May 22 14:39:37 hakai kernel: wait_on_page_bit+0x138/0x200 May 22 14:39:37 hakai kernel: ? add_to_page_cache_lru+0xc0/0xc0 May 22 14:39:37 hakai kernel: __filemap_fdatawait_range+0xb9/0x110 May 22 14:39:37 hakai kernel: filemap_fdatawait_range+0xe/0x20 May 22 14:39:37 hakai kernel: filemap_write_and_wait+0x47/0x70 May 22 14:39:37 hakai kernel: __blkdev_put+0x71/0x1e0 May 22 14:39:37 hakai kernel: blkdev_close+0x21/0x30 May 22 14:39:37 hakai kernel: __fput+0xa5/0x1d0 May 22 14:39:37 hakai kernel: task_work_run+0x8f/0xb0 May 22 14:39:37 hakai kernel: exit_to_usermode_loop+0xd3/0xe0 May 22 14:39:37 hakai kernel: do_syscall_64+0x157/0x180 May 22 14:39:37 hakai kernel: entry_SYSCALL_64_after_hwframe+0x44/0xa9 May 22 14:39:37 hakai kernel: RIP: 0033:0x7f13c4a66348 May 22 14:39:37 hakai kernel: Code: Bad RIP value. May 22 14:39:37 hakai kernel: RSP: 002b:00007ffcde214058 EFLAGS: 00000246 ORIG_RAX: 0000000000000003 May 22 14:39:37 hakai kernel: RAX: 0000000000000000 RBX: 0000562ed16d5160 RCX: 00007f13c4a66348 May 22 14:39:37 hakai kernel: RDX: 0000000000080000 RSI: 0000000000000000 RDI: 0000000000000001 May 22 14:39:37 hakai kernel: RBP: 0000000000000001 R08: 00000000ffffffff R09: 0000000000000000 May 22 14:39:37 hakai kernel: R10: 0000000000000022 R11: 0000000000000246 R12: 0000000000000001 May 22 14:39:37 hakai kernel: R13: 0000000000000000 R14: 0000000000000000 R15: 00007f13c4301000 May 22 14:39:37 hakai kernel: INFO: task systemd-udevd:313 blocked for more than 245 seconds. May 22 14:39:37 hakai kernel: Not tainted 5.1.3-arch1-1-ARCH #1 May 22 14:39:37 hakai kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. May 22 14:39:37 hakai kernel: systemd-udevd D 0 313 1 0x00000104 May 22 14:39:37 hakai kernel: Call Trace: May 22 14:39:37 hakai kernel: ? __schedule+0x30b/0x8b0 May 22 14:39:37 hakai kernel: schedule+0x32/0x80 May 22 14:39:37 hakai kernel: schedule_preempt_disabled+0x14/0x20 May 22 14:39:37 hakai kernel: __mutex_lock.isra.1+0x217/0x520 May 22 14:39:37 hakai kernel: ? kobj_lookup+0xf1/0x160 May 22 14:39:37 hakai kernel: __blkdev_get+0x83/0x540 May 22 14:39:37 hakai kernel: blkdev_get+0x108/0x340 May 22 14:39:37 hakai kernel: ? preempt_count_add+0x79/0xb0 May 22 14:39:37 hakai kernel: ? _raw_spin_lock+0x13/0x30 May 22 14:39:37 hakai kernel: ? bd_acquire+0xc0/0xc0 May 22 14:39:37 hakai kernel: do_dentry_open+0x13a/0x380 May 22 14:39:37 hakai kernel: path_openat+0x2d6/0x1500 May 22 14:39:37 hakai kernel: ? mntput_no_expire+0x11/0x1a0 May 22 14:39:37 hakai kernel: ? terminate_walk+0xeb/0x100 May 22 14:39:37 hakai kernel: do_filp_open+0x93/0x100 May 22 14:39:37 hakai kernel: ? __check_object_size+0xc1/0x175 May 22 14:39:37 hakai kernel: ? _raw_spin_unlock+0x16/0x30 May 22 14:39:37 hakai kernel: do_sys_open+0x186/0x220 May 22 14:39:37 hakai kernel: do_syscall_64+0x5b/0x180 May 22 14:39:37 hakai kernel: entry_SYSCALL_64_after_hwframe+0x44/0xa9 May 22 14:39:37 hakai kernel: RIP: 0033:0x7ff24fe549ff May 22 14:39:37 hakai kernel: Code: Bad RIP value. May 22 14:39:37 hakai kernel: RSP: 002b:00007ffe7b7ce060 EFLAGS: 00000246 ORIG_RAX: 0000000000000101 May 22 14:39:37 hakai kernel: RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007ff24fe549ff May 22 14:39:37 hakai kernel: RDX: 00000000000a0800 RSI: 00005578d289be10 RDI: 00000000ffffff9c May 22 14:39:37 hakai kernel: RBP: 00005578d28b3ea0 R08: 00007ff24fb896d0 R09: 00005578d2891fe0 May 22 14:39:37 hakai kernel: R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 May 22 14:39:37 hakai kernel: R13: 00007ffe7b7cf198 R14: 00007ffe7b7cf188 R15: 00005578d27e1254
