On 07/12/2018 08:59 PM, Qu Wenruo wrote:


On 2018年07月12日 20:33, Anand Jain wrote:


On 07/12/2018 01:43 PM, Qu Wenruo wrote:


On 2018年07月11日 15:50, Anand Jain wrote:


BTRFS Volume operations, Device Lists and Locks all in one page:

Devices are managed in two contexts, the scan context and the mounted
context. In scan context the threads originate from the btrfs_control
ioctl and in the mounted context the threads originates from the mount
point ioctl.
Apart from these two context, there also can be two transient state
where device state are transitioning from the scan to the mount context
or from the mount to the scan context.

Device List and Locks:-

   Count: btrfs_fs_devices::num_devices
   List : btrfs_fs_devices::devices -> btrfs_devices::dev_list
   Lock : btrfs_fs_devices::device_list_mutex

   Count: btrfs_fs_devices::rw_devices

So btrfs_fs_devices::num_devices = btrfs_fs_devices::rw_devices + RO
devices.
How seed and ro devices are different in this case?

  Given:
  btrfs_fs_devices::total_devices = btrfs_super_num_devices(disk_super);

  Consider no missing devices, no replace target, no seeding. Then,
    btrfs_fs_devices::total_devices == btrfs_fs_devices::num_devices

  And in case of seeding.
    btrfs_fs_devices::total_devices  == (btrfs_fs_devices::num_devices +
                                 btrfs_fs_devices::seed::total_devices

    All devices in the list [1] are RW/Sprout
      [1] fs_info::btrfs_fs_devices::devices
    All devices in the list [2] are RO/Seed

 to avoid confusion I shall remove RO here
       All devices in the list [2] are Seed

      [2] fs_info::btrfs_fs_devices::seed::devices


  Thanks for asking will add this part to the doc.

Another question is, what if a device is RO but not seed?

E.g. loopback device set to RO.
IMHO it won't be mounted RW for single device case, but not sure for
multi device case.

 RO devices are different from the seed devices. If any one device is
 RO then FS is mounted in RO.

 And the btrfs_fs_devices::seed will still be NULL.



   List : btrfs_fs_devices::alloc_list -> btrfs_devices::dev_alloc_list
   Lock : btrfs_fs_info::chunk_mutex

At least the chunk_mutex is also shared with chunk allocator,

  Right.

or we
should have some mutex in btrfs_fs_devices other than fs_info.
Right?

  More locks? no. But some of the locks-and-flags are wrongly
  belong to fs_info instead it should have been in fs_devices.
  When the dust settles planning to propose to migrate them
  to fs_devices.

OK, migrating to fs_devices looks good to me then.


   Lock: set_bit btrfs_fs_info::flags::BTRFS_FS_EXCL_OP

FSID List and Lock:-

   Count : None
   HEAD  : Global::fs_uuids -> btrfs_fs_devices::fs_list
   Lock  : Global::uuid_mutex


After the fs_devices is mounted, the btrfs_fs_devices::opened > 0.

fs_devices::opended should be btrfs_fs_devices::num_devices if no device
is missing and -1 or -2 for degraded case, right?

  No. I think you are getting confused with
     btrfs_fs_devices::open_devices

  btrfs_fs_devices::opened
   indicate how many times the volume is opened. And in reality it would
  stay at 1 always. (except for a short duration of time during
  subsequent subvol mount).

Thanks, this makes sense.



In the scan context we have the following device operations..

Device SCAN:-  which creates the btrfs_fs_devices and its corresponding
btrfs_device entries, also checks and frees the duplicate device
entries.
Lock: uuid_mutex
    SCAN
    if (found_duplicate && btrfs_fs_devices::opened == 0)
       Free_duplicate
Unlock: uuid_mutex

Device READY:- check if the volume is ready. Also does an implicit scan
and duplicate device free as in Device SCAN.
Lock: uuid_mutex
    SCAN
    if (found_duplicate && btrfs_fs_devices::opened == 0)
       Free_duplicate
    Check READY
Unlock: uuid_mutex

Device FORGET:- (planned) free a given or all unmounted devices and
empty fs_devices if any.
Lock: uuid_mutex
    if (found_duplicate && btrfs_fs_devices::opened == 0)
      Free duplicate
Unlock: uuid_mutex

Device mount operation -> A Transient state leading to the mounted
context
Lock: uuid_mutex
   Find, SCAN, btrfs_fs_devices::opened++
Unlock: uuid_mutex

Device umount operation -> A transient state leading to the unmounted
context or scan context
Lock: uuid_mutex
    btrfs_fs_devices::opened--
Unlock: uuid_mutex


In the mounted context we have the following device operations..

Device Rename through SCAN:- This is a special case where the device
path gets renamed after its been mounted. (Ubuntu changes the boot path
during boot up so we need this feature). Currently, this is part of
Device SCAN as above. And we need the locks as below, because the
dynamic disappearing device might cleanup the btrfs_device::name
Lock: btrfs_fs_devices::device_list_mutex
     Rename
Unlock: btrfs_fs_devices::device_list_mutex

Commit Transaction:- Write All supers.
Lock: btrfs_fs_devices::device_list_mutex
    Write all super of btrfs_devices::dev_list
Unlock: btrfs_fs_devices::device_list_mutex

Device add:- Add a new device to the existing mounted volume.
set_bit: btrfs_fs_info::flags::BTRFS_FS_EXCL_OP
Lock: btrfs_fs_devices::device_list_mutex
Lock: btrfs_fs_info::chunk_mutex
     List_add btrfs_devices::dev_list
     List_add btrfs_devices::dev_alloc_list
Unlock: btrfs_fs_info::chunk_mutex
Unlock: btrfs_fs_devices::device_list_mutex

Device remove:- Remove a device from the mounted volume.
set_bit: btrfs_fs_info::flags::BTRFS_FS_EXCL_OP
Lock: btrfs_fs_devices::device_list_mutex
Lock: btrfs_fs_info::chunk_mutex
     List_del btrfs_devices::dev_list
     List_del btrfs_devices::dev_alloc_list
Unlock: btrfs_fs_info::chunk_mutex
Unlock: btrfs_fs_devices::device_list_mutex

Device Replace:- Replace a device.
set_bit: btrfs_fs_info::flags::BTRFS_FS_EXCL_OP
Lock: btrfs_fs_devices::device_list_mutex
Lock: btrfs_fs_info::chunk_mutex
     List_update btrfs_devices::dev_list

Here we still just add a new device but not deleting the existing one
until the replace is finished.

  Right I did not elaborate that part. List_update: I meant add/delete
  accordingly.

     List_update btrfs_devices::dev_alloc_list
Unlock: btrfs_fs_info::chunk_mutex
Unlock: btrfs_fs_devices::device_list_mutex

Sprouting:- Add a RW device to the mounted RO seed device, so to make
the mount point writable.
The following steps are used to hold the seed and sprout fs_devices.
(first two steps are not necessary for the sprouting, they are there to
ensure the seed device remains scanned, and it might change)
. Clone the (mounted) fs_devices, lets call it as old_devices
. Now add old_devices to fs_uuids (yeah, there is duplicate fsid in the
list but we change the other fsid before we release the uuid_mutex, so
its fine).

. Alloc a new fs_devices, lets call it as seed_devices
. Copy fs_devices into the seed_devices
. Move fs_deviecs devices list into seed_devices
. Bring seed_devices to under fs_devices (fs_devices->seed =
seed_devices)
. Assign a new FSID to the fs_devices and add the new writable device to
the fs_devices.

In the unmounted context the fs_devices::seed is always NULL.
We alloc the fs_devices::seed only at the time of mount and or at
sprouting. And free at the time of umount or if the seed device is
replaced or deleted.

Locks: Sprouting:
Lock: uuid_mutex <-- because fsid rename and Device SCAN
Reuses Device Add code

Locks: Splitting: (Delete OR Replace a seed device)
uuid_mutex is not required as fs_devices::seed which is local to
fs_devices is being altered.
Reuses Device replace code


Device resize:- Resize the given volume or device.
Lock: btrfs_fs_info::chunk_mutex
     Update
Unlock: btrfs_fs_info::chunk_mutex


(Planned) Dynamic Device missing/reappearing:- A missing device might
reappear after its volume been mounted, we have the same btrfs_control
ioctl which does the scan of the reappearing device but in the mounted
context. In the contrary a device of a volume in a mounted context can
go missing as well, and still the volume will continue in the mounted
context.
Missing:
Lock: btrfs_fs_devices::device_list_mutex
Lock: btrfs_fs_info::chunk_mutex
    List_del: btrfs_devices::dev_alloc_list
    Close_bdev
    btrfs_device::bdev == NULL
    btrfs_device::name = NULL
    set_bit BTRFS_DEV_STATE_MISSING
    set_bit BTRFS_VOL_STATE_DEGRADED
Unlock: btrfs_fs_info::chunk_mutex
Unlock: btrfs_fs_devices::device_list_mutex

Reappearing:
Lock: btrfs_fs_devices::device_list_mutex
Lock: btrfs_fs_info::chunk_mutex
    Open_bdev
    btrfs_device::name = PATH
    clear_bit BTRFS_DEV_STATE_MISSING
    clear_bit BTRFS_VOL_STATE_DEGRADED
    List_add: btrfs_devices::dev_alloc_list
    set_bit BTRFS_VOL_STATE_RESILVERING
    kthread_run HEALTH_CHECK

For this part, I'm planning to add scrub support for certain generation
range, so just scrub for certain block groups which is newer than the
last generation of the re-appeared device should be enough.

However I'm wondering if it's possible to reuse btrfS_balance_args, as
we really have a lot of similarity when specifying block groups to
relocate/scrub.

  What you proposed sounds interesting. But how about failed writes
  at some generation number and not necessarily at the last generation?

In this case, it depends on when and how we mark the device resilvering.
If we record the generation of write error happens, then just initial a
scrub for generation greater than that generation.

 If we record all the degraded transactions then yes. Not just the last
 failed transaction.

In the list, some guys mentioned that for LVM/mdraid they will record
the generation when some device(s) get write error or missing, and do
self cure.
>

  I have been scratching on fix for this [3] for some time now. Thanks
  for the participation. In my understanding we are missing across-tree
  parent transid verification at the lowest possible granular OR

Maybe the newly added first_key and level check could help detect such
mismatch?

  other approach is to modify Liubo approach to provide a list of
  degraded chunks but without a journal disk.

Currently, DEV_ITEM::generation is seldom used. (only for seed sprout case)
Maybe we could reuse that member to record the last successful written
transaction to that device and do above purposed LVM/mdraid style self cure?

 Record of just the last successful transaction won't help. OR its an
 overkill to fix a write hole.

 Transactions: 10 11 [12] [13] [14] <---- write hole ----> [19] [20]
 In the above example
  disk disappeared at transaction 11 and when it reappeared at
  the transaction 19, there were new writes as well as the resilver
  writes, so we were able to write 12 13 14 and 19 20 and then
  the disk disappears again leaving a write hole. Now next time when
  disk reappears the last transaction indicates 20 on both-disks
  but leaving a write hole in one of disk. But if you are planning to
  record and start at transaction [14] then its an overkill because
  transaction [19 and [20] are already in the disk.

Thanks, Anand


Thanks,
Qu

    [3] https://patchwork.kernel.org/patch/10403311/

  Further, as we do a self adapting chunk allocation in RAID1, it needs
  balance-convert to fix. IMO at some point we have to provide degraded
  raid1 chunk allocation and also modify the scrub to be chunk granular.

Thanks, Anand

Any idea on this?

Thanks,
Qu

Unlock: btrfs_fs_info::chunk_mutex
Unlock: btrfs_fs_devices::device_list_mutex

-----------------------------------------------------------------------

Thanks, Anand
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