btrfs: remove the dev stats item when removing a device

[MINOR BUG]
The following script will cause DEV_STATS item to be left after the
corresponding device is removed:

  # mkfs.btrfs -f $dev1
  # mount $dev1 $mnt
  # btrfs dev add $dev2 $mnt
  # umount $mnt

  ## Without real errors, only at mount time btrfs will update
  ## dev->dev_stats_ccnt, thus we need a mount cycle to create the
  ## DEV_STATS item for the new device.

  # mount $dev1 $mnt
  # touch $mnt/foobar
  # sync
  # btrfs dev remove $dev2 $mnt
  # umount $mnt

This will result the DEV_STATS item for devid 2 still left in device
tree:

  device tree key (DEV_TREE ROOT_ITEM 0)
  leaf 31064064 items 7 free space 15788 generation 18 owner DEV_TREE
  leaf 31064064 flags 0x1(WRITTEN) backref revision 1
  fs uuid 4bd853ed-f6ef-45fd-bbf1-1c3a2d9987cb
  chunk uuid b496eab1-ec23-46b5-81c1-2f1b3503ca07
         item 0 key (DEV_STATS PERSISTENT_ITEM 1) itemoff 16243 itemsize 40
         	persistent item objectid DEV_STATS offset 1
         	device stats
         	write_errs 0 read_errs 0 flush_errs 0 corruption_errs 0 generation 0
         item 1 key (DEV_STATS PERSISTENT_ITEM 2) itemoff 16203 itemsize 40
         	persistent item objectid DEV_STATS offset 2
         	device stats
         	write_errs 0 read_errs 0 flush_errs 0 corruption_errs 0 generation 0

This is not a huge problem, but if the existing DEV_STATS contains
errors, and a new device is added into the fs taking the old devid, then
after a mount cycle, the new device will suddenly inherit old errors
which can give false alerts.

[CAUSE]
Btrfs never has the ability to delete DEV_STATS items.

It either create a new one through update_dev_stat_item(), or read an
existing one through btrfs_device_init_dev_stats().

However update_dev_stat_item() is only called lazily, if a new device is
created and no new update to dev stats, then it will skip the update of
the on-disk item.

So if the old DEV_STATS item exists and a new device is added, and no
errors during the remaining operations, the old DEV_STATS will not be
updated.

Then at the next mount cycle, btrfs_device_init_dev_stats() is called at
mount time, which will read out the old records, causing false alerts to
the newly added device.

[FIX]
Manually remove the DEV_STATS item during btrfs_rm_device().

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index bfbb63cf14f559e22831cd3e5437acea3c364aa3..5733b964ab7e57068e9cce1695ca04430054b170 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -2397,6 +2397,12 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info,
                return ret;
        }
 
+       ret = btrfs_remove_dev_stat_item(trans, device->devid);
+       if (unlikely(ret)) {
+               btrfs_abort_transaction(trans, ret);
+               btrfs_end_transaction(trans);
+               return ret;
+       }
        clear_bit(BTRFS_DEV_STATE_IN_FS_METADATA, &device->dev_state);
        btrfs_scrub_cancel_dev(device);