Re: [PATCH v3 3/5] btrfs: remove identified alien btrfs device in open_fs_devices

From: Anand Jain <anand.jain@oracle.com>
To: Josef Bacik <josef@toxicpanda.com>, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v3 3/5] btrfs: remove identified alien btrfs device in open_fs_devices
Date: Fri, 17 Jan 2020 17:10:52 +0800	[thread overview]
Message-ID: <f04ad491-ad34-0a0d-e2ba-11ad3713bedd@oracle.com> (raw)
In-Reply-To: <550c6454-be48-51bb-1196-40586e3649b7@toxicpanda.com>

On 1/16/20 11:56 PM, Josef Bacik wrote:
> On 10/7/19 5:45 AM, Anand Jain wrote:
>> In open_fs_devices() we identify alien device but we don't reset its
>> the device::name. So progs device list does not show the device missing
>> as shown in the script below.
>>
>> mkfs.btrfs -fq /dev/sdd && mount /dev/sdd /btrfs
>> mkfs.btrfs -fq -draid1 -mraid1 /dev/sdc /dev/sdb
>> sleep 3 # avoid racing with udev's useless scans if needed
>> btrfs dev add -f /dev/sdb /btrfs
>> mount -o degraded /dev/sdc /btrfs1
>>
>> No missing device:
>> btrfs fi show -m /btrfs1
>> Label: none  uuid: 3eb7cd50-4594-458f-9d68-c243cc49954d
>>     Total devices 2 FS bytes used 128.00KiB
>>     devid    1 size 12.00GiB used 1.26GiB path /dev/sdc
>>     devid    2 size 12.00GiB used 1.26GiB path /dev/sdb
>>
>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> 
> Why not just remove the device if there's any error?  I'm not sure why 
> these particular checks make a difference from any other error?  Thanks,

  That's interesting, but disadvantage is user has to re-run the
  device scan if we remove the device for a non-alien device which can
  fail temporarily in btrfs_open_one_device() function stack such as

     *bdev = blkdev_get_by_path(device_path, flags, holder);

   If user land has opened the device with O_EXCL this shall
   fail with -EBUSY. So here we shouldn't remove.

      ret = set_blocksize(*bdev, BTRFS_BDEV_BLOCKSIZE);

   This can fail if the bdev does not accept the blocksize and its
   rather a good idea to remove the device as we won't be able to
   use this device any time. So as this is not a temporary issue,
   here we could remove the device.

         *bh = btrfs_read_dev_super(*bdev);

   This function is still an incomplete (because we don't yet handle
   the corrupted super block #1, there is a patch in the ML but
   in dispute, I think). Needs clarity on how a completed function
   will look like. So here it depends on when this function completes.

      bh = __bread(bdev, bytenr / BTRFS_BDEV_BLOCKSIZE, 
BTRFS_SUPER_INFO_SIZE);

   Read can fail momentarily for transport/disconnect/plug-out issue
   and which can reappears and assume if there isn't systemd auto scan
   so here we shouldn't remove.

Thanks, Anand

> Josef