[PATCH v4 3/3] btrfs: free alien device due to device add

[Anand Jain <anand.jain@oracle.com>]

When the old device has new fsid through btrfs device add -f <dev> our
fs_devices list has an alien device in one of the fs_devices. So this is
a trigger and not the root cause of the issue. This patch fixes the trigger.

By having an alien device in fs_devices, we have two issues so far

1. missing device is not shows as missing in the userland

Which is due to cracks in the function btrfs_open_one_device() and
hardened by the pending patches in the ml.
 btrfs: remove identified alien device in open_fs_devices
 btrfs: remove identified alien btrfs device in open_fs_devices

2. mount of a degraded fs_devices fails

Which is due to cracks in the function btrfs_free_extra_devids() and
hardened by patch (included in the set).
 btrfs: include non-missing as a qualifier for the latest_bdev

The trigger for both of this issue is that there is an alien (does not
contain the intended fsid/btrfs_magic) device in the fs_devices.

We know a device can be scanned/added through
btrfs-control::BTRFS_IOC_SCAN_DEV|BTRFS_IOC_DEVICES_READY
or by
ioctl::BTRFS_IOC_ADD_DEV

And device coming through btrfs-control is checked against the all other
devices in btrfs kernel but not coming through BTRFS_IOC_ADD_DEV.

This patch checks if the device add is alienating any other scanned
device and deletes it.

In fact, this patch fixes both the issues 1 and 2 (above) by eliminating
the trigger of the issue, but still they have their own patch as well
because its the right way to harden the functions and fill the cracks.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
---
v3-rebased: change log updated.
v4: use the helper btrfs_forget_devices().

 fs/btrfs/volumes.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index f93739afe681..8e6f110b375d 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -2664,6 +2664,15 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
 
 	/* Update ctime/mtime for libblkid */
 	update_dev_time(device_path);
+
+	/*
+	 * Now that we have written a new sb into this device, check all other
+	 * fs_devices list if device_path alienates any other scanned device.
+	 * Ignore the return as we are successfull in the core task - add
+	 * the device.
+	 */
+	btrfs_forget_devices(device_path);
+
 	return ret;
 
 error_sysfs:
-- 
2.25.1