btrfs: Handle another split brain scenario with metadata uuid feature

There is one more cases which isn't handled by the original metadata
uuid work. Namely, when a filesystem has METADATA_UUID incompat bit and
the user decides to change the FSID to the original one e.g. have
metadata_uuid and fsid match. In case of power failure while this
operation is in progress we could end up in a situation where some of
the disks have the incompat bit removed and the other half have both
METADATA_UUID_INCOMPAT and FSID_CHANGING_IN_PROGRESS flags.

This patch handles the case where a disk that has successfully changed
its FSID such that it equals METADATA_UUID is scanned first.
Subsequently when a disk with both
METADATA_UUID_INCOMPAT/FSID_CHANGING_IN_PROGRESS flags is scanned
find_fsid_changed won't be able to find an appropriate btrfs_fs_devices.
This is done by extending find_fsid_changed to correctly find
btrfs_fs_devices whose metadata_uuid/fsid are the same and they match
the metadata_uuid of the currently scanned device.

Fixes: cc5de4e70256 ("btrfs: Handle final split-brain possibility during fsid change")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reported-by: Su Yue <Damenly_Su@gmx.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index d48274451452fa67c84b49190b249e264d68a6c5..7a30dd8ffdb5e116776629f6ae2733a9629dbd80 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -710,17 +710,28 @@ static struct btrfs_fs_devices *find_fsid_changed(
        /*
         * Handles the case where scanned device is part of an fs that had
         * multiple successful changes of FSID but curently device didn't
-        * observe it. Meaning our fsid will be different than theirs.
+        * observe it. Meaning our fsid will be different than theirs. We need
+        * to handle two subcases :
+        *  1 - The fs still continues to have different METADATA/FSID uuids.
+        *  2 - The fs is switched back to its original FSID (METADATA/FSID
+        *  are equal).
         */
        list_for_each_entry(fs_devices, &fs_uuids, fs_list) {
+               /* Changed UUIDs */
                if (memcmp(fs_devices->metadata_uuid, fs_devices->fsid,
                           BTRFS_FSID_SIZE) != 0 &&
                    memcmp(fs_devices->metadata_uuid, disk_super->metadata_uuid,
                           BTRFS_FSID_SIZE) == 0 &&
                    memcmp(fs_devices->fsid, disk_super->fsid,
-                          BTRFS_FSID_SIZE) != 0) {
+                          BTRFS_FSID_SIZE) != 0)
+                       return fs_devices;
+
+               /* Unchanged UUIDs */
+               if (memcmp(fs_devices->metadata_uuid, fs_devices->fsid,
+                          BTRFS_FSID_SIZE) == 0 &&
+                   memcmp(fs_devices->fsid, disk_super->metadata_uuid,
+                          BTRFS_FSID_SIZE) == 0)
                        return fs_devices;
-               }
        }
 
        return NULL;