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Subject[PATCH 5.4 143/309] btrfs: Handle another split brain scenario with metadata uuid feature
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From: Nikolay Borisov <nborisov@suse.com>

commit 05840710149c7d1a78ea85a2db5723f706e97d8f upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/btrfs/volumes.c | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)

--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -881,17 +881,28 @@ static struct btrfs_fs_devices *find_fsi
/*
* 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;

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