[PATCH] btrfs: allow single disk devices to mount with older generations

[Josef Bacik <josef@toxicpanda.com>]

We have this check to make sure we don't accidentally add older devices
that may have disappeared and re-appeared with an older generation from
being added to an fs_devices.  This makes sense, we don't want stale
disks in our file system.  However for single disks this doesn't really
make sense.  I've seen this in testing, but I was provided a reproducer
from a project that builds btrfs images on loopback devices.  The
loopback device gets cached with the new generation, and then if it is
re-used to generate a new file system we'll fail to mount it because the
new fs is "older" than what we have in cache.

Fix this by simply ignoring this check if we're a single disk file
system, as we're not going to cause problems for the fs by allowing the
disk to be mounted with an older generation than what is in our cache.

I've also added a error message for this case, as it was kind of
annoying to find originally.

Reported-by: Daan De Meyer <daandemeyer@fb.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/volumes.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 77b7da42c651..eb2cc27ef602 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -786,6 +786,7 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 	struct rcu_string *name;
 	u64 found_transid = btrfs_super_generation(disk_super);
 	u64 devid = btrfs_stack_device_id(&disk_super->dev_item);
+	bool multi_disk = btrfs_super_num_devices(disk_super) > 1;
 	bool has_metadata_uuid = (btrfs_super_incompat_flags(disk_super) &
 		BTRFS_FEATURE_INCOMPAT_METADATA_UUID);
 	bool fsid_change_in_progress = (btrfs_super_flags(disk_super) &
@@ -914,7 +915,8 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 		 * tracking a problem where systems fail mount by subvolume id
 		 * when we reject replacement on a mounted FS.
 		 */
-		if (!fs_devices->opened && found_transid < device->generation) {
+		if (multi_disk && !fs_devices->opened &&
+		    found_transid < device->generation) {
 			/*
 			 * That is if the FS is _not_ mounted and if you
 			 * are here, that means there is more than one
@@ -922,6 +924,10 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 			 * with larger generation number or the last-in if
 			 * generation are equal.
 			 */
+			btrfs_warn_in_rcu(device->fs_info,
+		  "old device %s not being added for fsid:devid for %pU:%llu",
+					  rcu_str_deref(device->name),
+					  disk_super->fsid, devid);
 			mutex_unlock(&fs_devices->device_list_mutex);
 			return ERR_PTR(-EEXIST);
 		}
-- 
2.26.2