btrfs: set update the uuid generation as soon as possible

In my EIO stress testing I noticed I was getting forced to rescan the
uuid tree pretty often, which was weird.  This is because my error
injection stuff would sometimes inject an error after log replay but
before we loaded the UUID tree.  If log replay committed the transaction
it wouldn't have updated the uuid tree generation, but the tree was
valid and didn't change, so there's no reason to not update the
generation here.

Fix this by setting the BTRFS_FS_UPDATE_UUID_TREE_GEN bit immediately
after reading all the fs roots if the uuid tree generation matches the
fs generation.  Then any transaction commits that happen during mount
won't screw up our uuid tree state, forcing us to do needless uuid
rescans.

Fixes: 70f8017547 ("Btrfs: check UUID tree during mount if required")
CC: stable@vger.kernel.org # 4.19+
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -3151,6 +3151,18 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 	if (ret)
 		goto fail_tree_roots;
 
+	/*
+	 * If we have a uuid root and we're not being told to rescan we need to
+	 * check the generation here so we can set the
+	 * BTRFS_FS_UPDATE_UUID_TREE_GEN bit.  Otherwise we could commit the
+	 * transaction during a balance or the log replay without updating the
+	 * uuid generation, and then if we crash we would rescan the uuid tree,
+	 * even though it was perfectly fine.
+	 */
+	if (fs_info->uuid_root && !btrfs_test_opt(fs_info, RESCAN_UUID_TREE) &&
+	    fs_info->generation == btrfs_super_uuid_tree_generation(disk_super))
+		set_bit(BTRFS_FS_UPDATE_UUID_TREE_GEN, &fs_info->flags);
+
 	ret = btrfs_verify_dev_extents(fs_info);
 	if (ret) {
 		btrfs_err(fs_info,
@@ -3375,8 +3387,6 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 			close_ctree(fs_info);
 			return ret;
 		}
-	} else {
-		set_bit(BTRFS_FS_UPDATE_UUID_TREE_GEN, &fs_info->flags);
 	}
 	set_bit(BTRFS_FS_OPEN, &fs_info->flags);