btrfs: clear free space tree on ro->rw remount

A user might want to revert to v1 or nospace_cache on a root filesystem,
and much like turning on the free space tree, that can only be done
remounting from ro->rw. Support clearing the free space tree on such
mounts by moving it into the shared remount logic.

Since the CLEAR_CACHE option sticks around across remounts, this change
would result in clearing the tree for ever on every remount, which is
not desirable. To fix that, add CLEAR_CACHE to the oneshot options we
clear at mount end, which has the other bonus of not cluttering the
/proc/mounts output with clear_cache.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -2878,6 +2878,7 @@ static int btrfs_check_uuid_tree(struct btrfs_fs_info *fs_info)
 void btrfs_clear_oneshot_options(struct btrfs_fs_info *fs_info)
 {
 	btrfs_clear_opt(fs_info->mount_opt, USEBACKUPROOT);
+	btrfs_clear_opt(fs_info->mount_opt, CLEAR_CACHE);
 }
 
 /*
@@ -2887,6 +2888,26 @@ void btrfs_clear_oneshot_options(struct btrfs_fs_info *fs_info)
 int btrfs_start_pre_rw_mount(struct btrfs_fs_info *fs_info)
 {
 	int ret;
+	bool clear_free_space_tree = false;
+
+	if (btrfs_test_opt(fs_info, CLEAR_CACHE) &&
+	    btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE)) {
+		clear_free_space_tree = true;
+	} else if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE) &&
+		   !btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID)) {
+		btrfs_warn(fs_info, "free space tree is invalid");
+		clear_free_space_tree = true;
+	}
+
+	if (clear_free_space_tree) {
+		btrfs_info(fs_info, "clearing free space tree");
+		ret = btrfs_clear_free_space_tree(fs_info);
+		if (ret) {
+			btrfs_warn(fs_info,
+				   "failed to clear free space tree: %d", ret);
+			goto out;
+		}
+	}
 
 	ret = btrfs_cleanup_fs_roots(fs_info);
 	if (ret)
@@ -2960,7 +2981,6 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 	struct btrfs_root *chunk_root;
 	int ret;
 	int err = -EINVAL;
-	int clear_free_space_tree = 0;
 	int level;
 
 	ret = init_mount_fs_info(fs_info, sb);
@@ -3371,26 +3391,6 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 	if (sb_rdonly(sb))
 		goto clear_oneshot;
 
-	if (btrfs_test_opt(fs_info, CLEAR_CACHE) &&
-	    btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE)) {
-		clear_free_space_tree = 1;
-	} else if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE) &&
-		   !btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID)) {
-		btrfs_warn(fs_info, "free space tree is invalid");
-		clear_free_space_tree = 1;
-	}
-
-	if (clear_free_space_tree) {
-		btrfs_info(fs_info, "clearing free space tree");
-		ret = btrfs_clear_free_space_tree(fs_info);
-		if (ret) {
-			btrfs_warn(fs_info,
-				   "failed to clear free space tree: %d", ret);
-			close_ctree(fs_info);
-			return ret;
-		}
-	}
-
 	ret = btrfs_start_pre_rw_mount(fs_info);
 	if (ret) {
 		close_ctree(fs_info);