btrfs-progs: do not use btrfs_commit_transaction() just to update super blocks

There are several call sites utilizing btrfs_commit_transaction() just
to update members in super blocks, without any metadata update.

This can be problematic for some simple call sites, like zero_log_tree()
or check_and_repair_super_num_devs().

If we have big problems preventing the fs to be mounted in the first
place, and need to clear the log or super block size, but by some other
problems in extent tree, we're unable to allocate new blocks.

Then we fall into a deadlock that, we need to mount (even
ro,rescue=all) to collect extra info, but btrfs-progs can not do any
super block updates.

Fix the problem by allowing the following super blocks only operations
to be done without using btrfs_commit_transaction():

- btrfs_fix_super_size()
- check_and_repair_super_num_devs()
- zero_log_tree().

There are some exceptions in btrfstune.c, related to the csum type
conversion and seed flags.

In those btrfstune cases, we in fact wants to proper error report in
btrfs_commit_transaction(), as those operations are not mount critical,
and any early error can be helpful to expose any problems in the fs.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

check/main.c

@@ -9659,17 +9659,12 @@ out:
 
 static int zero_log_tree(struct btrfs_root *root)
 {
-	struct btrfs_trans_handle *trans;
 	int ret;
 
-	trans = btrfs_start_transaction(root, 1);
-	if (IS_ERR(trans)) {
-		ret = PTR_ERR(trans);
-		return ret;
-	}
 	btrfs_set_super_log_root(gfs_info->super_copy, 0);
 	btrfs_set_super_log_root_level(gfs_info->super_copy, 0);
-	ret = btrfs_commit_transaction(trans, root);
+	/* Don't use transaction for overwriting only the super block */
+	ret = write_all_supers(gfs_info);
 	return ret;
 }
 

check/mode-common.c

@@ -1628,7 +1628,6 @@ static int get_num_devs_in_chunk_tree(struct btrfs_fs_info *fs_info)
 
 int check_and_repair_super_num_devs(struct btrfs_fs_info *fs_info)
 {
-	struct btrfs_trans_handle *trans;
 	int found_devs;
 	int ret;
 
@@ -1650,23 +1649,14 @@ int check_and_repair_super_num_devs(struct btrfs_fs_info *fs_info)
 		return -EUCLEAN;
 
 	/*
-	 * Repair is pretty simple, just reset the super block value and
-	 * commit a new transaction.
+	 * Repair is simple, reset the super block value and write back all the
+	 * super blocks. Do not use transaction for that.
 	 */
-	trans = btrfs_start_transaction(fs_info->tree_root, 0);
-	if (IS_ERR(trans)) {
-		ret = PTR_ERR(trans);
-		errno = -ret;
-		error("failed to start trans: %m");
-		return ret;
-	}
-
 	btrfs_set_super_num_devices(fs_info->super_copy, found_devs);
-	ret = btrfs_commit_transaction(trans, fs_info->tree_root);
+	ret = write_all_supers(fs_info);
 	if (ret < 0) {
 		errno = -ret;
-		error("failed to commit trans: %m");
-		btrfs_abort_transaction(trans, ret);
+		error("failed to write super blocks: %m");
 		return ret;
 	}
 	printf("Successfully reset super num devices to %u\n", found_devs);

kernel-shared/volumes.c

@@ -2861,7 +2861,6 @@ err:
  */
 int btrfs_fix_super_size(struct btrfs_fs_info *fs_info)
 {
-	struct btrfs_trans_handle *trans;
 	struct btrfs_device *device;
 	struct list_head *dev_list = &fs_info->fs_devices->devices;
 	u64 total_bytes = 0;
@@ -2886,19 +2885,11 @@ int btrfs_fix_super_size(struct btrfs_fs_info *fs_info)
 		return 0;
 
 	btrfs_set_super_total_bytes(fs_info->super_copy, total_bytes);
-
-	/* Commit transaction to update all super blocks */
-	trans = btrfs_start_transaction(fs_info->tree_root, 1);
-	if (IS_ERR(trans)) {
-		ret = PTR_ERR(trans);
-		errno = -ret;
-		error("error starting transaction: %d (%m)", ret);
-		return ret;
-	}
-	ret = btrfs_commit_transaction(trans, fs_info->tree_root);
+	/* Do not use transaction for overwriting only the super block */
+	ret = write_all_supers(fs_info);
 	if (ret < 0) {
 		errno = -ret;
-		error("failed to commit current transaction: %d (%m)", ret);
+		error("failed to write super blocks: %d (%m)", ret);
 		return ret;
 	}
 	printf("Fixed super total bytes, old size: %llu new size: %llu\n",