Btrfs: fix regression in raid level conversion

commit 153c35b6 upstream.

Commit 2f081088 changed
btrfs_set_block_group_ro to avoid trying to allocate new chunks with the
new raid profile during conversion.  This fixed failures when there was
no space on the drive to allocate a new chunk, but the metadata
reserves were sufficient to continue the conversion.

But this ended up causing a regression when the drive had plenty of
space to allocate new chunks, mostly because reduce_alloc_profile isn't
using the new raid profile.

Fixing btrfs_reduce_alloc_profile is a bigger patch.  For now, do a
partial revert of 2f081088, and don't error out if we hit ENOSPC.
Signed-off-by: Chris Mason <clm@fb.com>
Tested-by: Dave Sterba <dsterba@suse.cz>
Reported-by: Holger Hoffstaette <holger.hoffstaette@googlemail.com>
[adapted for stable kernel branch, v4.0.5]
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/btrfs/extent-tree.c

@@ -8535,6 +8535,24 @@ int btrfs_set_block_group_ro(struct btrfs_root *root,
 	trans = btrfs_join_transaction(root);
 	if (IS_ERR(trans))
 		return PTR_ERR(trans);
+	/*
+	 * if we are changing raid levels, try to allocate a corresponding
+	 * block group with the new raid level.
+	 */
+	alloc_flags = update_block_group_flags(root, cache->flags);
+	if (alloc_flags != cache->flags) {
+		ret = do_chunk_alloc(trans, root, alloc_flags,
+				     CHUNK_ALLOC_FORCE);
+		/*
+		 * ENOSPC is allowed here, we may have enough space
+		 * already allocated at the new raid level to
+		 * carry on
+		 */
+		if (ret == -ENOSPC)
+			ret = 0;
+		if (ret < 0)
+			goto out;
+	}
 
 	ret = set_block_group_ro(cache, 0);
 	if (!ret)