btrfs: Fix early enospc because 'unused' calculated with wrong sign.

'unused' calculated with wrong sign in reserve_metadata_bytes().
This might have lead to unwanted over-reservations.

Signed-off-by: Arne Jansen <sensille@gmx.net>
Reviewed-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index a541bc8..ddaf634 100644 (file)
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3413,7 +3413,7 @@ again:
         * our reservation.
         */
        if (unused <= space_info->total_bytes) {
-               unused -= space_info->total_bytes;
+               unused = space_info->total_bytes - unused;
                if (unused >= num_bytes) {
                        if (!reserved)
                                space_info->bytes_reserved += orig_bytes;