xfs: reserve AG space for the refcount btree root

Reduce the max AG usable space size so that we always have space for
the refcount btree root.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index be7e3fcefd73..effb64cf714f 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c
@@ -128,6 +128,8 @@ xfs_alloc_ag_max_usable(
                 blocks++;               /* finobt root block */
         if (xfs_sb_version_hasrmapbt(&mp->m_sb))
                 blocks++;               /* rmap root block */
+        if (xfs_sb_version_hasreflink(&mp->m_sb))
+                blocks++;               /* refcount root block */
 
         return mp->m_sb.sb_agblocks - blocks;
 }