xfs: compute absolute maximum nlevels for each btree type

Add code for all five btree types so that we can compute the absolute
maximum possible btree height for each btree type.  This is a setup for
the next patch, which makes every btree type have its own cursor cache.

The functions are exported so that we can have xfs_db report the
absolute maximum btree heights for each btree type, rather than making
everyone run their own ad-hoc computations.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

1 files changed, 28 insertions, 3 deletions

diff --git a/fs/xfs/libxfs/xfs_bmap_btree.c b/fs/xfs/libxfs/xfs_bmap_btree.c
index b90122de0df0..59d146696a62 100644
--- a/fs/xfs/libxfs/xfs_bmap_btree.c
+++ b/fs/xfs/libxfs/xfs_bmap_btree.c
@@ -571,6 +571,17 @@ xfs_bmbt_init_cursor(
 	return cur;
 }
 
+/* Calculate number of records in a block mapping btree block. */
+static inline unsigned int
+xfs_bmbt_block_maxrecs(
+	unsigned int		blocklen,
+	bool			leaf)
+{
+	if (leaf)
+		return blocklen / sizeof(xfs_bmbt_rec_t);
+	return blocklen / (sizeof(xfs_bmbt_key_t) + sizeof(xfs_bmbt_ptr_t));
+}
+
 /*
  * Calculate number of records in a bmap btree block.
  */
@@ -581,10 +592,24 @@ xfs_bmbt_maxrecs(
 	int			leaf)
 {
 	blocklen -= XFS_BMBT_BLOCK_LEN(mp);
+	return xfs_bmbt_block_maxrecs(blocklen, leaf);
+}
 
-	if (leaf)
-		return blocklen / sizeof(xfs_bmbt_rec_t);
-	return blocklen / (sizeof(xfs_bmbt_key_t) + sizeof(xfs_bmbt_ptr_t));
+/* Compute the max possible height for block mapping btrees. */
+unsigned int
+xfs_bmbt_maxlevels_ondisk(void)
+{
+	unsigned int		minrecs[2];
+	unsigned int		blocklen;
+
+	blocklen = min(XFS_MIN_BLOCKSIZE - XFS_BTREE_SBLOCK_LEN,
+		       XFS_MIN_CRC_BLOCKSIZE - XFS_BTREE_SBLOCK_CRC_LEN);
+
+	minrecs[0] = xfs_bmbt_block_maxrecs(blocklen, true) / 2;
+	minrecs[1] = xfs_bmbt_block_maxrecs(blocklen, false) / 2;
+
+	/* One extra level for the inode root. */
+	return xfs_btree_compute_maxlevels(minrecs, MAXEXTNUM) + 1;
 }
 
 /*