xfs: compute rtrmap btree max levels when reflink enabled

Compute the maximum possible height of the realtime rmap btree when
reflink is enabled.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>

diff --git a/fs/xfs/libxfs/xfs_rtrmap_btree.c b/fs/xfs/libxfs/xfs_rtrmap_btree.c
index 83b0eba46ae352e021a7602734e47ad3dcae441d..a516b3b909a917912c184e3083a7febd956c2be8 100644 (file)
--- a/fs/xfs/libxfs/xfs_rtrmap_btree.c
+++ b/fs/xfs/libxfs/xfs_rtrmap_btree.c
@@ -702,6 +702,7 @@ xfs_rtrmapbt_maxrecs(
 unsigned int
 xfs_rtrmapbt_maxlevels_ondisk(void)
 {
+       unsigned long long      max_dblocks;
        unsigned int            minrecs[2];
        unsigned int            blocklen;
 
@@ -710,8 +711,20 @@ xfs_rtrmapbt_maxlevels_ondisk(void)
        minrecs[0] = xfs_rtrmapbt_block_maxrecs(blocklen, true) / 2;
        minrecs[1] = xfs_rtrmapbt_block_maxrecs(blocklen, false) / 2;
 
-       /* We need at most one record for every block in an rt group. */
-       return xfs_btree_compute_maxlevels(minrecs, XFS_MAX_RGBLOCKS);
+       /*
+        * Compute the asymptotic maxlevels for an rtrmapbt on any rtreflink fs.
+        *
+        * On a reflink filesystem, each block in an rtgroup can have up to
+        * 2^32 (per the refcount record format) owners, which means that
+        * theoretically we could face up to 2^64 rmap records.  However, we're
+        * likely to run out of blocks in the data device long before that
+        * happens, which means that we must compute the max height based on
+        * what the btree will look like if it consumes almost all the blocks
+        * in the data device due to maximal sharing factor.
+        */
+       max_dblocks = -1U; /* max ag count */
+       max_dblocks *= XFS_MAX_CRC_AG_BLOCKS;
+       return xfs_btree_space_to_height(minrecs, max_dblocks);
 }
 
 int __init
@@ -750,9 +763,20 @@ xfs_rtrmapbt_compute_maxlevels(
         * maximum height is constrained by the size of the data device and
         * the height required to store one rmap record for each block in an
         * rt group.
+        *
+        * On a reflink filesystem, each rt block can have up to 2^32 (per the
+        * refcount record format) owners, which means that theoretically we
+        * could face up to 2^64 rmap records.  This makes the computation of
+        * maxlevels based on record count meaningless, so we only consider the
+        * size of the data device.
         */
        d_maxlevels = xfs_btree_space_to_height(mp->m_rtrmap_mnr,
                                mp->m_sb.sb_dblocks);
+       if (xfs_has_rtreflink(mp)) {
+               mp->m_rtrmap_maxlevels = d_maxlevels + 1;
+               return;
+       }
+
        r_maxlevels = xfs_btree_compute_maxlevels(mp->m_rtrmap_mnr,
                                mp->m_rgblocks);