[17/43] xfs: compute rtrmap btree max levels when reflink enabled

Commit Message

Darrick J. Wong Dec. 19, 2024, 7:37 p.m. UTC

From: Darrick J. Wong <[e-mail]>

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

Signed-off-by: "Darrick J. Wong" <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
---
 fs/xfs/libxfs/xfs_rtrmap_btree.c |   28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

Patch

diff --git a/fs/xfs/libxfs/xfs_rtrmap_btree.c b/fs/xfs/libxfs/xfs_rtrmap_btree.c
index 3cb8f126b9ce16..04b9c76380adb0 100644
--- a/fs/xfs/libxfs/xfs_rtrmap_btree.c
+++ b/fs/xfs/libxfs/xfs_rtrmap_btree.c
@@ -718,6 +718,7 @@  xfs_rtrmapbt_maxrecs(
 unsigned int
 xfs_rtrmapbt_maxlevels_ondisk(void)
 {
+	unsigned long long	max_dblocks;
 	unsigned int		minrecs[2];
 	unsigned int		blocklen;
 
@@ -726,8 +727,20 @@  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
@@ -766,9 +779,20 @@  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_groups[XG_TYPE_RTG].blocks);