xfs: compute the maximum height of the rmap btree when reflink enabled

Instead of assuming that the hardcoded XFS_BTREE_MAXLEVELS value is big
enough to handle the maximally tall rmap btree when all blocks are in
use and maximally shared, let's compute the maximum height assuming the
rmapbt consumes as many blocks as possible.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

1 files changed, 27 insertions, 18 deletions

diff --git a/fs/xfs/libxfs/xfs_rmap_btree.c b/fs/xfs/libxfs/xfs_rmap_btree.c
index 1b48b7b3ee30..7388201e48d2 100644
--- a/fs/xfs/libxfs/xfs_rmap_btree.c
+++ b/fs/xfs/libxfs/xfs_rmap_btree.c
@@ -540,26 +540,35 @@ void
 xfs_rmapbt_compute_maxlevels(
 	struct xfs_mount		*mp)
 {
-	/*
-	 * On a non-reflink filesystem, the maximum number of rmap
-	 * records is the number of blocks in the AG, hence the max
-	 * rmapbt height is log_$maxrecs($agblocks).  However, with
-	 * reflink each AG 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.
-	 *
-	 * That effectively means that the max rmapbt height must be
-	 * XFS_BTREE_MAXLEVELS.  "Fortunately" we'll run out of AG
-	 * blocks to feed the rmapbt long before the rmapbt reaches
-	 * maximum height.  The reflink code uses ag_resv_critical to
-	 * disallow reflinking when less than 10% of the per-AG metadata
-	 * block reservation since the fallback is a regular file copy.
-	 */
-	if (xfs_has_reflink(mp))
-		mp->m_rmap_maxlevels = XFS_BTREE_MAXLEVELS;
-	else
+	if (!xfs_has_rmapbt(mp)) {
+		mp->m_rmap_maxlevels = 0;
+		return;
+	}
+
+	if (xfs_has_reflink(mp)) {
+		/*
+		 * Compute the asymptotic maxlevels for an rmap btree on a
+		 * filesystem that supports reflink.
+		 *
+		 * On a reflink filesystem, each AG 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.
+		 * However, we're likely to run out of blocks in the AG 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 AG due to maximal
+		 * sharing factor.
+		 */
+		mp->m_rmap_maxlevels = xfs_btree_space_to_height(mp->m_rmap_mnr,
+				mp->m_sb.sb_agblocks);
+	} else {
+		/*
+		 * If there's no block sharing, compute the maximum rmapbt
+		 * height assuming one rmap record per AG block.
+		 */
 		mp->m_rmap_maxlevels = xfs_btree_compute_maxlevels(
 				mp->m_rmap_mnr, mp->m_sb.sb_agblocks);
+	}
 }
 
 /* Calculate the refcount btree size for some records. */