diff --git a/include/xfs_mount.h b/include/xfs_mount.h
index 2102009aa8df73..1d36e3986ead2f 100644
--- a/include/xfs_mount.h
+++ b/include/xfs_mount.h
@@ -26,6 +26,32 @@ enum {
 
 struct xfs_groups {
 	struct xarray		xa;
+
+	/*
+	 * Maximum capacity of the group in FSBs.
+	 *
+	 * Each group is laid out densely in the daddr space.  For the
+	 * degenerate case of a pre-rtgroups filesystem, the incore rtgroup
+	 * pretends to have a zero-block and zero-blklog rtgroup.
+	 */
+	uint32_t		blocks;
+
+	/*
+	 * Log(2) of the logical size of each group.
+	 *
+	 * Compared to the blocks field above this is rounded up to the next
+	 * power of two, and thus lays out the xfs_fsblock_t/xfs_rtblock_t
+	 * space sparsely with a hole from blocks to (1 << blklog) at the end
+	 * of each group.
+	 */
+	uint8_t			blklog;
+
+	/*
+	 * Mask to extract the group-relative block number from a FSB.
+	 * For a pre-rtgroups filesystem we pretend to have one very large
+	 * rtgroup, so this mask must be 64-bit.
+	 */
+	uint64_t		blkmask;
 };
 
 /*
diff --git a/libxfs/libxfs_api_defs.h b/libxfs/libxfs_api_defs.h
index 8f3e9e8694675d..483a7a9a4cbf45 100644
--- a/libxfs/libxfs_api_defs.h
+++ b/libxfs/libxfs_api_defs.h
@@ -168,6 +168,7 @@
 #define xfs_free_extent_later		libxfs_free_extent_later
 #define xfs_free_perag_range		libxfs_free_perag_range
 #define xfs_fs_geometry			libxfs_fs_geometry
+#define xfs_gbno_to_fsb			libxfs_gbno_to_fsb
 #define xfs_get_initial_prid		libxfs_get_initial_prid
 #define xfs_highbit32			libxfs_highbit32
 #define xfs_highbit64			libxfs_highbit64
diff --git a/libxfs/xfs_group.c b/libxfs/xfs_group.c
index 9c7fa99d00b802..80b6993cc9916e 100644
--- a/libxfs/xfs_group.c
+++ b/libxfs/xfs_group.c
@@ -212,3 +212,12 @@ xfs_group_insert(
 #endif
 	return error;
 }
+
+struct xfs_group *
+xfs_group_get_by_fsb(
+	struct xfs_mount	*mp,
+	xfs_fsblock_t		fsbno,
+	enum xfs_group_type	type)
+{
+	return xfs_group_get(mp, xfs_fsb_to_gno(mp, fsbno, type), type);
+}
diff --git a/libxfs/xfs_group.h b/libxfs/xfs_group.h
index 0ff6e1d5635cb1..5b7362277c3f7a 100644
--- a/libxfs/xfs_group.h
+++ b/libxfs/xfs_group.h
@@ -46,6 +46,8 @@ struct xfs_group {
 
 struct xfs_group *xfs_group_get(struct xfs_mount *mp, uint32_t index,
 		enum xfs_group_type type);
+struct xfs_group *xfs_group_get_by_fsb(struct xfs_mount *mp,
+		xfs_fsblock_t fsbno, enum xfs_group_type type);
 struct xfs_group *xfs_group_hold(struct xfs_group *xg);
 void xfs_group_put(struct xfs_group *xg);
 
@@ -72,4 +74,58 @@ int xfs_group_insert(struct xfs_mount *mp, struct xfs_group *xg,
 #define xfs_group_marked(_mp, _type, _mark) \
 	xa_marked(&(_mp)->m_groups[(_type)].xa, (_mark))
 
+static inline xfs_agblock_t
+xfs_group_max_blocks(
+	struct xfs_group	*xg)
+{
+	return xg->xg_mount->m_groups[xg->xg_type].blocks;
+}
+
+static inline xfs_fsblock_t
+xfs_group_start_fsb(
+	struct xfs_group	*xg)
+{
+	return ((xfs_fsblock_t)xg->xg_gno) <<
+		xg->xg_mount->m_groups[xg->xg_type].blklog;
+}
+
+static inline xfs_fsblock_t
+xfs_gbno_to_fsb(
+	struct xfs_group	*xg,
+	xfs_agblock_t		gbno)
+{
+	return xfs_group_start_fsb(xg) | gbno;
+}
+
+static inline xfs_daddr_t
+xfs_gbno_to_daddr(
+	struct xfs_group	*xg,
+	xfs_agblock_t		gbno)
+{
+	struct xfs_mount	*mp = xg->xg_mount;
+	uint32_t		blocks = mp->m_groups[xg->xg_type].blocks;
+
+	return XFS_FSB_TO_BB(mp, (xfs_fsblock_t)xg->xg_gno * blocks + gbno);
+}
+
+static inline uint32_t
+xfs_fsb_to_gno(
+	struct xfs_mount	*mp,
+	xfs_fsblock_t		fsbno,
+	enum xfs_group_type	type)
+{
+	if (!mp->m_groups[type].blklog)
+		return 0;
+	return fsbno >> mp->m_groups[type].blklog;
+}
+
+static inline xfs_agblock_t
+xfs_fsb_to_gbno(
+	struct xfs_mount	*mp,
+	xfs_fsblock_t		fsbno,
+	enum xfs_group_type	type)
+{
+	return fsbno & mp->m_groups[type].blkmask;
+}
+
 #endif /* __LIBXFS_GROUP_H */
diff --git a/libxfs/xfs_sb.c b/libxfs/xfs_sb.c
index d32f789037389f..9120a3377735b0 100644
--- a/libxfs/xfs_sb.c
+++ b/libxfs/xfs_sb.c
@@ -999,6 +999,8 @@ xfs_sb_mount_common(
 	struct xfs_mount	*mp,
 	struct xfs_sb		*sbp)
 {
+	struct xfs_groups	*ags = &mp->m_groups[XG_TYPE_AG];
+
 	mp->m_agfrotor = 0;
 	atomic_set(&mp->m_agirotor, 0);
 	mp->m_maxagi = mp->m_sb.sb_agcount;
@@ -1009,6 +1011,11 @@ xfs_sb_mount_common(
 	mp->m_blockmask = sbp->sb_blocksize - 1;
 	mp->m_blockwsize = sbp->sb_blocksize >> XFS_WORDLOG;
 	mp->m_blockwmask = mp->m_blockwsize - 1;
+
+	ags->blocks = mp->m_sb.sb_agblocks;
+	ags->blklog = mp->m_sb.sb_agblklog;
+	ags->blkmask = xfs_mask32lo(mp->m_sb.sb_agblklog);
+
 	xfs_mount_sb_set_rextsize(mp, sbp);
 
 	mp->m_alloc_mxr[0] = xfs_allocbt_maxrecs(mp, sbp->sb_blocksize, true);
diff --git a/repair/bmap_repair.c b/repair/bmap_repair.c
index f052b5dcddff08..7e7c2a39f5724b 100644
--- a/repair/bmap_repair.c
+++ b/repair/bmap_repair.c
@@ -154,7 +154,7 @@ xrep_bmap_walk_rmap(
 	    !(rec->rm_flags & XFS_RMAP_ATTR_FORK))
 		return 0;
 
-	fsbno = xfs_agbno_to_fsb(to_perag(cur->bc_group), rec->rm_startblock);
+	fsbno = libxfs_gbno_to_fsb(cur->bc_group, rec->rm_startblock);
 
 	if (rec->rm_flags & XFS_RMAP_BMBT_BLOCK) {
 		rb->old_bmbt_block_count += rec->rm_blockcount;
