diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
index de6d585c00f1..0930c441159d 100644
--- a/fs/xfs/libxfs/xfs_bmap.c
+++ b/fs/xfs/libxfs/xfs_bmap.c
@@ -4242,7 +4242,7 @@ xfs_bmapi_convert_unwritten(
 	return 0;
 }
 
-static inline xfs_extlen_t
+xfs_extlen_t
 xfs_bmapi_minleft(
 	struct xfs_trans	*tp,
 	struct xfs_inode	*ip,
diff --git a/fs/xfs/libxfs/xfs_bmap.h b/fs/xfs/libxfs/xfs_bmap.h
index 01c2df35c3e3..524912f276f8 100644
--- a/fs/xfs/libxfs/xfs_bmap.h
+++ b/fs/xfs/libxfs/xfs_bmap.h
@@ -220,6 +220,8 @@ int	xfs_bmap_add_extent_unwritten_real(struct xfs_trans *tp,
 		struct xfs_inode *ip, int whichfork,
 		struct xfs_iext_cursor *icur, struct xfs_btree_cur **curp,
 		struct xfs_bmbt_irec *new, int *logflagsp);
+xfs_extlen_t xfs_bmapi_minleft(struct xfs_trans *tp, struct xfs_inode *ip,
+		int fork);
 
 enum xfs_bmap_intent_type {
 	XFS_BMAP_MAP = 1,
diff --git a/fs/xfs/libxfs/xfs_bmap_btree.c b/fs/xfs/libxfs/xfs_bmap_btree.c
index cfa052d40105..18de4fbfef4e 100644
--- a/fs/xfs/libxfs/xfs_bmap_btree.c
+++ b/fs/xfs/libxfs/xfs_bmap_btree.c
@@ -213,18 +213,16 @@ xfs_bmbt_alloc_block(
 	if (args.fsbno == NULLFSBLOCK) {
 		args.fsbno = be64_to_cpu(start->l);
 		args.type = XFS_ALLOCTYPE_START_BNO;
+
 		/*
-		 * Make sure there is sufficient room left in the AG to
-		 * complete a full tree split for an extent insert.  If
-		 * we are converting the middle part of an extent then
-		 * we may need space for two tree splits.
-		 *
-		 * We are relying on the caller to make the correct block
-		 * reservation for this operation to succeed.  If the
-		 * reservation amount is insufficient then we may fail a
-		 * block allocation here and corrupt the filesystem.
+		 * If we are coming here from something like unwritten extent
+		 * conversion, there has been no data extent allocation already
+		 * done, so we have to ensure that we attempt to locate the
+		 * entire set of bmbt allocations in the same AG, as
+		 * xfs_bmapi_write() would have reserved.
 		 */
-		args.minleft = args.tp->t_blk_res;
+		args.minleft = xfs_bmapi_minleft(cur->bc_tp, cur->bc_ino.ip,
+						cur->bc_ino.whichfork);
 	} else if (cur->bc_tp->t_flags & XFS_TRANS_LOWMODE) {
 		args.type = XFS_ALLOCTYPE_START_BNO;
 	} else {
@@ -248,6 +246,7 @@ xfs_bmbt_alloc_block(
 		 * successful activate the lowspace algorithm.
 		 */
 		args.fsbno = 0;
+		args.minleft = 0;
 		args.type = XFS_ALLOCTYPE_FIRST_AG;
 		error = xfs_alloc_vextent(&args);
 		if (error)